138 Progress of Foreign Science. 
turally established an electrical current, since the plate of platinum 
permits the electricity to circulate from one liquid to the other. 
Suppose one of these liquids to be concentrated, and the other 
dilute nitric acid. On plunging into each tube an end of the cop- 
per wire of the galyanometer, the experiment will shew that the 
electrical current goes from the stronger acid to the other. Let us 
now substitute, in the place of one of these acids, ammonia, which 
dissolves the oxide of copper. At the instant of immersion of the 
two wires the current will go from the acid to the alkali, and will 
continue to move in the same direction, even when the acid shall 
be diluted with water.——Ann. de Chem. et de Phys. xxiii. p. 244. 
Heat. On the Property which some Metals possess of facilitating 
the Combination of Elastic Fluids. By MM. Dulong and 
Thenard. 
After exhibiting to the Academy of Sciences Dobereiner’s in- 
teresting experiment described in our last number, this gentleman 
proceeded to detail some modifications of it which they had de- 
vised. On immersing some spongy platinum into a mixture of 
two parts of hydrogen and one of oxygen, explosion takes place. 
If much azote be present, the water is slowly and actually formed. 
The sponge of platinum, when strongly calcined, loses the property 
of becoming incandescent; but in this state it produces slowly, 
and without any very sensible elevation of temperature, the com- 
bination of the gases. Platinum reduced into a very fine powder 
by a well known chemical process, has no action whatever at the 
ordinary temperature. The same is the result with wires or la- 
mine. It might thence be supposed that the porosity of the metal 
was an essential condition of the phenomenon, but the following 
facts destroy this conjecture. Platinum was reduced into leaves, 
as thin as the malleability of this metal allows. In this state the 
platinum acts at the ordinary temperature, on the mixture of oxy- 
gen and hydrogen, with the greater rapidity the thiner its leaf is, 
They procured some which caused detonation after some instants. 
But what renders this action still more extraordinary is the phy- 
sical condition indispensable for its development. A very thin leaf 
of platinum, rolled round a cylinder of glass, or suspended freely 
in an explosive mixture, produced no sensible effect at the end of 
several days. The same leaf crushed together like the wadding 
of a musket, acts instantaneously, making the mixture explode. 
Rolled leaves and wires at temperatures of from 200° to 500° cent. 
act slowly, but without explosion. 
Thin leaves of gold and silver act only at elevated temperatures ; 
but always below that of boiling mercury. Silver is less effica- 
cious than gold. In accordance with Sir H. Davy’s results with 
palladium and platinum in the safety-lamp these gentlemen found, 
