摘要:

A VERY interesting discussion followed the paper LA. on the training of engineers given by Sir Theodore Morison on Oct. 29 before the North-East Coast Institution of Engineers and Shipbuilders; and a report of it has now been published by the Institution. Those engaging in the discussion were representative of every branch of the profession, and while on some questions differences of opinion were manifest, there was a number of interesting points on which opinion appeared to be practically unanimous. Among these was the general agreement that all university students of engineering should have at least one year's works experience before beginning their university training, and that during this period evening classes should be taken in mathematics and physics with the view of conserving the habit of study engendered at school. This plan has been advocated for many years by a number of those engaged in university teaching, and it is of interest to know that such procedure now appears to be so generally approved among practising engineers who have studied the question of engineering training. Its advantages are many. It brings a student to his university course with a realisation of the bearing of its scientific training on his future career, such as, in the majority of cases, increases notably his keenness in the work. It gives him, at the most plastic stage of his life, intimate contact with the working man whom in future years he will probably have to control, while the routine work and drudgery which is usually connected with this first period of training is likely to weed out any one whose real interests are not in engineering, and to give him the opportunity of withdrawing before much time has been wasted. The discussion would have been important if this were the only point on which it has given a definite lead. The chief difficulty in applying it is that at present scarcely any engineering firm in Great Britain is prepared to take a youth for one year preliminary to his university training. If the North-East Coast Institution can persuade its members to adopt this course, arranging to take the youth back to complete his practical training after his graduation, it will have done a very important piece of work and made a real advance in the training of engineers.Another point emphasised in the discussion was the somewhat haphazard method adopted by the majority of firms in the shop training of their special apprentices. In this connexion matters have improved during the past fifteen years or so, but even now the number of firms which has arranged for the apprentices carefully graded courses of shop training, and see that this training is adhered to, is almost negligibly small. The sandwich system, in which a youth spends alternate periods of six months in works and at the university, received some support. It is, however, becoming more generally acknowledged that the value to a young engineer of a university course lies almost as much in the facilities which are afforded him for intercourse with fellow students in his own and especially in other branches of study, and in the general activities of student life, as in the technical training which he receives, and that the necessity for keeping terms different from those of the remainder of the university, which is necessary where the sandwich system is adopted, is, from this point of view, a serious objection to the system.Another point on which opinion was almost unanimous has reference to the requirements for entry to a university course. It was generally felt that the present matriculation regulations operate adversely in the case of many excellent students whose education prior to a shop apprenticeship has been obtained at an elementary school, and whose later work at evening technical schools has shown them to be excellent engineering material. While one or two universities have recognised the difficulty and have made special provision for such students, the majority of British universities still insist on the normal matriculation, except indeed for mature candidates, and there is little doubt that this insistence is barring each year from the universities a number of young engineers who would richly repay their training.As Sir Theodore Morison pointed out, engineering is becoming more and more dependent yearly on scientific developments, and its future in Great Britain will depend largely on the type of mind that can be recruited into the profession, and on the methods adopted in the training of engineers. These methods will be investigated by a committee of the North-East Coast Institution of Engineers and Shipbuilders, as suggested by Sir Theodore. It is now more than fifteen years since engineering education was discussed fully at a conference arranged by the Institution of Civil Engineers, and changing conditions have made desirable the re-examination of its many aspects. The recommendations of the committee set up by the NorthEast Coast Institution will be awaited, therefore, with much interest.

