388 4 New and Curious variety of Combustion. 
plunging a piece of this spongein a mixture of two parts 
hydrogen and one oxygen, explosion of course ensues; but 
if the proportions of the mixture be very different from that 
which forms water, or if a gas, foreign to the combination, 
be present, such as azote, the combination takes place slow- 
ly, the temperature rises a little, and water is soon conden- 
sed upon the glass. 
Platina sponge strongly calcined, loses the property of be- 
coming incandescent, but in this case it produces slowly and 
without any sensible elevation of temperature, the combina- 
tion of the two gases. Platina reduced to fine powder, has 
no action at common temperatures, nor has it in the form of 
wire or plates. But the French chemists, state, that 
when reduced to leaves as thin as the metal will admit, 
it acts at common temperature, and acts upon the mixed 
gases, with a rapidity proportionate to its. thinness, in 
some instances causing detonation. But one thing is re- 
markable, viz, that a very thin leaf of platina, rolled upon a 
glass cylinder, or suspended freely in a detonating mixture, 
produces no sensible effect, though exposed during several 
days; but the same leaf compressed into a wad, acts instant- 
ly, and produces detonation. _ 
eaves and wire too thick to produce any action, 
when cold, will nevertheless, as Sir H. Davy had_ before 
determined, act at the temperature of 2 to 300°, according 
to their thickness. Palladiumacts as well as Platina at 
the same temperature, and thickness. Rhodium occasion- 
ed the formation of water at 240° 
Gold and Silver in thin leaves act only at elevated a 
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. 
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It is well known that iron, copper, gold, silver and plati- 
na have the property of decomposing ammonia at a certain 
cient than copper, an copper more than silver, gold or pla- 
