33 VI.-Contributions to our Knowledge of the Chemical Action of Sunlight upon Sensitive Photographic Papers. By CHARLESR. WRIGHT, B.Sc. (Student in the Laboratory of Owens College Manchester) In a paper published in the journal of the Chemical Society (Series 11. vol. iii. p. 183) by Mr. A. McDougall a method of measuring the relative sensitiveness to light of photographic papers is described and the results detailed of a series of measure-ments giving the relative sensitiveness of papers salted with solu- tions of different strengths of sodium- chloride potassium-chloride ammonium-chloride and potassium-bromide and afterwards floated upon a strong silver-nitrate bath of constant composition. From these experiments it appears that the sensitiveness of a pho-tographic paper depends solely on the quantity of the halogen contained in the sa2ting solution and that it is not influenced by the metal with which the halogen is combined; for if the results of his experiments on papers salted in solutions of chlorides of varring strengths be graphically represented with percentages of chlorine as ordinates and relative sensitivenesses as abscissze the three curves obtained with solutions of sodium- potassium- and ammonium- chlorides will be found to coincide within the limits of experimental error.This conclusion is also borne out by the fact that papers prepared with solutions of potassium- sodium- ammonium- and bmiurn-chlorides in such a manner that equal quantities of chlorine were contained in each were all found to possess an equal degree of sensitiveness whilst the same was also found to be true for the corresponding bromides.It next becomes of interest to examine the relative sensitive- nesses of papers salted with different halogens; to compare for instance the sensitiveness of a paper salted with a solution con- taining 1per cent. of potassium-chloride with that of one salted with a solution containing an equivalent quantity of bromine or with one in which half the chlorine is replaced by its equivalent of bromine or iodine. The following investigation was undertaken VOL. XIX. D 34 WBIQHT ON THE CHEMIOAL ACTION OF SUNLIGHT at Dr. Roscoe’s suggestion for the purpose of examining these questions. The method described in the above-mentioned paper may be shortly explained thus Let us suppose that papers A B C &c.whose relative sensitivenesses are to be determined are exposed in the disc-photometer for known relative times t, t, t, &c. the points identical in shade with A B C &c. after exposure being read off on a pendulum-strip of standard paper (or on a fixed calibrated strip) ; let the readings be a b c &c. be millimetres and let the times corresponding to these readings (that is the relative times that standard paper would take to assume the tints gained by the papers A B C &c.) be called T, T2 T, &c. Hence the ratio of the time taken by paper A to assume its particular tint to that required by standard paper to assume the same tint is t T or 1 ?! ;and similarly the ratio of the time taken by paper B to t1 assume its peculiar tint to that required by standard paper to assume this same tint is 1 t-; T and so on for the others-that is, t!2 T the sensitiveness of A compared with standard paper is 2,and t that of B compared with standard paper is hence if the t,’ T2 4 similarly sensitiveness of A be taken as unity that of B is -; t!2*I the sensitiveness of C and the others with respect to A are obtained.It hence follows that when pieces of the same paper are exposed at once in the disc-photometer for the times respectively T T T, t, tz t, . . .. and read off as before the fractions 2 -2 -. 6 tl t2’ t, must be equal as each fraction represents the sensitiveness of the paper experimented on with respect to standard paper.It was however found on exposing papers prepared with different halogens in the photometer that the values thus obtained were by no means constant when the value of the t’s varied whilst if the values of the t’s remained the same constant values were obtained. Thus for instance when chlor-iodide and chloro-brom-iodide papers were exposed together the ratio of the t’s being 1.00 to 0.40; the following agreeing values were obtained for the sensi- tiveness of the latter paper as compared with that of the former. UPON SENBITIYE PHOTOGPAPHIC PBPEM. EXP.1.