28 THEORY OF HEAT. [CHAP. I. 



27. If a body of definite nature and weight (a kilogramme of 

 mercury) occupies a volume Fat temperature 0, it will oecupy a 

 greater volume F+ A, when it has acquired the temperature 1, 

 that is to say, when the heat which it contained at the tempera 

 ture has been increased by a new quantity C, equal to the 

 specific capacity of the body for heat. But if, instead of adding 

 this quantity C, a quantity z C is added (z being a number 

 positive or negative) the new volume will be F + B instead 

 of F + A. Now experiments shew that if | is equal to J, the 

 increase of volume 8 is only half the total increment A, and 

 that in general the value of B is ^A, when the quantity of heat 

 added is zC. 



28. The ratio z of the two quantities zG and C of heat added, 

 which is the same as the ratio of the two increments of volume 8 

 and A, is that which is called the temperature; hence the quantity 

 which expresses the actual temperature of a body represents the 

 excess of its actual volume over the volume which it would occupy 

 at the temperature of melting ice, unity representing the whole 

 excess of volume which corresponds to the boiling point of 

 water, over the volume which corresponds to the melting point 

 of ice. 



29. The increments of volume of bodies are in general pro 

 portional to the increments of the quantities of heat which 

 produce the dilatations, but it must be remarked that this propor 

 tion is exact only in the case where the bodies in question are 

 subjected to temperatures remote from those which determine 

 their change of state. The application of these results to all 

 liquids must not be relied on; and with respect to water in 

 particular, dilatations do not always follow augmentations of 

 heat. 



In general the temperatures are numbers proportional to the 

 quantities of heat added, and in the cases considered by us, 

 these numbers are proportional also to the increments of 

 volume. 



30. Suppose that a body bounded by a plane surface having 

 a certain area (a square metre) is maintained in any manner 



