CHLORIDE OF PLATINUM. The chloride of platinum usually spoken of is a bichloride (Pt. C1.2) easily soluble in water, ether, or alcohol. The protochloride (Pt. Cl.) is insoluble like the responding salt of gold. When the bichloride of platinum is mixed with lime water, or solution of carbonate of soda, and exposed to light, a white precipitate is caused in the lime water, and a red precipitate in the soda solution. The white powder has been deter mined by Weiss and Diibereiner to be a hydrated compound of chloride of calcium with lime and peroxide of platinum, having the formula Ca. Cl. + Ca. 0, 2 Pt. 02 + 7 H 0 ; and the red crystalline precipitate to be Na. 0, 3 Pt. 02, 6 H O. These two compounds are sufficiently complex and might seem to indicate a very intricate reaction when the solar rays fail upon the mixture. It will be found, however, on examination that the increase of attrac ,tion of chlorine for hydrogen in the light will account for the change in a very simple manner, thus :— 2 Ca. 0 + 2 Pt. C1.2 + 10 H 0 = Ca. Cl., Ca. 0, 2Pt. 02, + 7H0+3HC1.
and Na. 0 C 02 + 3 Pt. C1.2 + 12 H 0 = Na. 0, 3 Pt. 02,
+ 6H0+6HC1.+ CO2, from which formulEe it will be seen that in both cases hydrochloric acid is formed, and that the platinum becomes oxide instead of chloride. The light does not appear to have the power of causing water alone to decompose chloride of platinum as it does chloride of silver, but when other substances are present which have an affinity for oxide of platinum, and, therefore, may be supposed to exert some force in its formation, then the decomposition by light will take place. Paper has the power of slightly decomposing chloride of platinum in the light, for vvith some days' exposure, the effect is visible. In one of Sir John Herschel's experiments neutral bichloride of platinum was exposed to the sun and brushed over afterwards in the dark vvith nitrate of silver. The picture was weeks before it began to develope, but still it developed in all its details. It has been used as a toning agent both before fixing and also in the toning bath, just as chloride of gold, but it is not so energetic, and the tone is less pleasing.