-2*16 EXP.5.2'32 EXP. 9.-2.46 2.-2*17 , 6.-2.36 , 10.-2.53 , 3.-2.29 , 7.-2.38 , 11.-2*62 , 4.-2.31 , 8.-2.40 Mean 2.36.When however the ratio of the fs was 1.00to 0.63 the mean of five similarly agreeing determinations gave the value 1*65. It hence follows that for one or other of these two papers (or per- haps for both) the values of the fractions -T,J 5 5. . . . are not 4 ta' t3 constant which may be expressed by saying that the value of -Tn tn is for this paper not constant. Similarly on comparing bromide and brom-iodide papers the mean of twenty experiments gave the following numbers :-Ratio of sensitivenew of brom-iodide paper Ratio of the t's. to that of bromide. 1.00 to 2.552 0.297 to 1.00 , 1.867 0.393 ,j , 1.678 0.411 , , 0.720 0.706 ,) , 0.704 0-792 , Before pursuing this point any further it became necessary to ascertain (1)that the disc-photometer fulfils its functions and (2) that the phenomena of photochemical induction do not interfere with %he accuracy of' the results.(1.) In order to test the correct working of the photometer papers salted with potassium-chlor-iodide chloro-brom-iodide and chloro-bromide respectively were insolated (a) in the disc photometer and (6) by direct exposure for as nearly as possible the same relative times in both cases. The following numbers were obtained as the relative sensitiveness of these three papers as the mean of twelve experiments :-Disc photometer. Direct exposure. Chlor-iodide . . . . 1.00 1-00 Chloro-brom-iodide 2.30 2.32 Chloro-bromide . . 3.09 2.99 D2 36 WBIQHT ON THE CHEMICAL ACITION OF ElUNLIGHT Also papers soaked in solutions of sodium-chloride containing respectively 0.5 1-0 2.0 and 4.0 per cent.of that salt were similarly insolated giving the following numbers as the mean of eleven experiments :-Disc photometer. Direct exposure. 0.5 per cent. 0.781 0.821 1-0 , 1*000 1*ooo 2.0 1.787 1 788 jj 4.0 , 3,018 2.986 Further normal paper (that soaked in a 3 per cent. sodium- chloride solution) was insolated in the photometer and the values TT ofthe fractions 4' t &c. were found to be sensibly uniform whether the insolation were performed in bright siinlight or diffused daylight ; precisely similar results were obtained with 4 per cent. potassium-chloride paper. (2.)For the purpose of testing the influence of photochemical induction papers salted in potassium-chloride bromide and brom- iodide solutions were severally exposed to the action of direct sun- light for a known time; and at the same time portions of the same papers were exposed under a small disc (of which one-sixth of the area was cut away in the form of sectors) for six times that interval so that in the two cases the absolute time of exposure was identical but in the latter case the exposure was interrupted hy a succession of dark phases; hence the effect of photochemical induction if appreciable would be rendered manifest by the greater darkening of the paper in the second case.On rending off the various insolated papers on a pendulum-strip the following ratios were obtained for the tints assumed .-Chloride paper.Mean of 8 experiments. By direct exposure.. ...... 0.991 By disc photometer ...... 1.000 Bromide paper. Mean of 8 experiments. By direct exposure.. ...... 0.989 By disc photometer ...... 1-000 Brom-iodide paper. Mean of 6 experiments. By direct exposure.. ..... 0.917 By disc photometer ...... 1*000 UPON LYENSITIVE PHOTOQRAPHIC PAPEBI. The differences here observed are not greater in amount than can easily be accounted for by the action of the stray day-which must inevitably get under the small revolving disc; in mn-firmation of this it was observed that brom-iodide pap with which the greatest difference was thus obtained was considembiy blackened by exposure to light of so feeble an intensity & to have no appreciable action on either the corresponding chloride ~f bm mide paper.It is hence manifest that the differences in the relative mm-tivenesses of papers observed above are neither due to chemical induction nor to any abnormal action in tbe &m photometer and it therefore appears that they must hare bees caused by the differences in the ratios of the t’s; and hence we must conclude that papers prepared with different halogm do rrof all darken at the same relative rates. This