ISSN:0028-0836    

DOI:10.1038/119225a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

MR. NEVILLE JONES has earned our lil thanks for having brought together the evidence of various stone age cultures as derived from Rhodesia, together with some of the dnta culled from a wider South African area. He has produced a very readable book which contains some valuable information. The whole problem of the South African stone age cultures is every day becoming more interesting and suggestive; and now that search for relics is being conducted upon more scientific lines, we can see the opening of an archaeological chapter of high importance. The author's own researches at Sawmills (South Rhodesia) and at Tiger Kloof and Taungs (Cape Province) are valuable as evidencing culture-levels of lowerand mid-Palsoolithic facies, which can be clearly distinguished and placed in sequence order. He very rightly urges the paramount necessity of attacking the archaeological problems from the viewpoint of geological evidence. The numberless stone implements which may be picked up on the surface can teach us little unless we can identify the geological horizons to which they originally belonged. The spade alone can provide a clear idea as to the several cultures which may be distinguished, and their relative positions in the time-sequence. Mr. Jones has worked conscientiously and thoughtfully, and his results help to advance the subject. The value of the book is somewhat marred by unduly positive statements, which lack the support of evidence. To quote examples, which I hope may be modified in a future edition: On p. 12 (and cf. p. 19), in referring to mid-Paleolithic times, Mr. Jones says, "the core implement was thus gradually superseded by the flake implement, and finally disappeared altogether." In actual fact, core implements persisted in great abundance all through the Neolithic period, while within the Palaeolithic period they form one of the most striking features of the Solutr6an culture-phase. On p. 23 he says, " In Europe the stone implement, in Palheolithic times at any rate, was invariably made of flint," a statement which is quite untrue, since chert, quartzite, and other materials were frequently employed. On p. 19, in his diagram (adapted from Osborn), he asserts that the " bone industry begins " in Solutr6an times. Why does he ignore the bone implements so characteristic of the Aurignacian culture ? On p. 44 he states that " in the Maramba . . . the only large paleoliths found in its bed are of diorite." This is not my own experience, for I have picked up in the bed of this tributary of the Zambezi typical large palveoliths of chalcedony and other materials, in addition to those of diorite. It is rather unduly committal at the present stage of our knowledge to assert (p. 21) that " we know for a fact that South Africa was Aurignacian in culture until the Bantu immigrants introduced the use of iron." So positive a dogma is rather premature and may tend to hamper unbiassed research in a field which is essentially complex.I find it sometimes difficult to follow the author in his descriptions of stone-working technique and in some of his diagnoses of implement types. The 'tortoise-core ' figured on p. 53 appears to be quite aberrant and to lack the main characteristics of this specialised core-type. Mr. Jones remarks (p. 26) that " the Mousterian culture has not yet been demonstrated in Rhodesia," but if the 'tortoise-core' industry is mainly suggestive of that culture, as many maintain, it may interest him to know that I have found on the Zambezi banks excellent examples of the 'Levallois ' flakes struck from such prepared cores. The author's description of the probable process whereby the leaf-shaped blades from the Cape Flats were made can scarcely carry conviction. Has he ever tried to effect the " removal, probably by pressure [italics mine], of minute chips over the entire surface " of a large piece of quartzite ? If so, has he ever produced an effect similar to the surface-flaking of this type of implement ? I venture to doubt it. He compares (pp. 56 and 113) the finest of the Cape Flats implements with the best examples of Solutrean technique; but this seems to do scant justice to the marvellous skill exhibited by Solutr6an man. Surely no blades in any way comparable with those of Volgu have as yet been found in South Africa, and the Cape Flats examples, for all their high quality, exhibit, to my mind, a far easier technique. The author does not refer to the discovery in Rhodesia of minute implements of Tardenoisian type, though it is many years since the first example was found in Buluwayo. Tardenoisian types are now being discovered abundantly in South Africa, and are a noteworthy feature of the region.The illustrations of implements in this book vary much in value. Some (e.g. Fig. 39) are quite good; others are so lacking in detail and 'crispness ' that they convey little beyond mere outline and nothing as regards technique. The description (p. 54) and the figure referred to (Fig. 13, No.26) appear to be totally unrelated. Apart, however, from these and several other points which invite criticism, the book is a welcome one, dealing with problems of ever-increasing interest. The author endeavours to approach his subject without bias, and to treat the evidence as he finds it. The task of correlating the South African stone age cultures with those of Europe is beset with pitfalls, but is none the less attractive on that account.The author's diagnosis of Bushman art is carefully thought out and is in agreement with the most widely held views. His statement of the case for magic versus ' art for art's sake,' as the primary incentive, would have been greatly reinforced had he brought to bear upon it the evidence afforded by late Palheolithic art in Europe. It is interesting to note that he adopts the theory that the gravels lying along the top of the gorge of the Zambezi, below the Victoria Falls, are the remains of ancient gravels deposited by the Zambezi itself. My own investigations, conducted during three visits to the region (in 1905, 1907, and 1910), led me to adopt this view, which, pace Dr. Codrington, has been supported by several very able geologists. The Appendix III. in Mr. Jones's book, detailing the views of Dr. Hrdlicka upon this problem, should not be taken too seriously. The very hurried survey conducted by a distinguished physical anthropologist, in company with two American newspaper men and a South African engineer, does not carry conviction, nor does it dispose of the views of such capable geologists as Lamplugh, Zealley, and others. Further, close examination by trained geologists is needed to settle the matter finally. The age of these Zambezi gravels is one of the most important problems awaiting solution.

