A Manual Of Photography

126 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY, which it eventually attains will be considerably deeper than before.„ In this experiment we prove that, although the white salt or silver changes colour alone, the addition of nitrate of silver and organic matter considerably quickens the operation ; therefore, in preparing the papers, it is always necessary for the nitrate of silver to be in excess. Scheele, in his “ Experiments on Air and Fire,” has some ex- periments which are remarkably to the point. I precipitated a solution of silver by sal ammoniac; then I edulcorated and dried the precipitate, and exposed it on a piece of paper to the beams of the sun for the space of two weeks, when the surface of the white powder grew black ; after which I stirred the powder, and repeated the same several times. Hereupon I poured some caustic spirit of sal ammoniac on this, in all ap- pearance, black powder, and set it by for digestion. Ibis men- struum dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed, was for the greater part dissolved by a pure acid of nitre, which, by the operation, acquired volatility. This solu- tion I precipitated again by means of sal ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires is silver by reduction.” Experiment 4. To determine the character of the change set up by sunshine.—A solution, No. 1, is nitric acid and oxide of silver dissolved in water, and a solution, No. 2, is chlorine and sodium. These, when in solution, become hydrochloric (muriatic) acid, by the chlorine combining with the hydrogen of the water ; and soda, by the sodium absorbing the oxygen from the same fluid. When these solutions are mixed, a white precipitate — chloride of silver—falls. The chlorine of the common salt seizes the silver, and as this is nearly insoluble, it is precipitated: the nitric acid combines at the same time with the soda, and this remains in solution. The chloride of silver being carefully washed, is placed in very pure distilled water, to which a minute portion of organic matter has been added, and then exposed to sunshine. After it has darkened, remove the water, and it will be found to contain chlorine ; by adding some nitrate of silver, we shall obtain a fresh precipitate, and we may thus determine exactly the amount of decomposition which lias taken place. In the process, the strong affinity existing has been broken up. Metallic silver, in a state of very fine division, is produced; and the chlorine set free dissolves in the water, from which we can precipitate it by silver, and consequently readily ascertain its quantity.