240 



NATURE 



\yuly 13, 1876 



found by Sir John Herschel in sodium-hyposulphite. On apply- 

 ing this salt to the image, it was removed, and also one atom 

 of silver and one of chlorine from the sub-chloride molecule, 

 leaving the atom of metallic silver behind. The chemical 

 change that takes place on the silver chloride can be very dis- 

 tinctly shown by exposing it perfectly pure benfeath water. The 

 presence of the sub-chloride is shown by the colour, and that of 

 the chlorine can be exhibited by the usual chemical tests. 



In making an ordinary silver print on paper, we have, how- 

 ever, something more present than silver chloride ; we have an 

 organic salt known as the albuminate of silver, that is, a com- 

 bination between albumen and silver. I have in this test-tube 

 a little dilute albumen — the solid constituent of the white of an 

 egg. Into it I drop a little silver nitrate ; a flocculent precipitate 

 is at once apparent. The silver from the nitrate has combined 

 with the albumen, and on burning a piece of magnesium wire 

 before it the outer surface shows a darkening ; evidently, then, 

 the albuminate of silver is decomposed by light. For silver 

 printing purposes, paper is coated on one surface with a solu- 

 tion of albumen and sodium chloride, and the production of 

 the silver chloride and albuminate is effected by floating that 

 surface on a solution of silver nitrate. When dry, the paper 

 •which is now sensitive to light is ready for exposure beneath a 

 negative. Here we have two prints produced on paper so 

 prepared. If now I take one of them and disselve away the 

 insoluble salts in sodium hyposulphite, you see that the colour 

 is of a disagreeable foxy-red tint. To show you how this want 

 of a pleasing tone may be overcome, the other print is immersed 

 in a weak solution of gold, and by a well-known chemical action 

 the metallic gold is deposited on the darkened portions of the 

 picture. Now when gold is precipitated, it has not the well- 

 known yellow colour, but is of a bluish purple ; thus the depo- 

 sited gold mixes its peculiar tint with that of the silver, and 

 after immersion in the hyposulphite we obtain a print whose 

 beauty cannot be surpassed. 



I daresay that many of you may have been charmed with the 

 production of magic photographs, as they were called. Some few 

 years ago the sale of such was enormous, but now the curiosity 

 of the public seems to be satiated. The magic, as you may be 

 aware, consisted in being able to produce on a white piece of 

 paper a photograph of some unknown object. These mysterious 

 pieces of paper were generally supplied in packets, containing 

 with them a piece of blotting-paper. The directions stated that 

 the blotting-paper was to be damped, and whilst moist, to be 

 applied to the surface of one of the accompanying pieces of 

 blank paper, and then a photograph would shoot out. I will 

 endeavour to show you one method of their production. Here 

 I have an ordinary photographic print which has not been 

 treated with gold, but merely immersed in sodium hyposulphite 

 and then washed. I immerse it in a solution of mercurous 

 chloride which I have in this dish, and immediately a bleaching 

 action is set up. The action continues, and the paper is appa- 

 rently blank. What has happened ? Simply a white compound 

 of silver and mercury has been formed, which is indistinguish- 

 able from the paper. If I wash the paper and dry it, it is in 

 the state of the paper supplied in the packets. I have one 

 here washed and dried, and I immerse it in the sodium 

 hyposulphite. The image immediately reappears, a combina- 

 tion has taken place between the constituents of the hyposul- 

 phide, the mercury, and the silver. 



Need I say that the blotting-paper supplied is impregnated with 

 the same sodium salt ? In damping it the molecules of the latter 

 are so separated and mobile, that they are free to combine with 

 the white image. By similar treatment the picture may be made 

 to again disappear and once more reappear. 



Besides silver there are various other metals which will give a 

 photographic image. This paper, which has a slightly yellow 

 tint, h?s been brushed over with ferric chloride, more commonly 

 known as perchloride of iron, in which we have the maximum 

 number of colours of chlorine combined with metallic iron. 

 Allowing ordinary white light to act upon it, the waves cause a 

 disturbance between the iron and the chlorine atoms, and one of 

 the latter is shaken off, leaving ordinary ferrous chloride, or 

 muriate of iron behind. A piece of paper, similarly prepared, 

 has been exposed beneath a negative, and the reduction of the 

 ferric chloride to the ferrous state can be demonstrated by 

 floating it on a solution of potassium ferricyanide. The com- 

 binatioii between the lowest type of the iron salt and this salt 

 results in the formation of a deep blue precipitate known as 

 Tumbull's blue. You see, after applying It, we have the lines 

 of this map, of which this is the negative, of an intense blue. 