ISSN:0028-0836    

DOI:10.1038/119226a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

"AS the artist, after painstaking effort, steps llhack from his easel to view his picture as a whole, so it may not be unprofitable for the scientist to forsake from time to time his own specialty and survey the general trend of science." So writes the author of this volume of Silliman lectures. Not only is such action not unprofitable, but also it is of the highest importance that leading men of science who, like Prof. G. N. Lewis, possess in such high degree wide scientific knowledge, clear philosophic insight, and the power of lucid and interesting expression, should from time to time examine the existing state of science and the directions along which our knowledge of the universe is advancing. The author's purpose in this volume is to present " a kind of contemporaneous cross section showing the inner structure of science," and the book is addressed to those " who are interested not so much in the products of science as in its methods." By all such, these lectures will be given a hearty welcome. In the lecture on space and geometry are discussedthenon-Euclideangeometries, more especially Minkowski's geometry of asymptotic rotation, the details of which have been worked out by the author and Prof. E. B. Wilson, and the geometry of shear rotation. According to Minkowski's geometry, all points on the hyperbolic curve are equidistant from the point of intersection of the asymptotes, and any interval marked off on one of these asymptotes is of zero length. To a Euclidean mind such language is nonsense, but, as the author points out, each geometry has its own definitions and postulates just as each game has its own rules; and to the query whether such geometry is true or false, the author replies that the question has no meaning in science. All that one is concerned with, as in the case of all scientific theories, is its usefulness; and the usefulness of the Minkowski geometry is demonstrated by the author in connexion with the new kinematics based on the Einstein theory of relativity-a kinematics which Euclidean geometry is powerless to express.In discussing light and the quantum, the author introduces his theory of absorption of light and interference, an account of which has already been published in NATURE (117, 236, 1926). In this theory the assumption is made that an atom never emits light except to another atom, and one can no longer regard the one atom as an active agent and the other as a passive recipient. According to this theory, when one arranges an optical apparatus so that interference bands are produced, one not only brings it about that no light is received on the dark band, but also the light which otherwise would have fallen in the region of the dark band is not emitted. The author recognises that such an idea, which is really equivalent to saying that we can influence events which are already past, is offensive to our established notions of causality and temporal sequence, but he holds that we need not therefore be deterred from making use of the hypothesis so long as no actual fact of experiment or observation can be brought against it, especially since by its means " certain inconsistencies between prevailing physical ideas and the geometry which so admirably interprets the kinematics of relativity are removed." This view can of course be accepted, but does it not imply that we are here dealing merely with a mathematical device which may later have to be discarded or receive interpretation by a more complete physics ? In the concluding lecture, the author boldly, but apparently with a certain amount of trepidation, attacks the problem of life. While professing ignorance of the meaning of the term 'vitalist,' the author proclaims that " it is indisputable that many of the characteristic properties of living beings are not only far beyond the reach of existing physical science, but are not even suggested by the most remote extrapolation of the laws and theories that we have made to fit the inorganic world." Of the phenomena which have no counterpart in inanimate Nature, one of the most striking is known as the struggle for existence. In concluding his lectures the author says: " The sciences of physics and biology comprise sets of man-made postulates and laws which no more need to be compatible with one another than do the geometries of Euclid and Lobachevski. The science of physics rests upon the postulate of determinism; the science of biology, unless it is to ignore deliberately the phenomena of behaviour, must abandon this postulate and substitute therefor a postulate of choice or freedom."The book is most refreshing to read, owing to the sense of humour of the author and the clearness with which he expresses his views, and it is also most stimulating and provocative of thought. It suffers, however, from over-condensation. The author confesses that the collection of " simple essays " has been brought " by the publishers to the dimensions of a book only by generous spacing between the lines," and he obviously expects the reader to read rather extensively between these lines. There are, however, very many people, not all of them laymen in the domains of science and philosophy, who are intensely curious concerning the subjects treated in these lectures, but will find it very difficult, if not impossible, to make the necessary interjections. May we not express the hope, therefore, that the author will place the world still more in his debt by giving to it an expansion of this excellent outline of a system of philosophy ? Meanwhile, we are grateful for this smaller work.

