118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 2 66 REVIEWS OF BOOKS REVIEWS OF BOOKS. Stainless Iron and Steel. By J. H. G. MONYPENNY, F.INsT.P. (London, Price 21s.) The remarkable development of stainless steels since their introduction by Brearley in I913 is an indication of the demand for engineering materials possessing the range of strength and ductility obtainable only in steel, combined with a greatly increased resistance to corrosion and oxidation.The annual cost to the world of the corrodibility of steel amounts to figures which at first sight seem fantastically high. It is only the relatively high first cost of the stainless steels which stands in the way of a far more rapid increase in their use. The author, by his wide experience in the study and manufacture of stainless steels, in association with Brearley, was exceptionally well qualified to write the first comprehensive book on the subject and has discharged his task with conspicuous success. The point of view is practical but the author’s treatment is thoroughly scientific wherever possible ; it must be admitted that much of our knowledge of the stainless steels is still empirical. The book is packed with 1926: Chapman & Hall Ltd.Pp. ix + 304 with 22 plates.REVIEWS OF BOOKS 2 6 7 information and quantitative data, on nearly all aspects of the subject, gained in the laboratory and steel works with which the author is associated or in the works of other manufacturers. It is specially strong on the metallographical side. The relation of properties to structure and the effect on both of these of heat-treatment and the position and character of the thermal critical points are held in view throughout. All the stainless steels owe their resistance to corrosion to their chromium content in the first place and it is only in the newer ‘‘ austenitic” varieties that the presence of other alloy elements, nickel in particular, is necessary to secure the special type of non-corrodibility required.The stainless steels used for cutlery and for many engineering purposes differ little from the composition first developed by Brearley, with about 1 3 per cent. chromium and 0.3 per cent. carbon, while “stainless iron,” otherwise similar in composition but with about 0.1 per cent. of carbon, has its own field of application owing to its greater ductility and its capacity for being cold-worked without losing resistance to corrosion. A knowledge of the iron-chromium-carbon series of alloys is there- fore of primary importance, though the effect of additions of nickel, molybdenum, etc., and of variation in silicon, manganese and sulphur content may be lsrge.The consideration of the relative corrodibility of different stainless steels and the observation that no variety resists all types of corrosive attack may obscure the fact that the corrodibility of steel is enormously diminished by a chromium content exceeding 1 2 per cent., to such an extent indeed that it becomes practically incorrodible in the conditions of many kinds of actual service. An interesting historical sketch is followed by chapters on the influence of chromium on the structure and hardness of steel, the manufacture and treatment of stainless steel, mechanical and physical properties, influence of varying treatment and composition on resistance to corrosion, resistance to different corroding media, “ special ” stainless steels, and applications of stainless steels.The significance of the low carbon content, about 0.3 per cent., of tlie eutectoid (pearlite) and the importance of carbon content generally in stainless steels are rightly stressed by the author. I n general, resistance to corrosion increases with increasing chromium and diminishing carbon content ; the limita- tion of the operation of these factors by other considera:ions is clearly explained. The treatment and mechanical properties of the stainless steels are largely influenced by the effect of chromium in raising the temperature of the Ac, and Ar, critical points and retarding the separation of ferrite and carbide during cooling, and by the accentuation of the latter effect when the initial temperature is raised.The development of steels specially resistant to particular forms of corrosion and the wide range of mechanical properties, controlled by variation of composition and treatment, now give a choice of stainless steels comparable in variety with all the other alloy steels taken together. Among the useful purposes served by Mi-. Monypenny’s book are the restoration of order in this seeming confusion, and guidance in the selection of suitable types of stainless steel according to the purpose in view. The author makes some interesting observations on the patent position, and due credit is given to Strauss and Maurer for their independent development of austenitic stainless steels. The book is written in a clear and direct style and is very readable.On some points the explanations given are perhaps longer than is necessary for the scientific metallurgist, but these are probably well worth while in view of the wide circle of readers to whom the book will appeal. The use of “Brinell” as a verb (p. 124 <‘they were quenched out in water and268 REVIEWS OF BOOKS subsequently Brinelled”) is more appropriate to the workshop than to a scientific treatise. The abundant illustrations do justice to the well-known skill of the author in photomicrography and the printing and production of the book are excellent. In this respect indeed, the publishers have gone to extremes. The book is not a large one if judged by its contents, yet it weighs 24 lbs. Of the 304 numbered pages 14 are completely blank and 10 contain only the title of a chapter which appears again in its proper place. A stout, heavily loaded paper is necessary for the photomicrographs but not for the rest of the contents.A book of this weight is physically burdensome to read if held in the hand. The value of its contents is such as to render quite unnecessary any attempt to make the book appear more substantial than it really is. It is probably safe to assume that every maker of stainless steel already has Mr. Monypenny’s book. All who are interested in the use or study of any of the many varieties of this increasingly important material cannot do better than make themselves well acqainted with the book. Wireless Pictures and Television. By T. THORNE BAKER. (London : The author is an enthusiast in his subject and his survey of the progress which has so far been made in the telegraphy of pictures reflects his keen interest.He has, however, found difficulty in deciding as to what type of reader he is addressing. Modern developments in wireless telephony have led to the growth of a large class of ‘( amateurs ” with very limited scientific know- ledge. The book appears to be addressed mainly to this class ; much matter of an elementary nature is introduced in order to make the story intelligible to such readers, who, nevertheless, are unlikely to appreciate the occasional mathematical treatment. Perhaps the most striking instance, of the manner in which the author thus falls between two stools, occurs in Chapter III., where, after the author feels it necessary to explain the nature of a galvanometer, he proceeds in the next few pages to discuss the Einthoven galvanometer and the cathode ray oscillograph. The ample references to original papers, however, render the book useful to the more serious worker. It is well written and free from serious printers’ errors. The illustrations are admirably reproduced. Report of the Committee of t h e Privy Council for Scientific and Industrial Research, for t h e year 1925.1926. (London, H.M.S.O. ; Paper wpr. Pp. iv -F 178. 3s. net.) This publication gives, inter alia, the report of the Advisory Council and a summary of the work of the Research Organisations of the Department, and deals with developments in the organisation of research in other parts of the ‘Empire. The Development and Present Position of Chemical Analysls by Emission Spectra. By F. TWYMAN. (London: Adam Hilger, Ltd. Pp. 43. 2 s . 6d. net.) The booklet comprises two lectures delivered before the Sheffield Metal- H. M. Constable & Co. Ltd. Pp. 88, x. Price 6s. 6d. net.) lurgical Association in December, 1924. ABERDEEN : THE UNIVERSITY PREES