and on Dissociation. 455 



This mode of expression introduces great simplicity into the 

 scientific language relating to this question, but it does not 

 involve a new idea. It has often guided me in connecting phe- 

 nomena of total or partial decomposition, phenomena of ebulli- 

 tion, and of the formation of vapours with variable tensions in 

 solids or liquids. I shall use this formula in explaining the 

 meaning and the range of some experiments which I have made 

 during the last winter. It is, however, to be understood that 

 I simply rely on the analogies which I have established between 

 affinity and the forces of cohesion ; and I reserve to myself the 

 opportunity of showing how these two forces (which are exerted 

 on two kinds of distinct molecules — the constituent molecules, 

 and the integrant molecules, of compound bodies) produce in 

 analogous cases identical effects. 



When two volumes of hydrogen and one volume of oxygen 

 are combined in a medium whose temperature is 100°, two 

 volumes of aqueous vapour are produced, whose temperature for 

 a few moments is at least 2500°. The two effects observed are 

 the contraction, and the elevation of temperature, of the water; 

 the apparent physical condition (gaseous) of the three bodies, 

 water, hydrogen, and oxygen, has not changed. Either, then, 

 the contraction must have produced all the heat, or the hy- 

 drogen and oxygen have- given out from themselves the heat 

 which is disengaged. But the heat due to contraction would 

 simply raise the temperature of aqueous vapour to 137° instead 

 of to 2500°; hence the second hypothesis can alone be ad- 

 mitted. 



Further, chlorine and hydrogen combine without condensa- 

 tion ; a simple ray of light is sufficient to produce an enormous 

 elevation of temperature in the gaseous mixture. As hydro- 

 chloric acid could only borrow from its elements the heat which 

 it has manifested, and consequently lost at the moment of combi- 

 nation, it is clear that it does not possess the physical state, of 

 one at least, of the two gases which compose it; and as the 

 distance between the molecules has not changed, it is in the 

 constituent molecule itself that the modification must be sought 

 from which the disengaged heat results. There are then two 

 sources of heat in the combination of gases : 1st, the contraction 

 in volume, the effect of which can be calculated with precision ; 

 2ndly, an intimate modification in the constitution : with this 

 latter I am now concerned. 



In order to elucidate at least one part of the question, I have 

 sought to ascertain what would be the influence of the heat due 

 to contraction when the state of bodies which combine evidently 

 Undergoes no change, when, for example, two liquids combine to 

 form a compound which is also a liquid. In order to solve this 



