USES OF THE PRINCIPLE. 371 



which, while it acknowledges the principle of conservation, 

 leaves space for research. When employed in changing the 

 state of bodies, the appearance and disappearance of the heat 

 is provided for consistently by the assumption of enlarged or 

 diminished motion, or else space is left by the term " capa- 

 city" for the partial views which remain to be developed. 

 When converted into mechanical force, in the steam or air 

 engine, and so brought into direct contact with gravity, being 

 then easily placed in relation to it, still the conservation of 

 force is fully respected and wonderfully sustained. The con- 

 stant amount of heat developed in the whole of a voltaic cur- 

 rent described by M. P. Favre,* and the present state of the 

 knowledge of thermo-electricity, are again fine, partial, or 

 subordinate illustrations of the principles of conservation. 

 Even when rendered radiant, and for the time giving no trace 

 or signs of ordinary heat action, the assumptions regarding 

 its nature have provided for the belief in the conservation of 

 force, by admitting either that it throws the ether into an 

 equivalent state, in sustaining which for the time the power 

 is engaged ; or else, that the motion of the particles of heat 

 is employed altogether in their own transit from place to 

 place. 



It is true that heat often becomes evident or insensible in 

 a manner unknown to us ; and we have a right to ask what 

 is happening when the heat disappears in one part, as of the 

 thermo-voltaic current, and appears in another ; or when it 

 enlarges or changes the state of bodies ; or what would hap- 

 pen, if the heat being presented, such changes were purposely 

 opposed. We have a right to ask these questions, but not to 

 ignore or deny the conservation of force ; and one of the 

 highest uses of the principle is to suggest such inquiries. Ex- 

 plications of similar points are continually produced, and will 

 be most abundant from the hands of those who, not desiring 



* Comtes Rendus 1854, vol. xxxix., p. 1212. 



