and the Mode of its Communication. 109 



order to their taking the fluid form, cannot be lost, but 

 must continue to reside in the liquid, and must again 

 make its appearance when the liquid changes its form 

 and becomes a solid. 



It is well known that a certain quantity of heat is 

 requisite to melt a solid, which quantity disappears or 

 remains latent in the liquid produced in that process ; 

 and that the same quantity of heat reappears when this 

 liquid is congealed and becomes a solid body. 



But before I proceed any farther in these abstruse 

 speculations, I shall endeavour to investigate some of 

 the consequences which would necessarily result from 

 the radiations of hot and of cold bodies, supposing 

 those radiations to exist, and their motions and actions 

 to be regulated by certain assumed laws. 



And first, it is evident that the intensity of the rays 

 emitted by a luminous point, in a perfectly transparent 

 medium, is everywhere as the squares of the distance 

 from that point inversely ; for the intensity of those 

 rays must be as their condensation ; and their con- 

 densation being diminished in proportion as the space 

 they occupy is increased, if we suppose all the rays 

 which proceed in all directions from any point to set 

 out at the same instant and to move with the same 

 velocity in right lines, these simultaneous rays (or un- 

 dulations) will in their progress form a sphere, which 

 sphere will increase continually in size as the rays ad- 

 vance ; and as all the rays must be found at the surface 

 of this sphere, their intensity or condensation must 

 necessarily be as the surface of the sphere inversely, or 

 as the squares of the distance inversely from the centre 

 of the sphere, or, which is the same thing, from the 

 luminous point from which these rays proceed ; the 



