April, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



91 



Photography. 



Pure arvd Applied. 



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



Toning with ferricyanides. — It is very desirable, at least 

 for the serious worker, to know exactly what chemical 

 changes take place during photographic operations, so 

 that the operations may be intelligently controlled and 

 the character of the product understood. It was there- 

 fore with a pleasurable expectancy that I read a few 

 weeks ago a communication from Messrs. A. and L. 

 Lumifere and A. Seyewetz on the composition of the 

 resulting image when a silver image is toned by means 

 of a solution of potassium ferricyanide mixed with either 

 a ferric, copper, or uranium salt. But it is with a feeling 

 of disappointment that I refer to the paper, for the authors 

 appear to have done little more than begin to find the 

 difficulties of the investigation. They show that when 

 finely divided metallic silver is acted on by a solution of 

 potassium ferricyanide, the silver does not simply attach 

 itself to the ferricyanide to form a double ferrocyanide, 

 thus- KjFeCys + Ag = KjAgFeCyg, 



but that two salts are formed, 



4 KjFeCye + 4 Ag = Ag^FeCye + 3 K^FeCye, 

 and that the potassium ferrocyanide may be washed 

 away, leaving the silver ferrocyanide. But when the 

 metallic silver is suspended in a gelatine film, as it is in 

 an ordinary developed image, they get a quite different 

 result by the action of potassium ferricyanide upon it. The 

 product then contains about twice as much iron as it 

 ought in proportion to the silver, after allowing for a very 

 small quantity of potassium which appears to be due to in- 

 complete washing. The presence of this extraordinary pro- 

 portion of iron (or deficiency of silver) remains a mystery. 

 When the ferricyanide of potassium is mixed with 

 ferric citrate as in iron toning or blue toning, the ferric 

 ferricyanide produced might be expected to combine 

 with silver directly forming silver ferric ferrocyanide 

 (AgFe'^FeCye), or if the potassium ferricyanide first 

 forms silver ferrocyanide as shown above and this reacts 

 with the ferric citrate, the silver might be entirely replaced 

 by iron and Prussian blue result (Fe^(FeCyf,),). But the 

 analysis of the product shows about five times as much 

 silver in proportion to the iron as represented by the first 

 formula. Probably a large amount of the silver in the 

 original image is unattacked. 



When a copper toning solution is used we might 

 similarly expect to get either a double ferrocyanide of 

 copper and silver (Cu3Ag(FeCyo),) or merely ferrocyanide 

 of copper (Cu.FeCyo), a chocolate coloured substance to 

 which the colour produced on toning is generally supposed 

 to be due. A considerable quantity of silver was found 

 but the proportion of iron was nearly double that required 

 according to either formula. The approximately double 

 proportion of iron in this case, and also when the simple 

 potassium ferricyanide acts on finely divided silver in 

 gelatine, seem to point to a reaction that would repay 

 investigation. 



Variations in Platinum Printing. — The platinum process 

 has many advantages, the chief of which are the per- 

 manency and the beauty of the results that it furnishes. 

 But to let well alone is not the nature of photographers, 

 and it is too often the case that those who try to im- 

 prove processes have only an empirical knowledge of 

 them, and know nothing about the suggestions that they 

 make and the modifications that they propose, except 

 that the prints they get are different from ordinary'prints. 



The methods initiated by such workers must always 

 be unsafe until they have been properly investigated. 



The image in a platinum print consists of metallic 

 platinum, and therefore it can only be affected by adding 

 something to it. Many methods of toning, and so on, 

 have been suggested, but they all, except perhaps one 

 in which gold is used, consist in depositing upon the 

 image substances that cannot be compared with platinum 

 for permanency. No reliance can be placed upon such 

 compound images, and it is not right to call them platinum 

 prints, for the great advantage of platinum, itsunchange- 

 ableness, has been compromised. By adding a small 

 proportion of certain salts, especially salts of mercury, 

 to the mixture with which the paper is coated, or, less 

 advantageously, to the developing solution, the colour of 

 the deposited platinum may be modified to a warmer 

 tint. This applies particularly to hot development. 

 Here also the image consists, I believe, of pure platinum, 

 for it behaves as if it were so, and neither metallic 

 mercury nor mercurous chloride can exist in contact 

 with the platinum salt without immediately depositing 

 metallic platinum. The paper supplied commercially 

 for sepia prints gives images that seem to be as un- 

 changeable as the ordinary black platinum image. But 

 suggestions have been made and formula; given for 

 adding comparatively large quantities of extraneous 

 salts to the coating solution or the developing solution, 

 and there is practically no doubt that in many of these 

 the limit of safety has been passed and the image is not 

 platinum and not permanent. I belie\ethatitmay betruly 

 stated that if the image is affected by any reagent that 

 the paper it rests on will withstand, it is not a genuine 

 platinum print. Such reagents as hydrochloric acid, 

 chlorine water, and potassium cyanide may be used. 



Received. — The Thornton-Pickard Company send their 

 new catalogue, in which the important novelties described 

 are the " Royal " shutter, similar to their " Standard " " time 

 and instantaneous " shutter, but with the mechanism inside 

 the case, and so protected from dust and other damage ; 

 and a bellows-form of ball compressor, which is better than 

 the simple ball in that it delivers always the same volume 

 of air, and thus contributes to the uniform working of the 

 time exposure valve. A prize competition is announced. 



Mr. William Hume, of i, Lothian Street, Edinburgh, 

 pubhshes a list of his enlarging apparatus in their many 

 varieties, and with almost innumerable accessories, 

 including also valuable suggestions as to the selection 

 and using of them ; indeed, it is a guide book as well as 

 a list. The application of modern illuminants such as 

 acetylene, arc lamps, Nernst lamps, and incandescent gas, 

 as well as oil lamps and the limelight, are fully dealt 

 with. Mr. Hume was the first to use the word " canti- 

 lever " in this connection as a " selling name," and has 

 specialised in enlarging apparatus since the year 1888. 



Messrs. Taylor, Taylor, and Hobson, of Leicester, 

 have just published their new catalogue of lenses, &c., 

 which includes other items of information likely to be 

 useful to photographers, and will be sent to any applicant 

 who mentions the name of this journal. They have 

 introduced two new series of Cooke lenses. Series II. are 

 portrait lenses 'with a maximum aperture of f 4"5, and 

 will give sharp or softened definition at will. Series IV. 

 have an aperture of f 5-6, and are specially suitable for 

 high-speed shutter work in general. 



Messrs. Kodak are again inviting competition for 

 several valuable money prizes for work done with Kodak 

 apparatus and materials. About one half are reserved 

 for those who have not yet won a prize in such competi- 

 tions. Entries will be received up to the end of September, 

 and full particulars may be obtained from the Company. 



