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



the physical properties of bodies must be known, and therefore 

 be determined whenever they are unknown. Hence all the 

 complications which would be a source of trouble in calculating 

 the effects observed (for instance, the latent heat of the fusion 

 of solids) must be removed at the outset in this investigation. 

 Heuce my researches have hitherto been limited to the determi- 

 nation of the calorific phenomena manifested on the contact of 

 liquids which combine or dissolve and produce a liquid. 



In general, two bodies which dissolve, contract. I shall begin 

 by defining what I call heat of contraction, either in the parti- 

 cular case of liquids or in the general case. 



Suppose we take a body whose weight is unity : knowing the 

 law of its expansion as a function of the temperature, we can 

 calculate the temperature at which this body would lose a given 

 fraction of its volume ; and knowing the specific heat of this 

 body within the limits of experiment, we can calculate the heat 

 of contraction corresponding to this diminution of volume. 

 Hence we can obtain the quantity of heat necessary for a given 

 variation of the density. That will be the heat of contraction. 



Suppose we take water and sulphuric acid at 0° superposed 

 in a spherical flask provided with a perfectly cylindrical narrow 

 neck ; suppose that the two surfaces of contact are separated by 

 an obstacle easy to break, such, for instance, as a spider's web ; 

 and suppose, further, that the vessel is athermanous, and can 

 neither be heated nor cooled — in other words, that its specific 

 heat is zero. 



The level of the upper of the two liquids being at A, they are 

 mixed in an infinitely short time, and in a complete manner. 

 Heat at once manifests itself, and assumes a maximum value, 

 which is indicated by a thermometer (whose weight can be neg- 

 lected) placed in the interior of the liquid; this temperature 

 will be / degrees. 



At the same moment, the temperature being supposed to be 

 equal and invariable, the liquid will sink in the narrow neck to 

 the level B. Finally, cooling the acid to its original tempera- 

 ture of 0°, its volume will again diminish until the surface is 

 level with the point C. The volume of the cylindrical space 

 A C divided by the original volume of the elements (water and 

 acid), which I shall call V, will represent the contraction. Call- 

 ing v the volume of the acid after mixture, we shall have for this 



contraction the value * „ 



1— — . 

 V 



What I call heat of contraction is the quantity of heat necessary 

 to restore the volume of the mixture v to the volume V. Know- 



* S being the section of the cylinder, we have V— # = ACxS. 



