284- Note hy MM. Dulong and Tlienard 



goes on slowly, the temperature rises little, and water soon 

 appears condensing on the vessel. Platinum in the spongy state 

 strongly calcined, loses the property of becoming incande- 

 scent; but in this case, it causes the combination of the two 

 gases slowly and without a veiy sensible raising of the tem- 

 perature. The finely-divided platina, obtained by a well- 

 known chemical process, has no action, not even the slowest, 

 at the ordinary temperature. The result with wires or laminae 

 is the same. The comparison of these observations might 

 give rise to the idea that the porousness of the metal was an 

 essential condition of the pheenomenon ; but the following 

 facts destroy this conjecture. 



We caused some platinum to be reduced into leaves as thin 

 as the malleability of this metal admits of. In this state the 

 platinum acts, at the ordinary temperature, on the mixture of 

 hydrogen and oxj'gen, and with a rapidity proportioned to 

 the tenuity of the leaf. We obtainecl some that caused de- 

 tonation after some moments. But what renders this action 

 still more extraordinary, is the physical state indispensable for 

 its development. A very thin leaf of platinum, rolled round a 

 cylinder of glass or suspended freely in a detonating mixture, 

 produced no sensible effect at the end of several days. The 

 same leaf crumpled like the wadding of a gun, acted instantly, 

 and made the miixture detonate. 



Leaves prepared as we have just mentioned, and which ax-e 

 then without effect at an ordinary temperature, wires, powder, 

 and thick plates of platinum, whose action is always null, in the 

 same circumstance, act slowly and without producing explo- 

 sion at a temperature of from 400° to 572° F. according to 

 their thickness. 



We have observed that some other metals possess the same 

 property as platinum. The very remarkable fact which Sir H. 

 Davy discovered in the course of his researches on the safety- 

 lamp, namely, that wires of platinum and palladium heated to a 

 dull red become incandescent when plunged in a detonating 

 mixture, having appeared to us referable to the same cause 

 with the phasnomenon in question, we were immediately led 

 to try palladium. 



The piece which we employed had been given to one of 

 lis by Dr. WoUaston ; it must have been free from alloy ; we 

 were not able, however, to obtain very thin leaves from it ; it 

 shattered under the hammer of the beater. To this circum- 

 stance we attribute its inaction at the temperature of the 

 atmosphere: however, it acts at least as well as platinum, 

 of the same thickness, at an elevated temperature. Rhodium, 

 being brittle, could not be subjected to the same operation ; 



but 