conclusion may be ex-pressed as follows :-If standard paper be exposed to a constant source of light it assumes a definite series of tints in the relative times 1 2 3,4 . . . . ; if however bromide paper be expoaed W this same source of light the relative times in which it 8138uuIQ# the same series of tints are not 1 2 3 4 ... ,but arein-aQIPd other proportion; and the same is (probably) true for all paw prepared with other halogens. Before proceeding to determine what this proportion is for d paper it becomes necessary to examine whether this proportioic,$r the same for a paper salted with a strong solution of any he as for one salted with a weak solution of the same halogen. Fo? this purpose papers salted in a 10 per cent. sodium-chloride wb tion were exposed in the disc-photometer and read off on a 6 brated strip hence the ratios of the t’s are known from tb positions of the papers in the photometer whilst the ratios of h T’s are obtained from the calibration table of the strip. !& following numbers were obtained as the mean of four agr-determinations :-(The column headed t represents the t’s all reduced to tb greatest of them as unity; that headed T denotes the %?8 similarly reduced ; the column headed R represents the r~th crf the numbers in the t column to the corresponding ones in the T column; while that headed Diff.denotes the + or -dig-of these ratios from their mean.) WRIGIFIT ON TEE CHBMXUAL ACTION OF 8DlqLIOHT t. T. R. Diff. 1*000 1*000 1*000 -0.06 0742 *781 1-05 -0.01 0590 *588 1*oo -0.06 *490 *534 1-08 + 0-02 -420 *477 1.13 + 0.07 *392 *427 1.09 + 0.03 In a precisely similar manner the following numbers were ob- tained with 0.5 per cent. sodium-chloride paper as the mean of four agreeing experiments :-t.T. R. Diff. 1*ooo 1*000 1OOO -0.04 -742 .787 1006 + 0.02 -590 0611 1.03 -0.01 *490 527 1.07 + 0.03 -420 445 1.06 + 0-02 -392 -409 1005 + 0.01 Hence it appears that with these chloride solutions which present great differences in their strengths the numbers in the t column do not differ appreciably from the corresponding ones in the T column hence it follows that the relative rafes of darkening of the 10 per cent. 3 per cent. and 0-5 per cent. sodium-chloride papers are within the limits of experimental error identical. If denote the values of the ratios of the numbers in the t (+-) column to the corresponding ones in the T column this may be expressed by saying that the value of the fraction (5) (which Mr. McDougall’s experiments have shown not to be a function of the metal with which the halogen of the salting solution is com-bined) is a function of neither the strength of the salting solution finorof the time of expobure in the case of chloride papers A similar set of experiments performed with bromide solutions yielded the following numbers as the mean of nine experiments :- UPON IIBNBITIVE PHOTOQRAPHIC PAPERS (A) 6.386 per cent.potassium-bromide paper. t. T. R. Diff. 1-000 1.000 1-00 -0.51 -742 *920 1.24 -0.27 -590 0858 1*45 -0.06 -490 -838 1.71 + 0-20 773 1-84 + 0.33 ,420 *392 0721 1.84 + 0-33 (B)0.25per cent. potassium-bromide paper. t. T. R. Diff. 1*ooo 1*ooo 1*oo -0.56 -742 0935 1.25 -0.31. 0590 0871 1*47 -0.09 *490 -843 1.72 + 0.16 0420 -813 1.93 +0.37 0392 * 782 2.00 + 0.44 From these numbers it appears that with bromide papers the value of the fraction ( +),(1) is afunction of the time of ex-posure and (2) is not a function of the strength of the salting solu-tion,-since the numbers in the column of ratios are within experi- mental error limits identical both with the strong and with the weak solutions.It is therefore clear that if the relative sensitivenesses of papers are to be compared some particular tint must be taken as a standard of reference. If we know the relation between the relative times taken by papers prepared with chlorine as halogen and those by the papers experimented on to gain a certain definite series of tints and also the relative sensitivenesses of these papers with respect to any one of these tints we can then calculate their relative sensitivenesses with respect to any other of these tints.In order to determine the relative times taken by papers pre- pared with various halogens to gain the same definite series of tints papers salted in the undermentioned solutions were insolated