PLATINUM. 327 



ordinary metals; its density is 21*50. It is very malleable and ductile, 

 and can be beaten into very thin sheets, and into wires as slender as wires 

 of gold. Platinum wires have even been made so fine that the eye can 

 scarcely perceive them ; these are known as Wollaston's invisible wires. 

 Platinum resists the action of the most intense fire, and we can only fuse it 

 by means of a blow- pipe and hydro-oxide gas. Its inalterability and the 

 resistance it opposes to fire render it very valuable for use in the laboratory. 

 Small crucibles are made of it, which are used by chemists to calcine their 

 precipitates in analytical operations, or to bring about reactions under the 

 influence of a high temperature. Platinum may be reduced to very small 

 particles ; it then takes the form of a black powder. In this pulverulent 

 condition it absorbs gases with great rapidity, to such an extent that a 

 cubic centimetre can condense seven hundred and fifty times its own volume 



Fig. 316. Gold-leaf exposed to the fumes of mercury. 



of hydrogen gas. It also condenses oxygen, and in a number of cases acts 

 as a powerful agent. Platinum is also obtained in porous masses (" spongy 

 platinum"), which produce phenomena of oxidation. 



A very ingenious little lamp may be constructed which lights of itself 

 without the help of a flame. It contains a bell of glass, which is filled with 

 hydrogen gas, produced by the action exercised by a foundation of zinc on 

 acidulated water. If the knob on the upper part of the apparatus is 

 pressed, the hydrogen escapes, and comes in contact with a piece of spongy 

 platinum, which, acting by oxidation, becomes ignited. The flame produced 

 sets fire to a small oil lamp, which is opposite the jet of gas. This very 

 ingenious lamp is known under the name of Gay-Lussac's lamp. Platinum 

 can also produce, by mere contact, a great number of chemical reactions. 

 Place in a test glass an explosive mixture formed of two volumes of hydrogen 

 and one volume of oxygen ; in this gas plunge a small piece of spongy platinum, 



