452 M. H. Deville on tlie Decomposition of Bodies by Heat, 



vapour which forms an atmosphere round the metal ; besides, as 

 the greater part of the gas recombines by the slow cooling of its 

 molecules, only that portion of gas is liberated which by a sudden 

 cooling escapes the action of the decreasing temperature. The 

 phenomenon is in relation with the rapidity of its development, 

 as I have shown in the case of the dissociation of potash, soda, 

 of chloride of magnesium, and of water itself under other cir- 

 cumstances. 



I have shown that water comports itself like a substance 

 whose molecules are separated, when it is submitted to a tem- 

 perature of about 1000° (fusion of silver) ; on the other hand, 

 hydrogen and oxygen separated produce, in combining, more 

 heat than is necessary for melting platinum and even iridium, 

 whose fusing-point may be taken at least at 2500° (I shall 

 assume in what follows, but simply for facility of expression, that 

 these 2500 degrees exactly represent the temperature of the 

 detonating mixture itself at the moment of combustion) . How 

 is it then that the combination of hydrogen and oxygen produces 

 sufficient heat to melt platinum, while this melted platinum can 

 itself decompose water, and restore it to its elements, hydrogen 

 and oxygen ? Much more, according to my experiments, does 

 this property which water possesses of dissociating, exist at a 

 temperature far below 2000°, a temperature which I consider to be 

 approximately the fusing-point of platinum ; it takes place even 

 at a temperature of the fusion of silver. How then can water be 

 decomposed at a temperature far below that developed by the 

 combination of hydrogen and oxygen ? This contradiction must 

 be explained ; it appears to me to escape all the theories which 

 have hitherto been propounded to interpret the different actions 

 of heat on chemical combinations. 



It is difficult to regard the integrant molecule of a compound 

 body other than as composed of an assemblage of constituent 

 molecules, separated from each other by a space, finite, though 

 extremely small, and which further varies with the affinities of 

 the elements concerned. Thus each compound molecule is a 

 group of simple molecules separated from each other by finite 

 distances, as compound molecules themselves are. Affinity is the 

 force which keeps the constituent molecules united : it causes 

 the stability of a compound ; the force of cohesion opposes the 

 separation of the integrant molecules, and it is clear that affinity 

 and the cohesive force would cease to act when the molecules are 

 at a certain distance from each other. 



The action of heat tends to diminish affinity as well as the 

 force of cohesion ; by dilating bodies, it increases the distance 

 between compound or integrant molecules until cohesion is 

 nil; it appears to me that it ought also to increase the di- 