in the disc-photometer and read off on a carefully calibrated strip :-Potassium Bromide containing 6.386 per cent. of KBr. ,) Brorn-iodide 4.452 per cent. of KI and 3.193 of KBr. and 3.193 of KBr. )) Chloro-bromide )> 2*000per cent. of KCl 40 WRIGHT ON THE CHEMIUAL ACTION OF lgUNLIUHT Potassium-Chlor-iodide containing 2.000 per cent. of KCl and 4.452of KI. 2.128of KBr. and 2.969 of KI., Chloro-brom-iodide , 1.333 percent. of KC1 All the above solutions contain precisely as much potassium as exists in a solution containing 4 per cent. of potassium-chloride. The mixed solutions (brom-iodide &c.) contain equal quantities of potassium combined with different halogens. In order to verify beyond doubt the results thus obtained the same papers were exposed in the pendulum photometer and read off as before; whilst as a third check the same papers were directly exposed to sunlight for known times and read off. In order to estimate the amount of accuracy attainable by these latter methods 4 per cent. potassium-chloride papers were thus insolated and read off giving the following numbers :-(The same symbols t T R and Diff.are used as the headings of the columns and with the same meanings as in the previous experiments.) (A) Insolated in the pendulum photometer; mean of three agreeing experiments. t. T. R. Diff. 1*ooo 1.000 1*ooo -0*002 *8?6 0871 -995 -0.007 -765 0764 *999 -0.003 -655 0657 1*003 +0*001 541 -548 1.012 + 0*010 -414 04'16 1*005 + 0.003 (B) Insolated by direct exposure ; mean of four agreeing ex-periments. t. T. R. Diff. 0 1*000 1.000 1*000 -0.004 0857 0842 *982 -0.022 0800 *810 1.012 + OW8 0750 0742 *989 -0.015 -714 .757 1*060 + 0.056 -600 *606 1*009 + 0.005 0571 *584 1*023 + 0.019 0500 0501 1*O02 -o'ooz *429 -435 1.014 + 0*010 -400 0380 *950 -0.054 UPON SENSITIVE PHOTOCBAPHIC PAPERS. The smallness of the diiTerences thus obtained shows that fair results may be anticipated by each method.The following are the numbers obtained with bromide paper :-1. By the disc-photometer (mean of five experiments). 2. By the pendulum-photometer (mean of four experiments). 3. By direct exposure (mean of eight experiments). (1) By the disc-photometer :- (2) By the pendulum-photometer :- t. T. R. 1*000 1*000 1*000 *742 *920 1.240 *590 *858 1.455 *490 *838 P711 -420 *773 1.840 -392 0721 1.839 t. T. R. 1*000 l*OOo 1*000 0876 0961 1-097 765 0921 1.204 *655 -890 1-359 0541 -839 1.551 0414 -786 1.904 (3)By direct exposure :- 1*ooot. T. 1*000 R. 1-000 *750 -907 1*208 *667 -828 1.242 600 0894 1-489 -500 *713 1425 ,400 740 1*850 *333 -677 2.031 *250 0486 I -94.3 -2W -541 2.704 The following values are obtained for the mean bromide mrvc by graphical interpolation the numbers in the column t being represented as abscissz and those in the column.T as ordi- nates :- 42 WBIGST ON T3E CHEMICAL ACTION OF BUNLIGHT MEANBROMIDE CURVE. t. T. R 1*OOO 1*000 1.000 *950 0988 1Q40 *goo -975 1-084 2350 *960 1 129 *so0 945 1.181 -750 0930 1.240 700 0910 1.300 0650 *890 1.369 0600 *860 1.433 0550 *830 1.509 *500 0790 1.580 -450 740 1.644 0400 -685 1.713 -350 -625 1986 -300 0565 1.887 *250 -500 2*000 -200 *430 2.150 These numbers signify that if two pieces of any paper prepared with chlorine as halogen gain certain tints on exposure to sun-light for the relative times 1*000and 0.790,two pieces of any paper prepared with bromine as halogen would gain the same tints on exposure to the same light in the relative times 1*000 and 0.500;and so on.The following table gives the values for the mean brom-iodide curve obtained in a precisely similar way from four experiments with the disc-photometer four with the pendulum photometer and four by direct exposure :-MEANBROM-IODIDE CURVE. t. T. R. 1*000 1*ooo 1*000 -900 *980 1.089 *800 *960 1-200 -700 *930 1.367 .600 *goo 1500 -500 -850 1.700 *400 *780 1-950 0300 0700 2.333 -200 0600 3.000 UPON 8ENgITIVE PSOTOGIRAPHIO PAPEM.a It appears from these numbers that if the sensitivenesses of a chloride and a brom-iodide paper be compared the number ex- pressing the sensitiveness of the latter with respect to the former will be three times as great with respect to a tint gained by the chloride paper in a time 0.6,as that with respect to a tint gained by the chloride paper in a time 1.0 for that obtained in the first instance will be 0.60' x