Instead of demonstrating the change of the iron salt by this 

 means, I may float it on a weak solution of silver nitrate. The 

 ferrous salt of iron will reduce the silver, whilst the ferric salts 

 are wholly inoperative to produce the same effect. Here we 

 have such a print. 



The principal investigator of the action of light on iron com- 

 pounds was Sir John Herschel, and he employed a variety of 

 different combinations. Perhaps one of the most interesting 

 exhibits in the Photographic Section is that old list of Fellows 

 of the Royal Society on which were pasted, by the hand of that 

 distinguished philosopher, the actual solar spectrum prints made 

 during his researches on these and other metallic salts. 



Uranium salts are also capable of being reduced to less com- 

 plex forms by the action of light. I will not enter into a de- 

 tailed description of the decomposition, but will simply exhibit 

 the method of producing a print with the salt. The paper has 

 been coated with uranic nitrate and exposed to light, beneath 

 the same negative before shown to you. The image is made 

 visible by a solution of potassium ferricyanide, as in the case of 

 the iron salt. 



In the cases of photographs are shown some interesting speci- 

 mens of iron and uranium prints, made by Niepce de St. Victor. 

 I believe they were presented to Sir Charles Wheatstone by 

 that ardent experimentalist. The subdued br«wn tones of the 

 latter were probably obtained by the admixture of a little iron 

 with the uranium. 



Within the last couple of years the salts of iron have been put 

 to practical photographic printing purposes by Mr. W. Willis, 

 jun., of Birmingham, and a valuable process has resulted from 

 his labours. The sensitive salt employed is an organic salt of 

 iron known as ferric oxalate, and Mr. Willis made the discovery 

 that amongst other metals platinum could be reduced to the 

 metallic state from a double chloride of potassium and platinum, 

 by ferrous oxalate in the presence of a potassic oxalate. A 

 piece of paper is floated on a weak solution of silver nitrate and 

 dried ; and over the surface is brushed a mixture of the platinum 

 salt and the ferric oxalate. After exposure to light (which 

 produces the ferrous salts) beneath a negative, the paper is 

 floated on a solution of neutral potassium oxalate, when the 

 image at once appears formed of platinum black, a substance 

 at once durable and incapable of being acted upon by atmo- 

 spheiic influence. Such an exposed paper I have here, and 

 floating it on oxalate solution, you see the image is imme- 

 diately developed. The unreduced iron salt can be eliminated 

 by soaking the print in the oxalate solution, and a rinse and 

 hyposulphite removes all traces of silver nitrate. After a few 

 changes of water, the print may be dried, and is permanent. 

 I should explain that the paper is first coated with silver nitrate 

 in order to cause the platinum to adhere firmly to the surface of 

 the paper. When omitted, the fine black powder formed is apt 

 to precipitate in the bath. 



Before dwelling upon that metallic compound which in photo- 

 graphy is next in importance to silver, I must call your atten- 

 tion to the first vanadium print ever produced. Prof. Roscoe, 

 who has already delighted an audience in this room with an 

 admirable lecture on Dalton's apparatus and what he did with 

 it, has made a classical investigation of the compounds of 

 this metal, and amongst other interesting facts, has noticed that 

 the vanadium salts are reduced by light in a somewhat similar 

 manner to the uranium salts. 



We now have to consider the printing processes which are 

 due to the action of light on the dichromates of the alkalis in the 

 presence of organic matter. For our purpose to-night we may 

 take as a type potassium dichromate, a salt which readily 

 parts with its oxygen to those compounds that have an avidity 

 for it, more especially to certain carbon compounds under the 

 influence of the ether waves. 



To show that this salt is thus easily reducible by light in 

 the presence of organic matter, I have here a piece of paper 

 which has been brushed over with it, and exposed beneath 

 a print. For a moment I float it on a weak solution of 

 silver nitrate. The brilliant crimson colour of the part not 

 exposed to light tells us that silver dichromate has been formed, 

 but where the solar rays have acted, the colour remains un- 

 changed. A slight modification of this process now exhibited 

 to you is known as the chromatype, the offspring of Mr. Robert 

 Hunt, so well known in the scientific world for his researches on 

 light. Whilst experimenting with the chromatype process, Mr. 

 W. Willis, the father of the gentleman I have already men- 

 tioned, discovered what is known as the aniline process. It is 

 based on the fact that an acid in the presence of potassium 



