26 THEORY OF HEAT. [CHAP. I. 



same principles will serve to measure the conducibilities, proper or 

 relative, of different bodies, and their specific capacities, to dis 

 tinguish all the causes which modify the emission of heat at the 

 surface of solids, and to perfect thermometric instruments. 



The theory of heat will always attract the attention of ma 

 thematicians, by the rigorous exactness of its elements and the 

 analytical difficulties peculiar to it, and above all by the extent 

 and usefulness of its applications ; for all its consequences con 

 cern at the same time general physics, the operations of the arts, 

 domestic uses and civil economy. 



SECTION II. 



Preliminary definitions and general notions. 



22. OF the nature of heat uncertain hypotheses only could be 

 formed, but the knowledge of the mathematical laws to which its 

 effects are subject is independent of all hypothesis ; it requires only 

 an attentive examination of the chief facts which common obser 

 vations have indicated, and which have been confirmed by exact 

 experiments. 



It is necessary then to set forth, in the first place, the general 

 results of observation, to give exact definitions of all the elements 

 of the analysis, and to establish the principles upon which this 

 analysis ought to be founded. 



The action of heat tends to expand all bodies, solid, liquid or 

 gaseous ; this is the property which gives evidence of its presence. 

 Solids and liquids increase in volume^ if the quantity of heat which 

 they contain increases ; they contract if it diminishes. 



When all the parts of a solid homogeneous body, for example 

 those of a mass of metal, are equally heated, and preserve without 

 any change the same quantity of heat, they have also and retain 

 the same density. This state is expressed by saying that through 

 out the whole extent of the mass the molecules have a common 

 and permanent temperature. 



23. The thermometer is a body whose smallest changes of 

 volume can be appreciated ; it serves to measure temperatures by 