x (where x denotes the absolute ratioof 0.200 1-000 ~. the t to the T) ;and that in the second instance -1.000 The mean of four experiments with the disc-photometer gave the following numbers for chloro-bromide paper. MEANCHLORO-BROMIDE CTJEVE. t. T. R. 1*ooo 1*000 1*Ooo *900 0970 1.078 =800 0930 1.162 700 *880 1.257 *600 0820 1.367 -500 .730 1*460 *400 *610 1.525 *300 *480 1.600 The mean of four experiments with the disc photometer gave the following numbers for chlor-iodide paper :-MEAN CHLOR-IODIDE CURVE.t. T. R. 1*000 1*om 1*ooo -900 0980 1-089 *800 0950 1.178 TOO -910 1 *300 0600 *850 1 *417 GOO -770 1.540 *m *675 1*688 -300 *560 1-867 -200 420 2.100 The mean of four experiments with the disc-photometer gave the following numbers for chloro-brom-iodide paper :- 44 WRIGHT ON THE CHEMICAL ACTION OF SUNLIGHT MEANCHLOR-BROM-IODIDE CURVE. 8. 1*ooo T. 1.000 R. 1*ooo *goo ‘980 1*089 *800 *960 1*200 700 930 1.329 -600 ,900 1.500 $00 0860 1.720 0400 *800 2.000 0300 0730 2.433 *200 0640 3.200 The annexed illustration shows the curves thus obtained for these five papers the corresponding chloride line being represented by the straight diagonal line :-The values of these five curves may be all represented at once by the following table which gives the relative times that any of the above papers take to assume (when exposed to a constant source of light) the series of tints assumed by chloride paperin the times 1*000,0.975 0.950 &c.:- UPON SENSXTIVE PHOTOQBAPHIC PAPERS. Chloride. Bromide. Brom-iodide. Chloro-bromide Chlor-iodide. Chloro-brom-iodide. ~~ 1*ooo 1.000 1.000 1*000 1*000 1*ooo *975 *875 #810 -910 0875 -790 -950 -800 735 0840 800 0700 *925 *730 *660 -780 *735 *620 *goo,850 *800 -670 *585 *515 0580 -500 ~420 730 .650 0580 0680 -600 -535 -560 0470 -390 0750 *455 -350 -525 -480 0320 700 0410 290 *475 .425 0265 0650 *360 *240 *430 -375 -220 *600 -320 0200 390 *335 -650 *280 *355 -295 0500 -240 -320 0260 0450 *200 990 -230 *400 -200 These curves were obtained by the use of salting-solutions eqili- valent to a 4per cent.potassium-chloride solution; but it having been previously shown that the rates of darkening of all chloride and bromide payers are respectively identical whatever be the strengths of the salting solutions this may be assumed to be true for all the others hence the above numbers represent the relative rates of darkening of any papers prepared with these five halogens. If the numbers for chloride paper be represented generally by Tc,aud those for bromide papers by Tb,the connection between Toa.id Tbis approximately given by the formula- By the aid of this table the relative sensitivenesses of any of the above papers with respect to any given tint can be calculated when their relative sensitivenesses with respect to any other are known.For example let it be required to compare the relative sensitive- nesses of 4 per cent. chloride and the equivalent chlor-iodide chloro-bromide bromide and brom-iodide papers with respect to the tint taken as the normal in photometric observations (viz. that gained by 3 per cent. sodium-chloride paper on exposure for one second to light of the unit of intensity). As the mean of five ex- periments it was found that on exposure for the relative times 1-000 0.742,and 0.562,chloride chlor-iodide and bromide papers assumed a WRI6tHT QX TBH OHEHIOAL AUTION OF SfRJLIGHT tints which standard paper would have assumed (on exposure to light of the unit of intensity) in the times 0°509,0-682 and 0.905 seconds.Let x,y and x respectively represent the relative times in which the three papers would assume the normal tint then their relative sensitiveness with respect to this tint are respectively 2 2 1 and !; or 1-000,Y- and -. Thus-xy’ 2 z (1J x 1*000 1.000 0*509 1.000 -or 3 = -1.965. O4509 (2.) From the table the relative times in which chlor-iodide paper would gain tints assumed by chloride papers in the relative times 1.000 and 0.682 are 1.000 and 0,407.Hence y 0742 1.000 :0407 0.742 -or y = -1.822. 0.407 (3.) From the table the relative times in which bromide paper would gain tints assumed by chloride paper in the relative times 1*000and 0.906 are 1-000and 0-685. Hence z :0-562 1.000 0.685 0.562 -or z = -0.820. 0.685 5 1.965 P965 Hence -and-X are respectively -and -?