ISSN:0028-0836    

DOI:10.1038/119228a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

THIS comparatively large volume furnishes an 1excellent description of carbohydrate metabolism in general. This is only what might be expected from such a well-known author as Prof. Macleod, who has done so much useful work on this subject. To anybody wishing to get a fundamental idea of our present knowledge in this field of metabolism the book can be confidently recommended. Of all the volumes of literature on the difficult and evasive problem of carbohydrate changes in the animal body, the present appears certainly to be the best. It deals with the subject in a clear though fundamental manner, and embraces everything that is known as to the physiology of carbohydrate metabolism. The book opens with a chapter on the structure of the islets of Langerhans and their cytological characteristics. This is followed by a useful description of the structural changes which the pancreas undergoes under various experimental conditions. The various mechanisms of the different forms of glycosuria are concisely discussed, followed by a complete description of glycogen and the various changes this substance undergoes in the body. Methods for the estimation of blood sugar are mentioned, and the manner of existence of the sugar in the blood is discussed briefly. The author does not accept the view propounded a few years ago to the effect that the sugar in the blood of the healthy animal is not ordinary glucose, but a very reactive modification known as /-glucose. It is pointed out that the alleged evidence for the occurrence of y-glucose in the animal organism is exceedingly unsatisfactory, for this substance is so labile and unstable that even if it did occur in the body, it would be almost impossible, with our present methods, to isolate and identify it.Naturally, a great deal of the volume is devoted to a study of the action of insulin, and while this part of the book is done just as well as the rest of it, it must be admitted that a perusal of Prof. Macleod's critical compilation on this subject leaves the impression that there is yet very much to learn. The hopes held out a few years ago that the advent of insulin would solve many of our hitherto insoluble problems have been almost entirely unfulfilled. In spite of the very large amount of experimental research, both good and bad, carried out on the particular influence of insulin in the utilisation of carbohydrate in the animal body, it is safe to say that as yet but little advance has been made in this difficult field. When we consider that the products of sugar destruction are comparatively simple, and, from the chemical point of view, necessarily limited to a few substances, it is a matter for wonder that, so far, we have failed to trace the various steps through which sugar is broken down to carbon dioxide and water. It is true that some glimmerings of light have now and again appeared within the last few years, but nothing definite has so far transpired. Insulin has, on the whole, been a disappointment so far as it has afforded any insight into the physiological processes which the body employs. Very many papers have appeared, and are still appearing, which deal with the manner in which insulin acts, but the many controversiesand difficulties which have arisen seem to have been solved by the work of Dale and his collaborators, who state that insulin acts by changing some of the sugar of the body into glycogen, and by oxidising the rest of it. These authors believe that the disappearance of sugar, after insulin administration, can be accounted for by the sugar destroyed and the glycogen formed.This means, as was to have been anticipated from the very beginning, that insulin acts when injected just as it does when supplied normally by the pancreas. The problem is, how does insulin act when it is supplied in the natural manner to the healthy individual ? That problem still awaits solution, and we know little or nothing at present with regard to it. From the clinical point of view, it would appear that insulin in several cases produces changes in the diabetic which are not obvious in the normal individual, but it is quite possible that these changes may be dependent on the difficulty of administering the optimum dose when insulin is given artificially. However, the rapidity with which several diabetic patients become abnormally fat when taking insulin, suggests that insulin probably has some direct influence on the formation of fat from sugar in the body.To anybody interested in this fascinating though difficult subject of carbohydrate metabolism, a perusal of Prof. Macleod's volume will serve to give an exact and well-reasoned view of the present position of this subject. It would appear probable, however, that what is believed to be true to-day will undergo vast modification in the future. The subject, so far, has not yielded to attack, and whatever we write or think, it is certain that we know but little of the real facts.

