456 M. H. Deville on the Decomposition of Bodies by Heat, 



problem, I have verified directly the temperatures produced in 

 the liquids on which I have operated ; I thus obtained numbers 

 comparable with each other, and independent of the variation 

 in the specific heats of bodies. The latter is only introduced 

 into the calculation in the slight corrections which must be made 

 in the temperatures to allow for the heat absorbed by the vessels, 

 and for the heat expended in water evaporated in experiments 

 made at a high temperature. 



I assume that in a large number of gases (the specific heat of 

 the product is necessary for the corrections) there is determined 

 the temperature of combination of two liquid bodies, the density 

 of the two bodies at the initial temperature, and, lastly, the co- 

 efficient of expansion of the body from the initial temperature to 

 a point somewhat higher than the temperature of combination ; 

 we have then all the elements necessary to calculate — 



1st. The volume of the components before their combination. 



2ndly. The volume of the compound after combination, and in 

 consequence of contraction. 



3rdly. The heat of contraction, or the temperature at which the 

 compound would have the same volume as the components. 



It is clear that if there is no difference between this tempera- 

 ture of contraction thus calculated, and the temperature of com- 

 bination observed directly, the heat disengaged by the contrac- 

 tion is sufficient to explain the disengagement of heat observed 

 during combination. When this difference is positive (and this 

 has always been the case in my numerous experiments, except 

 with sulphate of soda which has 10 equivs. of water), it represents 

 the loss of temperature (of vis viva), or the cooling effected during 

 the act of combination, which always accompanies solution. 

 Thus a cooling is sometimes observed with bodies which really 

 combine, such as water and acetic acid. 



Hence the temperature corresponding to contraction is always 

 higher than the temperature observed directly during the act of 

 combination — and that with almost a single exception, the cause 

 of which is readily found : the heat disengaged in the combina- 

 tion of two liquids by contraction is then more than sufficient to 

 explain the effects produced, provided, of course, that there is no 

 change of condition. 



This observation will suggest that there is, on the contrary, 

 change of condition whenever two gases combine with disengage- 

 ment of heat, producing a body also gaseous, especially when 

 there is no contraction. In order to make my meaning clear, I 

 shall use the mode of expression which I have previously ex- 

 plained. 



Suppose the molecules of hydrogen and of chlorine are liquid, 

 and that the constituent molecules of hydrochloric acid are solid, 



