OF NATURAL PHILOSOPHY. 205 



come particular cases, either one of the other, 

 or all of one still more general, and, at length, blend 

 altogether in the point of view from which we learn 

 to regard them. An example will illustrate what we 

 mean. It is a general law, that all hot bodies 

 throw out or radiate heat in all directions, (by 

 which we mean, not that heat is an actual sub- 

 stance darted out from hot bodies, but only that 

 the laws of the transmission of heat to distant objects 

 are similar to those which would regulate the dis- 

 tribution of particles thrown forth in all directions,) 

 and that other colder bodies placed in their neigh- 

 bourhood become hot, as if they received the heat 

 so radiated. Again, all solid bodies which become 

 heated in one part conduct, or diffuse, the heat 

 from that part through their whole substance. 

 Here we have two modes of communicating heat, 

 by radiation, and by conduction; and both these 

 have their peculiar, and, to all appearance, very dif- 

 ferent laws. Now, let us bring a hot and a cold 

 body (of the same substance) gradually nearer 

 and nearer together, as they approach, the heat 

 will be communicated from the hot to the cold 

 one by the laws of radiation; and from the nearer 

 to the farther part of the colder one, as it gra- 

 dually grows warm, by those of conduction. Let 

 their distance be diminished till they just lightly 

 touch. How does the heat now pass from one to 

 the other? Doubtless, by radiation; for it may 

 be proved, that in such a contact there is yet 

 an interval. Let them then be forced together, 

 and it will seem clear that it must now be by 

 conduction. Yet their interval must diminish gra- 