ISSN:0028-0836    

DOI:10.1038/119229a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

PURSUING its work of standardisation, the 1 British Engineering Standards Association has issued a glossary in which the terms in common use in electrical engineering are defined, so as to provide engineers with a uniform nomenclature. With this end in view, the 6ompilers frequently give a list of the synonyms actually in use, indicating their preference by heavy type, and in some instances marking others as ' deprecated.' From this point of view, the sections on composite machines (i.e. converters of all types) and transformers, and that part which deals with the exact uses of such terms as ' protected,' ' enclosed,' and 'flame-proof,' will be valuable in giving exact meanings to terms which have not always been used in the same way by different writers. As is to be expected in such an undertaking, there are cases where the choice will not meet with general agreement; particularly is it to be hoped that the engineering world will not adopt the suggestion of calling the extreme range of a wave the ' double amplitude,' whilst it will be interesting to see if the terms ' active current ' (the tabulated alternatives being active component, energy component, power component, and in-phase component of the current) and ' reactive current ' (for the other component) succeed in ousting their rivals. We also note that the distinction between an oscillating and an alternating current is made to depend on whether the frequency is or is not dependent solely on the constants of the system in which it flows.As to the definitions themselves, they are for the most part adequate for their purpose, but it is doubtful whether much good is done by defining, for example, the blade of a switch, and the contact jaws, in a reciprocal manner, except in cases where it is desired to indicate that one of a number of alternatives is preferred. It is, however, in the general section that the weakest definitions occur. Apart from the vexed 4r question, we find separate definitions of quantity and of charge, and each of them is logically unsatisfactory, although no doubt adequate for the restricted purpose of this glossary. Quantity is given as the product of current and time, whilst current is earlier defined as quantity per second. Charge is stated to be 'an excess or deficiency of electrons on a body, causing electric effects in the neighbourhood'; but definition 1102 gives the meaning of ' electron' as the fundamental unit of negative electricity. On this subject of electrical units it seems rather a pity that the term selected for the B.T.U. should make the definition commence ' Unit of Electricity: a Unit of Electrical Energy.' It is also to be regretted that while the numerical value of the electronic charge is quoted, and values given for the B.Th.U. in joules, other useful figures, such as the ratio of the true to the international volt, and the E.M.F. of the Clark and cadmium cells, are omitted. One would also have liked more information on the International Candle, which is here stated simply to be a unit of luminous intensity, arrived at by common agreement between certain specified institutions. It is also to be regretted that the term ' thermoelectric effect ' has been restricted to one particular effect. Lest this catalogue of blemishes (impossible to avoid in reviewing a work in the form of a glossary) should convey a wrong impression, it must be pointed out that they concern but a small fraction of the total number of entries, most of which are very clear, and should materially assist in attaining the ideal of one definite term for each concept requiring expression. The book is excellently produced, is divided into sections, and provided with a very complete alphabetical index, in which all the terms are indexed, whether or not they are 'deprecated' in the body of the work. Misprints are very few, the following list including all that have been noted: in definition 1510 the second ' per sec.' is omitted, and in No. 1901 the figure 3-68 should be 36 8. In the index, under 'charge' the figures 1210 and 1310 should be 1201 and 1301.

ISSN:0028-0836    

DOI:10.1038/119230a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

THE present volume is a continuation of a volume on preliminaries of coal mining, etc., compiled and published in the same way as the present volume, which was reviewed in these columns in June last year. Practically the same criticism that was applied to the previous volume holds good for the one now before us. It deals almost entirely with American methods and American practice, though a few pages are devoted here and there to English methods by way of directing the attention of American engineers to them, though it cannot be said that the British illustrations which have been suggested are at all typical of the best British practice. The work is divided into four chapters: Stripping or surface mining; underground methods of mining, constituting the bulk of the book, so that more than half the text is contained in this one chapter; loading, loading machines, and conveyors; timbering and roof support. The first chapter is naturally of no use to the British colliery engineer, especially as the greater part of it is devoted to the stripping of anthracite. As regards the second chapter, it is of no use to engineers in England, and probably of but little use even to American engineers, because the most recent methods are not discussed. The chapter on loading machines and conveyors is decidedly disappointing; the compiler appears to have got his information on the subject of loaders mainly from the builders of the machines, rather than from their users, and gives the impression of not having been in touch with their practical use underground. He entirely ignores the existence of the gate-end loader which is making considerable headway in Great Britain, and omits from the discussion some of the modern American cutting and loading machines which are being tried out in West Virginia.The last chapter on timbering, etc., again is unsatisfactory. The compiler has attempted to work out certain formule for determining the size of timbers to be employed; he gives one of these on p. 307, and is apparently unaware of the fact that he is repeating the same formula only slightly altered in form and in the letters used in designation on p. 331. His confidence in his own formulae is best expressed by the statement that he solves a particular example, and shows that according to his formula " a 43-in. stick will support the load " he goes on to say, however, that " in this case it would be advisable to use a larger stick, say, 8 in. diameter."