/ 2 1.822 0*820’ or 1.078 and 2.396. That is the relative sensitivenesses of 4 per cent. chloride paper and its equivalent chlor-iodide and bromide papers with respect to the normal tint are 1.000 1-078 and 2.396 respectively. Again as the mean of 12 experiments it was found that chlor- iodide and chloro-bromide papers on exposure for the relative times 1*000and 0-431,gain tints which normal paper would (on expo- sure to light o€ the unit of intensity) gain in the times 0.483 and 0.628 seconds.Calculating as before the relative sensitivenesses of these two 1 and lm papers are found to be 4wo ’ or 1.078 and 4022. Again the mean of 10 experiments gave the following numbers for bromide and brom-iodide papers :- UPON SENBITIVE PBOTOBWHIU PAPERS Bromide. Brom. iodide. Bromide. BrOm-iodide. Bmm. Bromide. iodide Relative times of exposureTimee in which standard loO0O 1.867 1.000 1'678 1*000 2.652 paper would aesume the tints gained by the papers exposed.. ............ Relative sensitivenesses.. 2*396 4.624 0.943 2.396 0'648 4'002 0'732 2.396 0-553 2'612 Bromide. Brom-iodide. Mean relative senaitivenesses 2.396 4060 Hence the relative sensitivenesses of the above 5 papers With respect to the normal tint are-Chloride ..............loO0O Chlor-iodide ............ 1.078 Chloro.bromide .......... 4.022 Bromide .............. 2.396 Brom-iodide ............ 4.060 In a similar way their relative sensitivenesses with respect to any other tint may be calculated. It having been observed during these experiments that the same result was obtained whether the papers were exposed to the full noonday sun to the evening sun or in the shade it became inte- resting to observe what effects were produced upon each paper by variations in the intensity of the light acting upon them the time of exposure being constant. In order to observe this- (1.) A b~ass plate perforated with small circular holes the diame- ters of which were accurately measured by a micrometer was fitted light-tight into the wall of a dark room so that the sun could shine perpendicularly upon the plate.A sheet of prepared paper to receive the images of the sun formed by the small holes was placed at such a distance from the plate that the angle sub-tended by the largest hole from the paper was less than the appa- rent diameter of the sun; all the images were thus kept of the same size hence the intensity of light under each image was proportional to the square of the diameter of the hole forming that image. (2.) The prepared papers were exposed at the bottom of vertical cylinders coated internally with black paper and covered over with metal caps having round holes of varying sizes drilled in them; these holes varied in diameter between 77.5 and 25 millimetres the cylinders being 5 decirnetres high and 1deci- 48 WRIGHT ON TEE CHEXIOAL ACTION OF SUNLIGHT metre in diameter.By this means only diffused light from the zenith was employed the intensities of the light being as before proportional to the squares of the diameters of the holes. As a check on the accuracy of the results obtainable by these two methods 4 per cent. potassium-chloride paper was thus exposed with the following result,. (The column A represents the relative intensities of the acting light; the column T the relative times in which standard paper on exposure to a constant source of light would gain the tints assumed by the papers experimented on the largest being taken as unity; the column R gives the ratios of the numbers in the A column to the corresponding ones in the T column ;and the column Diff gives the differences of the numbers in the R column from their mean.In accordance with the law of Professors Bunsen and Roscoe the numbers in the column R should be sensibly constant provided that no serious imperfection exists in the working of the apparatus employed.) 