FOR MEASURING THE EXPANSION OF SOLIDS. 
457 
and the bar of platinum some pieces of iron, without being aware of the fact, 
which is known to all the workmen who manufacture platinum, that the mere 
presence of iron is enough to communicate brittleness to that metal.” 
Upon inquiry amongst workmen in this country I cannot find that such 
a property has ever been observed in the course of their experience ; and 
when I consider that the bar in the cavity of the register was perfectly pre- 
served from contact "with the iron nails ; and moreover, that it had actually 
been plunged into melted iron without any change of properties ; I cannot 
suppose that the alteration depended in any way upon this circumstance. 
To resolve these doubts I took 116 grains of the brittle platinum, which had 
been ground without difficulty to a fine powder in a steel mortar, and boiled 
them in nitro-muriatic acid till I had effected a complete solution ; — a little of 
this solution produced a scarcely perceptible cloudiness in a solution of muriate 
of baryta. This I have reason to think was owing to a slight impurity in the 
acids employed ; I infer therefore that there was no sulphur in the metal. I 
proceeded to evaporate the solution ; which towards the end of the process as- 
sumed a gelatinous appearance. When in this state, I poured alcohol upon it ; 
and as the acid still remained in excess, a violent reaction took place with extri- 
cation of nitrous gas. I then evaporated to dryness and continued the heat ; 
till the salt of platinum kindled spontaneously, and finally was left in a spongy 
state. This was again digested in nitro-muriatic acid, and the solution care- 
fully evaporated to dryness. The muriate of platinum was then dissolved in 
water, and a sandy residue remained ; which, when Avell washed and heated to 
redness, was of a grayish-white colour, and had all the properties of silica : it 
weighed 3.5 grains. There can therefore, I think, be little doubt that at the 
high temperature to which it was exposed, platinum took up as much as 3 per 
cent of silica ; or, more probably, a quantity of its base equivalent to that quan- 
tity of the earth, to which it owed all its change of character and properties. 
A temperature considerably above that of melting cast iron appears to be 
necessary to this combination ; which is analogous in many respects to the 
absorption of carbon by iron in the process of making steel by cementation. 
3 n 2 