ISSN:0028-0836    

DOI:10.1038/119231b0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

FOR the year 1925 the rainfall over the British Isles was very slightly above the average and can be taken as normal. In some districts abnormalities occurred; among the most remarkable of these was the exceptionally dry June, which was absolutely rainless in many localities and one of the driest months on record for the British Isles as a whole. The cold and cheerless summer was the fourth summer in succession of that nature. In spite of the large number of thunderstorms, especially in May and June, there were few days with heavy falls of rain. A special article is given showing a comparison of the fluctuations experienced at two standard stations-Oxford, fairly central in England, and Glenquoich in the western Highlands of Scotland -and maps are given showing the results of these statistical investigations, considering also their relation to other places in western Europe. A classified list appears of the papers published in " British Rainfall " in the last twenty-five years. A summary is also given of some experiments on the shielding of rain-gauges made at Valencia Observatory in Ireland. The question of overexposure is discussed as well as insufficient exposure. The effect of wind conditions is considered upon the catches in recording gauges.

ISSN:0028-0836    

DOI:10.1038/119232c0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

THIS book consists of a collection of the author's more important contributions to current medical literature extending over a period of thirty-three years. It comprises within the limits of 255 pages fifty separate items, made up of case records, clinical lectures, addresses to medical societies, and articles, more or less controversial, on current otological topics. It is the index of an industrious and intellectually active professional-career, which may well be a source of legitimate satisfaction to the author, and of interest to his colleagues and to the students who have received their training at his hands. There is not much in it that is of general scientific value at the present day. So far as the scientific side of his subject is concerned, we may cite some careful observations on tuningfork tests of hearing in various forms of deafness. The author lays emphasis on common sources of error arising from the personal factor in examiner and examinee. No doubt in the near future our estimate of the value and significance of the various methods of testing hearing will have to be recast in view of the recent introduction of more absolute methods of audiometry.

ISSN:0028-0836    

DOI:10.1038/119233c0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

IT is well known that associated with a medium containing free electrons of massmand chargeethere is a definite critical frequency νowhere νo2=Ne2/πm. This frequency is characterised by the fact that electric waves of frequency less than νocannot travel through the medium.The group and phase velocities of waves of greater frequency than V. are given by the relations Vphase = 2';group = c _1-2so that they tend to so and 0 respectively as: v VO. This relation: Ne2 z= v, or -71M^2which determines the critical frequency of the medium, can be expressed in a way which connects it up with the quantum theory. In a paper in the Philosophical MJagazine (July 1926, vol. 2, p. 267) I showed how to calculate the ratio g2 which is the ratio of the total energy of a quantum (engaged in an encounter with an electron) to the energy per unit volume. This ratio must therefore be of the dimensions of a volume which, for want of a better expression, was called " The Volume Occupied by the Quantum." The expression which represented this quantity was found to be:e2 VvaM = V say, so that the relation giving the critical frequency can be expressed in the simple form NV= 1. Since 1/N is the average volume occupied by each electron, we may say that the critical frequency occurs when this is such that the 'volume occupied by the quantum ' is equal to the average volume occupied by each electron. In order to conform with the ideas of statistical mechanics, of which long wave propagation appears to be an example, I prefer to consider that NV = V02/V2 represents the probability of a direct hit of the quantum and electron, or more precisely the fraction of quanta per unit volume which are in collision at any instant with the electrons in that volume. In the case of the critical frequency this probability is unity, and the relations of quantum mechanics gives a justification for the use of this description. For the analysis of the 'Compton scattering' shows that when a direct hit occurs the hv momentum given to the electron M = (Cos 6), where a is angle of scattering; the average of which may be considered to be: hV/c the momentum in the wave.It follows that when the probability of a direct hit is unity all the wave momentum will on the average be given up to the electrons and the wave can travel no farther, this state of affairs resulting in the zero group velocity which occurs at the critical frequency. In one respect this analysis seems to shatter the picture of a quantum as an objective particle of radiation, for such a picture would inevitably imply that the probability of a direct hit should be jointly proportional to the radiation energy density or density of quanta and the electron density, whereas the foregoing analysis shows this probability to be independent of the density of quanta.This aspect of the quantum theory has already been pointed out by Eddington in his discussion of Einstein's derivation of Planck's law of radiation (" The Internal Constitution of the Stars," p.