4 per cent. potassium-chloride paper exposed uuder the perforated plate :-MEANOF Two EXPERIMENTS. A. T. R. Diff. l*ooO 1*000 1*oo -0.07 701 *777 1.11 + 0.04 0663 *757 1.14 + 0.07 -653 -719 1-10 -t 0.03 *520 *496 -96 -0.11 -394 -399 1.01 -0.04 *299 *344 1.15 + 0.08 4 per cent. potassium-chloride paper exposed in the cylinders :-MEANOF FOUREXPERIMENTS.A. T. R. Diff. 1-000 1*000 1*oo -0.01 -719 *734 1-02 + 0.01 *595 0625 1-05 + 0.04 -444 0418 -94 -0.07 428 -443 1*03 + 0.02 -326 *330 1*01 4-0.00 The numbers in the columns of differences being in no case large tolerably accurate results may be anticipated by either method. C'PON SENSITIVE PHOTOGRAPHIC PAPERS. OAexposing bromide paper (equivalent to 4 per cent. potassium- chloride) the following results were obtained as the mean of five experiments with the perforated plate and of three with the cylinders. Experiments with the cylinders and bromide paper :-A. F. R. 1-000 l5O0O 1~000 -595 -881 1.4479 -326 0521 1.601 0145 *331 2-289 Experiments with the perforated plate and bromide paper :-A.T. R. 1*ooo l*C)OO 1000 *945 '093 1*051 -932 0992 1.065 759 0928 1.223 0741 0918 1*238 -701 *936 1.307 -663 *912 1.37'1 -653 0904 1 *384 *575 *842 1.$64 *5R2 *859 1.527 *520 -809 1.557 *426 -749 2-759 *394 0691 7.752 *299 ,569 1-902 *258 ,590 2.286 *I91 -390 2.041 *134 -377 2.814 On graphical representation the numbers in the columns A being represented as abscissae and those in the columns T as ordinates these results are found to coincide giving d mean curve identical with that derived from the experiments with the disc-photometer (p. 42); in other words bromide papers on exposure to a constant source of light for times t, t, t3. . . . gain certain tints ; and on exposure for the same time to lights of intensities A A A .. . . the same tints are gained then t A = t -4 = t3 A = . . . . . . = constant. That is the law enunciated by Profs. Bunsen and Roscoe for VOL. XIX. E 50 WRIGHT ON THE CHEMICAL ACTION OF SUNLIGHT standard chloride paper is also true for bromide papers ;viz. that the same tint is gained by exposing the papers for a time 1 to an intensity 10 as is gained on exposure for a time 10 to an intensity 1 or for a time 2 to an intensity 5; and so on. Hence if a pendulum strip be made with bromide paper and the same paper be exposed to varying intensities of light and the tints thus produced be read off on the bromide pendulum strip it should be found that the numbers from the strip calibratiou-table are to one another in the same prqortion as the numbers indicating the intensities of light.On trying the experiment however it was found impossible to read off on the bromide strip owing to the slight difference in tint between the two ends. On exposure of brom-iodide paper in the same ways the follow- ing numbers were obtained as the mean of six experiments by the aid of the perforated plate (direct sunlight) andof four by means of the cylinders (diffused light). Brom-iodide paper with direct sunlight :-A. I‘. R. 1*000 1*ooo 2.000 786 *914 1.163 0551 *843 1.530 O521 -824 1.581 0513 0823 1-604 409 *737 1-805 0310 *741 2.390 235 *610 2.707 0106 ,468 4.437 *060 -349 5.825 Brom-iodide paper with diffused daylight :-A.T. R. 1*000 1-000 1*ooo *719 *894 1 *244 0428 -765 1.787 0234 -644 2.751 .lo4 -406 3.899 As with the bromide paper these numbers are found on graphical representation to yield a mean curve identical with that derived from the previous experiments loith the disc-photometer qc. (that ie Bunsen and Roscoe’e law is likewise true for this paper). UPON SENSITIVE PHOTOGRAPHIC PAPERS. On endeavouring to read off brom-iodide papers exposed to varying interisities of light on a brom-iodide pendulum strip it was found impossible to get any trustworthy results owing to the slight difference in tint between the two ends of the strip. The following numbers were derived from the mean- of four ex- periments with chloro-bromide paper with diffused light from the zenith (cylinders) :-A.T. R. 1.000 1.000 1.000 *719 0837 1.164 *596 *844 1.4416 428 ‘601 1.404 -326 471 1-414 *I45 -357 2460 These numbers also on graphical representation g’ive a curve identical with that derived from the disc-phot onzeter experiments. On reading off papers exposed to varying intensities of light (in the cylinders) on a chloro-bromide pendulum strip the following numbers were obtained as the mean of four experiments. (The numbers in the column P are the relative times in which chloro- bromide paper would on exposure to light of uniform intensity gain the tints assumed by the papers esperimented on the columns A R and Diff. have the same meanings as hitherto.) A.P. R. D3. 