July, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



161 



Photography. 



Pure arvd Applied. 



By Chapman Jonks, F.I.C, F.C.S., &c. 



Inlensificatton with Chroiniiim Sa//s. — The method of 

 intensification referred to in the " Science Year Book " 

 ;is having been recently suggested by Mr. J. S. Teape 

 and by Messrs. Welborne Piper and D. J. Carnegie, 

 has been further examined by the latter gentle- 

 men. The process consists in rehalogenising the 

 silver image by means of a solution O'f potas- 

 sium bichromate containing hydrochloric acid, 

 cuid then reducing the silver salt formed with 

 a developer. The apparently anomalous result 

 that the image is so made much more dense is due. as 

 might have been anticipated, to the deposition of a 

 chromium compound, produced doubtless by the reduc- 

 tion of the chromate by the metallic silver. This is a 

 clear guide to the precautions that are necessary to 

 ensure success, and accounts for the very different re- 

 sults that different formulae give — from nothing up to 

 a very large gain in density. Messrs. Piper and 

 Carnegie in their last communication (" Amateur 

 Photographer," XLI., 453) recommend potassium 

 bichromate 10 grains, hydrochloric acid (s.g. 1.16) 5 

 minims, and water to one ounce, as the most generally 

 useful bleaching solution, an increase in acid diminish- 

 ing the increase of density, and a smaller quantity 

 (preferably with dilution of the solution) increasing it. 

 .Amidol is preferred to other developers because it is 

 rapid in action and needs little or no alkali. 



The authors consider this process far preferable to 

 any mercury process, including even the mercury and 

 ferrous oxalate method. Here I must join issue with 

 them, and for two distinct reasons, either of which 

 would, in my opinion, be sufficient to establish the 

 superiority of the mercury and ferrous oxalate method. 

 In the first place, the chromium compound that is 

 added to the image is soluble in acids and is produced 

 always in an acid solution. So far as at present 

 known, the presence of a solvent of the material that 

 constitutes the image renders the production of the 

 image uncertain. That is, one cannot be sure of the 

 same increase of density following the same procedure 

 so far as one is able to make it the same; and one can 

 never be sure that the presence of the solvent does not 

 lead to a reduction effect on the image, and if it should 

 do so it is scarcely possible for it to be proportional 

 throughout. The mercury and ammonia method has 

 a solvent present, namely, the ammonia, and this 

 method never gives a proportional result. Then, 

 secondly, the material added to the image has a 

 " brownish buff " colour, according to the authors, and 

 this is very much the colour that one would expect the 

 chromium compound to be. It is always undesirable 

 to introduce a coloured substance into a negative, be- 

 cause a coloured image will produce different results 

 according to the colour sensitiveness of the printing 

 paper, and also, it may be added, according to the nature 

 of the light used. A neutral tinted image graduates 

 all lights alike, but it is practically impossible to calcu- 

 late the effect of a coloured image. Therefore it seems 

 to me that while other methods have advantages in 

 special cases and are good enough for negatives that 

 have no particular value, the mercury and ferrous 

 oxalate still remains the only scientifically reliable 

 method of intensification. 



Reductinn with Cobaliic Salts. — Mr. Harry E. Smith 

 (jour. Royal Phot. Soc, May, p. 185) has been 

 experimenting with certain ammonio-cobaltic salts and 

 analogous compounds as reducers for negatives and 

 silver prints, and finds that the tetra-ammonio-cobaltic 

 potassium nitrite, a salt prepared by Erdmann a genera- 

 tion or so ago, is the most satisfactory of those he has 

 tried. Erdmann's salt will shortly be on the photo- 

 graphic market, its use as a reducer having been 

 patented. The formula suggested is a quarter per 

 cent, solution of the salt in a seven or eight per cent, 

 solution of sulphuric acid, and the negative or print 

 after reduction is soaked for three minutes in a ten 

 per cent, ammonia solution, and finally washed. Mr. 

 Smith claims for this reducer that it attacks the 

 " denser deposits of silver much more readily than the 

 half-tone and lighter deposits, so that it is particularly 

 useful in softening the scale of gradation of hard nega- 

 tives or prints." It appears to be distinctly slow in 

 action, from Mr. Smith's communication it is not 

 clear whether what he calls its " selective " action, that 

 is, the fact that it does not attack the thinner deposits 

 unduly as most reducers are liable to do, is due simply 

 to the slowness of the action or to some peculiar 

 property wherein it may be likened to the persulphates. 

 If the latter is the case, then an investigation of the 

 chemistry of the change during reduction may be of 

 considerable interest as helping to show why the per- 

 sulphates produce exceptional results, for we do not 

 yet know what the action of the persulphates is from 

 a chemical point of view. I consider that in all cases 

 we ought to know the chemistry of such changes as 

 these before trusting valuable negatives to the action 

 of the proposed reagents. Of course, negatives 

 that ha\e no permanent value do not require 

 such consideration. I hope that Mr. Smith will give 

 us the results of further investigation, and determine 

 the character of the brownish substance that is some- 

 times obvious after reduction, and for the removal of 

 which the ammonia bath is desirable. 



The Stability of Photographs. — The action of light 

 and air upon photographs is often regarded from a too 

 empirical point of view, its effect being judged of 

 merely by the visible change that results. I-ight, in 

 some cases, causes a loss of colour or bleaching, as in 

 the fading of dyes that is so obvious in curtains, car- 

 pets, clothes, and dyed fabrics in general; and in others 

 a production of colour as in some methods of photo- 

 graphic printing. If the simple object is either to 

 bleach or to produce colour, then, of course, the ob- 

 servation of the colour-change may be a sufficient guide 

 to the progress of the action, but if the aim is to test 

 for stability, neither the presence nor the absence of a 

 visible change is sufficient to justify any definite con- 

 clusion. There may be even much alteration in ap- 

 pearance while the image renrains unaffected, as in the 

 case of platinum prints carelessly made or pasted on to 

 inferior mounts; and on the other hand, there may be 

 considerable change that is not manifested by an\ 

 notable alteration in either tint or depth of colour. 

 The only way to settle such questions is to investigate 

 the composition of the image by chemical means as 

 well as its appearance by optical means. 



Correspondence. — Bryan, E. H. — Tlie method of 

 development you propose would not be advantageous 

 for seveial reasons. The practical aspects of photo- 

 graphy during the visit of the British Association to 

 South Afric-a will be dealt with in the special nun-.ber of 

 this journal that will shortly be issued. 



