combinations of platinum. 121 
such as paper-hangers use ; it had been recently prepared, 
and was, previous to its being dissolved in water, in the form 
of a tremulous jelly. The sulphate of platinum occasions, 
after a short time, a brown precipitate in astringent infusions ; 
but this substance I have not examined. 
7. On a grey oxide of platinum. 
In the course of my experiments to ascertain the compo- 
sition of fulminating platinum, I treated it with nitrous acid, 
and thus procured, as I have elsewhere stated, a grey oxide 
of platinum, which has not yet been described. It may be 
obtained by adding strong nitrous acid to fulminating plati- 
num, boiling it to dryness, and exposing the dry mass to a 
heat just below redness, so as to expel all the nitrous acid. 
The oxide of platinum remains. It is to be finely pulverized 
and boiled, first in pure water, and then in a weak solution 
of caustic alkali to separate the last portions of acid, which 
adhere with great tenacity to it. It is now to be well washed 
and dried at a heat not exceeding that of boiling mercury. I 
have usually made the experiment in a platinum crucible on a 
hot sand bath. The oxide thus prepared exhibits the following 
properties. 
8. Properties and composition of the grey oxide of platinum. 
Its colour is dark iron grey. It has the metallic lustre. It 
is sufficiently hard to cut brass, which it polishes, and when 
the polished surface is rubbed a little with the oxide, a deli- 
cate coating of platinum remains. It does not touch steel. 
It is not affected by cold or hot water, nor by the nitrous, 
sulphuric, or phosphoric acid at a boiling heat. It is insoluble 
mdcccxx. R 
