488 



KNOWLEDGE. 



December, 1910. 



colours are due to the optical resonance of the small particles, 

 these particles being of the same order as those obtaining in 

 colloid solutions. 



For gold gelatine solutions the theoretical relation between 

 the size and separation of the particles and the colour of the 

 solution, which was worked out by M. Planck, was experi- 

 mentally confirmed by Kirchmer and Zsigmondy. but with 

 silver suspensions the particles are larger and the separations 

 smaller, so that the colours are less pure and their maxima 

 not so sharp ; the silver suspensions are, however, affected by 

 swelling and drying in the way indicated by theory. 



The condition for the production of the coloured silver 

 images in development, instead of black deposits, is the 

 precipitation of the silver from a solution of silver salt of 

 considerable concentration (compared with the concentration 

 of silver bromide solution in an ordinary plate during dc\elop- 

 ment). For this reason silver chloride has always given such 

 "warm tones" with nmch greater ease than silver bromide; 

 a small addition of ammonium carbonate to the developer 

 being sufficient to produce a concentration of silver chloride 

 in solution, from which finely divided silver will be precipitated. 

 If, however, silver bromide be obtained in a sufficiently fine 

 state of subdivision, the same result can be obtained without 

 the use of silver chloride. Plates prepared with very fine 

 grained silver bromide will give "warm tones" with ammonium 

 bromide and ammonium carbonate in the developer. 



The tones obtained range through browns to purples, 

 carmine, and eventually yellow. When working along the 

 lines indicated above, it seemed to be worth while to find out 

 whether the addition of solvents of silver bromide to the 

 developer would aid in the production of warm tones. With 

 silver chloride and with ordinary silver bromide plates the 

 addition of such solvents to the developer usually induced a 

 general hea\y fog, often coloured, all over the plate. The very 

 fine grained bromide plates, however, proved resistant to this 

 general fog, which could be prevented by the addition of a 

 small amount of soluble bromide. 



The addition of small quantities of sodium thiosulphate 

 (" hypo ") to the developer was found simply to assist in the 

 production of warm tones ; but when thiocarbamide was tried 

 an entirely new range of colours was produced, " cold 

 tones" being obtained instead of "warm tones." These colours 

 ranged from dead blacks through blue blacks .md bhies to 

 violets, and eventually carmines. 



Most of the toning methods for lantern slides and bromide 

 papers depend upon the conversion of the silver image into 

 some coloured compound, but a method suggested by Mr. 

 Welborne Piper for obtaining warm tones on fast plates is 

 to bleach the image with bichromate and hydrochloric acid, 

 thus obtaining an image of silver chloride, and then to de\elop 

 this with a developer containing annnonium bromide and 

 ammonium carbonate. 



In the KoUoid-Zeitschrift for October, A. Neugschwender 

 publishes a new method of obtaining warm tones, and a trial 

 of the process seems to show that it is possible to produce 

 very fine results by means of it. If silver nitrate is added to 

 potassium ferrocyanide and the resulting precipitate of silver 

 ferrocyanide is treated with stannous chloride, and then 

 ammonia is added to the mass, a deep brown solution is 

 produced, which, after some standing, or immediately on boiling, 

 produces a dark brown precipitate. If the dark brown pre- 

 cipitate is treated with concentrated hydrochloric acid a yellow 

 solution is obtained which goes green, and finally blue. The 

 brown colour is removed from the solution by filtration through 

 parchment, and is also precipitated by acids. With potassium 

 cyanide it is first precipitated and then re-dissolved with the 

 formation of the double cyanide. 



All the evidence goes to show that the brown solution 

 contains colloidal silver, which is precipitated as a hydroge! 

 by acids or boiling. Now if a negative, lantern slide or 

 bromide print, be taken and treated with a solution of 

 potassium ferrocyanide the image is converted into silver 

 ferrocyanide. If the plate or print be then soaked in a 

 solution of stannous chloride, washed, and immersed in 



ammonia solution the image changes from the white of the 

 ferrocyanide to a bright brown colour of an intensity much 

 greater than that of the original. 



For negatives this forms an intensification process of 

 strength intermediate between the mercury and the uranium 

 processes. For lantern slides and bromide paper the tones 

 produced seem very satisfactory. 



The author has investigated the effect of modifications in 

 the process. No advantages appear to result from the use of 

 other silver salts than the ferrocyanide, other tin salts than the 

 chloride, or other reducing agents than stannous chloride ; 

 the use of other alkalis than ammonia, however, results in a 

 modification of the colour produced. Caustic soda produces 

 a chocolate brown colour instead of the bright burnt-sienna 

 colour obtained with ammonia, while caustic potash gives a 

 coffee colour. The addition of hydrochloric acid produces an 

 ugly yellow colour, but this looks as if it would give fine 

 results if toned with gold. This, however, has not been tried. 



THE "REVERSAL" OBTAINED WITH THIOCAR- 

 BAMIDE. — In 1890 Major-General Waterhouse showed that 

 if thiocarbamide be added to a developer a reversal is 

 obtained ; that is, on development a positive is produced 

 instead of a negative. The image resulting is of a brown colour 

 and evidently contains colloidal silver. No explanation 

 appears to have been offered of this reversal but it does not 

 seem to be very difficult to suggest one. In the first place the 

 probable cause of the general production of fog on the 

 unexposed portion of the plate seems to be the inoculation of 

 the silver bromide grains with a " germ " of siher sulphide. 

 In the same way a preliminary soaking of a plate in gold 

 chloride solution produces a "germ" of gold which causes 

 the plate to blacken all over at once when immersed in a 

 developer. The deposit produced with thiocarbamide is brown 

 in colour, because thiocarbamide is a solvent of silver bromide 

 and the silver consequently deposits from a solution rich in 

 salt. 



Now this deposition uf sil\'er sulphide "germs" is pre\ented 

 by the presence of a very small amount of soluble bromide, 

 which greatly lowers the solubility of the silver bromide, and 

 consequently the rate of deposition of the silver. When a 

 developer containing thiocarbamide is poured on to an exposed 

 plate it first begins to develop the ordinary negative image. 

 In doing this it impregnates the film wlwrcvcr the image is 

 developed with the soluble bromide formed as one of the 

 products of development. Consequently, when the general 

 deposition of the brown silver begins the places where the 

 most soluble bromide has been formed, that is, where the 

 greatest exposure was received, will receive the least deposit, 

 so that we shall get a positive. 



This explanation is confirmed by the fact that the positive 

 image is always a little indistinct in outline, and that there is 

 always a tendency for such objects as a church spire pro- 

 jecting into the sky, which should appear dark, to fail to 

 develop owing to the diffusion of the soluble bromide sideways 

 in the film. 



PHYSICS. 



By W. D. Eggar, M.A. 



A NEW METHOD FOR PRODUCING HIGH TENSION 

 DISCHARGES.— Professor Ernest Wilson and Mr. W. H. 

 Wilson exhibited to the Physical Society, on October 28th, 

 a new method of operating the primary of an induction coil. 

 An alternating or continuous current is allowed to surge into 

 an inductance coil, which forms with a condenser a low 

 frequency oscillating circuit. There is a contact-maker 

 driven by a small motor, which first connects the inductance 

 to the supply. When the contact is broken the whole energy 

 of the inductance finds its way into the condenser, which is 

 at this moment mechanically connected to the primary of the 

 spark coil, the inductance coil being simultaneously short 

 circuited. 'Hie condenser then discharges itself rapidly 