ISSN:0028-0836    

DOI:10.1038/119234a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature

摘要:

IT is well known that the flame of ammonia emits several characteristic bands. The appearance of the bands is strongly influenced by the temperature of the source. Thus the spectrum of the cool flame of ammonia is dominated by the diffuse so-called agroup in the yellow region. In the spectrum of a hot source of ammonia, burning in the electric are, the β-group with its intensity maxima at λλ 3370, 3360 is photographically by far the most intense. From investigations on the exciting conditions and on the structure of these bands, one may suppose that the very complicated α-group is emitted by the excited NH3or NH2molecules, or a mixture of both, as one can distinguish with some degree of certainty two different systems in α. Judging from some very distinct structural relations holding between the β-group and the long series of known hydride spectra, we refer the origin of this group to the NH dipole. This will be discussed later. Other bands of the ammonia-oxygen flame are referred by Eder to NO compounds. Very recently, K. Gleu, in an interesting paper on the chemi-luminescence of chlorine trinitride in hydrogen trinitride (Zeits. f. Phys., 38, p. 176, 1926), reported some new bands of nitrogen hydride. From their symmetric positions around the β-group and their comparatively simple structure we suggest that they also belong to the spectrum of the NH dipole.We have lately studied the structure of the b-group, using Fowler and Gregory's measurements (Phil. Trans. Roy. Soc. London, A, 218, p. 351, 1919) as well as our own, taken from spectrograms of high dispersion (1 A.U. = 12 mm.). Fig. 1 gives a rough view of the system arranged in a v-n diagram. The characteristic triplets on both sides of the intensity maximum at X 3360 are shown as Pand R-branches, while the intensity maximum itself is supposed to be made up of the overlapping lines of the corresponding Qbranch. The long-wave component (Q,), especially in the Q-triplets, thus causes the region which is still unresolved with a very high dispersion at X 3360 31. As in the cases of many other band spectra, this arrangement gives rise to a 'combination defect' which has to be explained by assuming a separate set of rotational terms for the Q-branches vis-d-vis with those of the Pand R-branches. From a closer analysis of the spectral terms, it appears that the initial state contains an electronic triplet, while the final state is a singlet. The final state, however, includes a rotational triplet of unknown nature in which the very small term separation increases linearly with m (0-010 in). Accurate figures for the electronic frequencies cannot be obtained (29750 cm.-') owing to the difficulties in computing the series down to their origins. If we denote by aand e the quantum numbers of the total electronic angular momenta parallel and perpendicular to the figure axis of the dipole, it appears from the analysis that the initial term contains distinct Ao. a components, while in the final state, a= 0, E = W. The nuclear spacing of the molecule in its final state is r0= 1 10 x 10-8 cm. The above statements are in agreement with the hypothesis on the appearance of Q-branches in band spectra, and also point to some clear relations holding between this spectrum and those of the hydrides as they appear in the periodic table. All the known hydride spectra are associated with electronic transitions of the type S -*S or S P. These two types can sometimes be distinguished from each other by the fact that bands of the first type contain only Pand R-branches, while those of the second type have Q-branches in addition. This seems to be a general rule holding for all band spectra. Consequently we assign to the 13-group of the NH spectrum a 3P --->IS transition, where 3P separations should agree in magnitude with those in the spectrum of the preceding atom, here carbon, which spectrum, however, is still imperfectly known. While the spectrum discussed above apparently forms the spectrum of the non-vibrating molecule (n2 = 0, to = 0), the secondary maximum at X 3370 with the faint triplets on both sides of it can be explained in detail as the corresponding first vibration spectrum (n2=1, n2=).The triplets in the 13-group are of the same type as those forming the bands of the second positive group of nitrogen. It is, however, interesting to note here that the regular alternation of intensity observed in the short-waved component (a narrow doublet) of the N2 triplets has no counterpart in the NH triplets, the components here being all single so far as can be judged from our spectrograms. This also confirms the assumption of Mecke, according to which such anomalies are to be found in the spectra of symmetric molecules as H2, He2, HCCH, N, (see also Slater's hypothesis on this problem, NATURE, April 17, 1926, p

ISSN:0028-0836    

DOI:10.1038/119235a0

出版商:Nature Publishing Group    

年代:1927

数据来源: Nature