The form of the metal known as ,,puicgy platinum has been already noticed. The metal may, however, be obtained in a state of subdivision much finer than that in which , it is left on heating the double chloride of platinum and atnruouium—namely, in the state known as platinum black. In this form it resembles soot. It may be prepared in various ways, of which one of the simplest is to boil a solution of bichloride of platinum, to which an excess of carbonate of soda and a quantity of sugar have been added, until the precipitate formed after a little time becomes perfectly black, and the supernatant liquid colorless. The black powder is then collected on a filter, washed, and dried by a gentle heat. In its finely comminuted state, either as spongy platinum or platinum black, it possesses a remarkable power of condensing and absorbing gasses, one volume of plat inum black being able to absorb more than 100 volumes of oxygen. This absorption appears to be accompanied by a conversion of some or all of the oxygen into the modifis cation known as ozone (q.v.), since the metal becomes capable of exerting the most ener getic oxidizing action, even at ordinary temperatures. For example, it can cause the combustion of a jet of hydrogen, can oxidize sulphurous acid into sulphuric acid, ammonia into nitric acid, and alcohol into acetic acid, the rise of temperature in the last case being often sufficiently great to cause inflammation. Platinum in the compact form, as foil or wire, possesses similar powers, but in a far lower degree.
Platinum may be easily alloyed with most of the metals, the alloys being in general much more fusible than pure platinum. Hence care must be taken not to heat the oxides of metals of easy reduction, such as lead and bismuth, in platinum crucibles, as, if any reduction took place, the crucible would be destroyed by the fusion of the resulting alloy_ An alloy of platinum, iridium, and rhodium is found, by the investigations of Deville and Debray to be harder, and capable of resisting a higher temperature than the pure metal; and hence is admirably adapted for the formation of crucibles, etc.
There are 2 oxides of platinum, a protoxides PtO, and a binoxide, neither of which can be formed by the direct union of the elements. Excepting that the change which platinum vessels undergo when containing the caustic alkalies, etc., and exposed to a red heat, is due to the formation of it superficial layer of oxide (probably binoxide), these compounds are of little interest. The sulphides and chlorides correspond in number and composition to the oxides. Of these compounds, the bichloride alone requires notice. It is formed by dissolving platinum in aqua regia, and evaporating the solution to dryness; and it is obtained as a deliquescent, reddish-brown mass, which forms an orange-colored solution in water, from which, on evaporation, it crystallizes in prisms. It is also freely soluble in alcohol and ether. A. solution of this salt is much used for the recognition and determination of potash and ammonia.
By the action of ammonia on protochloride of platinum (which is obtained by heating a solution of the bichloride to a temperature of 450°), several remarkable compounds are formed, which possess strong basic characters, and are of great interest in a theoretical point of view, such as platosamine platinamine etc.