M. H. Sainte-Claire Deville on Affinity and Heat. 377 



Combination is almost always produced by the destruction of 

 motion, sometimes by the transformation of heat into motion. 

 In the first case there is a disengagement of heat, in the second 

 a cooling or absorption of heat. In the second category are to 

 be included all those bodies which I have proposed to call explo- 

 sive, that is, which render into sensible heat the motion they 

 have acquired in absorbing latent heat. Formic acid, a great 

 number of organic compounds, as well as the explosive com- 

 pounds of nitrogen, are in the latter category ; and these are 

 rarely produced by the direct union of the elements, but are ob- 

 tained by the interchange of their elements in the body of more 

 or less dilute solutions. It is assumed that then the molecules 

 are in contact in the nascent state. Here we must be on our 

 guard ; this term still includes the idea of an occult cause. It 

 must be employed with extreme reserve; and by it must be 

 understood a system of circumstances in which the molecules 

 may change their state of equilibrium on finding about them 

 latent heat or, in general, the causes of motion necessary for 

 producing and exciting this change of state. The origin of the 

 expression which renders this idea implies an hypothesis which 

 may give rise to a vicious circle in th e m { n ^ s of young chemists, 

 or of persons who have not maturely reflected on these defini- 

 tions. It is seen, from the explanations which I h ave g lven J tnat a 

 more or less dilute solution really contains a cert aui quantity of 

 heat, arising either from gain by contraction, or b y cooling by 

 expansion. Hence most of the combinations which are due to tne 



nascent state ought to take place in solutions esP ec * a ^y those 



which take place under cooling — those which give e xpl° sive sub- 

 stances*. 



From what has been adduced in this chapter, it will be seen 

 how great in their origin are the differences between the calo rific 

 effects produced when gases combine with each other, and tne 

 calorific effects developed by the combination or solution of liquids 

 with each other. In the latter case the heat of contraction 

 has almost always been sufficient, and more than sufficient, to ac- 

 count for all. As to gases, this heat of contraction, which may 

 be calculated by formulae given above, is always very small as 

 compared with the heat disengaged during combination ; it majr, 

 indeed, be zero, as in the case of hydrogen and chlorine, and, 

 indeed, whenever gases combine in equal volumes, and hence 

 without condensation. Therefore we must admit that gases con- 

 tain of themselves, and in the latent state, the principle of motion 

 or of heat which is manifested at the moment of combination. 



* I call in general those bodies explosive which at the moment of their 

 decomposition contain more heat than the bodies into which they are re- 

 duced require to exist in the condition of simple substances. 



