110 REPORT — 1859. 



solution, which even when dilute is brownish-red, and appears opaque when 

 concentrated. Ammonia added to this liquid diminishes its opacity and 

 gives it an orange hue. 



In inquiring what the character of the change effected in these bodies is, 

 we would direct attention to a process analogous to that by which the citrate 

 of silver was examined. If hydrogen be freely passed over the albuminate 

 of silver in a water bath, this becomes converted into a red body resembling 

 in all essential particulars the red substance into which the light converts the 

 same albuminate. In each case the reaction with the different tests is the 

 same. That, in fact, a suboxide is in each case formed, and that this sub- 

 oxide is in combination with the albuminous or gelatinous substance, seems 

 the natural conclusion from what has preceded, no less than from the re- 

 actions of the bodies themselves. 



The silver cannot be there in the metallic form ; else, why should potash 

 dissolve it, and why should ammonia convert it into a paler body ? More- 

 over, metallic mercury does not amalgamate with it. One reaction, indeed, 

 might be urged as militating against this view. The hyposulphite of soda 

 has but little action on the red compound, whereas it dissevers the consti- 

 tuent elements of suboxide of silver as dissolved oxide of silver and residuary 

 metal. But we have shown that silver is not entirely precipitated from its 

 gelatinous nor from its albuminous compound by such tests as chlorides 

 or iodides, and one will hardly therefore see with wonder that the albuminate 

 or gelatinate of the suboxide resists the action of the alkaline hyposulphite. 

 Nor would it be out of place here to hint, as our colleague Mr. Hardwich has 

 done, at the high probability of the suboxide of silver associating itself with 

 organic substances such as cellulose, albumen, gelatine, &c, in a manner ana- 

 logous to that in which other metallic salts, in which the metallic element is 

 not entirely saturated by metalloid elements, act the part of conjugate bodies, 

 annexing themselves to the organic substances alluded to, and to colouring 

 matters of various kinds. The action of these mordants belongs still to an 

 obscure chapter of chemistry, but it is highly probable that the compounds 

 under consideration are closely allied to them. 



Finally, we have to bear in mind that the fixing agent modifies the image 

 formed by the light in the materials we have been considering. 



The alkaline hyposulphite, like ammonia, acts on the subchloride or the 

 suboxide of silver, splitting the one into metallic silver and chloride which 

 becomes dissolved, and the other into oxide and metal. 



Obviously the conversion of an image formed of either of the intensely 

 colorific subcompounds of silver into a pale metallic deposit containing only 

 half the amount of metal, and possessing none of the remarkable colorific 

 energy of the suboxide or subchloride, is a conversion that can only be ex- 

 pected to exhibit a great loss of tone. Practically the singular immunity from 

 this dissevering action which the organic matter, combined with or conjugated 

 to the subcompound of silver, extends to that subcompound, comes in to help 

 the photographist from losing the beautiful result which the light itself pro- 

 duces. And what little he still must lose he can almost restore again by 

 the remarkable toning methods which he has recourse to. 



The rationale of these toning methods is to be sought in the chemistry of 

 each different process. The deposit of gold from a solution of that metal 

 is in its broad features a simple reaction — a deposit of a more electro-posi- 

 tive metal in substitution of one less so, — but the precise details of each 

 method of using a gold toning-bath doubtless involve more refined chemical 

 explanations. Without attempting to go into these, we would invite atten- 

 tion, however, to the sulphuretting baths by which this toning is sometimes 