1-000 1.000 1-00 -0.08 -719 702 *98 -0.10 595 *658 1.10 $-0.02 -428 *450 105 -0.03 *326 *A10 1.26 $-0.18 Considering the great difficulty of reading off correctly on a chloro-bromide pendulum strip which exhibits much less grada- tion of tint than a chloride one the differences are sufficiently small to show that Bunsen and Roscoe’s law is obeyed by this paper. On exposing chloriodide papers to diffused light of varying intensities (cylinders) the following numbers were obtained as the mean of four experiments :- 52 WRIGET ON THE CHEMICAL ACTION OF SUNLIGHT A. T. R. 1-090 1.000 1-900 *718 -924 1.286 -428 *719 1.680 :234 *463 1.979 *lo4 *314 3.019 These numbers also on graphical representation are found to yield a mean curve identicnl with that derived from the disc-photometer experiments .Chlor-iodide papers exposed to varying intensities of light (cylinders) arid read off on a chlor-iodide pendulum strip yielded the following numbers as the mean of three experiments. (The column Q iridicates the relative times in which chlor-iodide papers would assume the tints gained by those experimented on on exposure to n constant source of light.) A. Q. R. DiK 1*000 1*000 1-00 -0-00 *719 0636 938 -0.12 -595 -579 '97 -0.03 0428 *425 *99 -0.01 -326 -882 1.17 + 0.17 As the chloriodide pendulum strip shows much less gradation in tint than the chloride strip thesenumbers show that Bunsen and Roscoe's law is obeyedby this paper. By exposing chloro-brom-iodide pape*r to diffused light of vary-ing intensities the following numbers were obtained as the mean of four experiments :-A.T. R. 1-000 1.000 1-000 *718 *921 1-280 -428 *a37 1.955 -234 *716 3-061 *lo4 *494 4-74.9 On graphical representation these numbers are found to give a mean curve identical with that derived from the disc-photometer experiments. On endeavouring to read off chloro-brom-iodide papers exposed to varying intensities of light on a chloro-brom-iodide pendulum strip no good results were obtainable from the slight differences in tint between the two ends of the strip. UPON SENSITIVE PHOTOGRAPHIC PAPERS Since all the above papers obey Bunsen and Roscoe's law the table given on p. 45 also denotes the relative intensities of light to which pieces of any papers prepared with the above halogens must be exposed for the same time in order to gain the same series Of tints.The results obtained by the foregoing experiments together with those previously obtained by Mr. McDougall may be briefly summed up thus :-(1.) Papers prepared with solutions of different alkaline chlorides darken to the same extents in the same relatiwe times no matter with what alkaline metal the chlorine be combined and no matter what be the constant percentage of chlorine in the salting folution. (2.) The same holds true for papers prepared with bromide solutions (and hence probably for all the others). (3.) Photo-chemical induction does not exist to any appreciable extent with chloride bromide .and brom-iodide papers (and hence probably with none of the others).(4.) The relative times for which pieces of any one of the above papers must be exposed to light of a constant intensity in order to gain a definite series of tints vary with each paper. (5.) The relative intensities of light to which pieces of any one of the above papers must be exposed for a constant time in order to gain a definite series of tints vary with each paper. (6.) The relative times for which pieces of' any one of the above papers must be exposed to light of a coilstarit intensity in order to gain a definite series of tints are in the same ratio to one another as are the relative intensities of light to which the same pieces must be exposed for a constant time in order to gain the same series of tints.(7.) The relative times for which pieces of any one of the above papem must be exposed to a constant source of light in order to gain a given series of tints or the relative intensities of light to which they must be exposed for a constant time to gain the same series of tints are given by the table on p. 45. (8.) By the aid of this table the relative sensitivenesses of any of the above papers with respect to any particular tint can be cal-culated when their relative sensitivenesses with respect to some one given tint are known. In conclusion I beg to tender sincere thanks to Professor Roscoe for the valuable advice and assistance which he has given me in carrying out the above investigation.