500 PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 



nature of heat was not correct. Even at that time his hypothesis only 

 spoke of the concealing' and reappearing of a caloric, but was unable 

 to throw any light on the cause of this disappearing and returning 

 of heat, and here the mechanical theory of heat shows itself in all its 

 glory. If the latent and specific heat quantities are nothing else but 

 the quantities of kinetic molecular energy applied to the disengaging 

 of the molecules which mutually attract each other, then it is plain 

 that, as long as heated bodies do not begin to melt or boil (which means 

 evaporation inside), a part only of the supplied heat suffices for the 

 loosening of the cohesion of the molecules (specific heat), and that in 

 connection with it an increase of the oscillation- work, i. e. an increase 

 of temperature, takes place. As soon however as all the newly sup- 

 plied quantity of heat is used entirely and exclusively in the separation 

 of the molecules during the process of melting and boiling (interior 

 evaporation) then no increase of the oscillation- work of the molecules, 

 i. e. no rising of temperature, can take place. 



In conclusion let us review the acquisitions made by this lecture. 

 We shall find that it resulted in the certain knowledge that heat is not 

 a substance, but the internal motion of the molecules of bodies. The 

 molecules of all bodies of the universe, as well as that of the universal 

 aether, are constantly in that peculiar state of motion which is termed 

 heat. Mechanical work is convertible into heat according to a defi- 

 nite equivalent. We shall endeavor to prove in the next lecture that 

 under certain restrictions heat may, by a reversal of the process, be 

 turned into mechanical work (thermodynamic machines); we shall 

 moreover demonstrate that the kinetic energy of the molecules of bodies 

 and the aether, termed heat, may be converted and again reduced to 

 other energies, called light, electricity, magnetism, and chemical action, 

 in such a manner that all these energies appear only as the transforma- 

 tions of one single indestructible energy. The doctrine of the inde- 

 structibility and increatibility of energy forms the basis of the modern 

 'fundamental principle in physics ; it has transformed the latter as thor- 

 oughly and perspicuously as had been done in a similar manner in re- 

 gard to chemistry toward the close of the last century, by the proposi- 



invaluablo researches, to designate the heat which (by every available test) he ob- 

 served to have disappeared — "latent calorie." The modern conception of the fact is 

 however entirely different. Heat, as a very special "mode of motion,*' is just as de- 

 structible as any other special form of motion, audit is no more irrational to designato 

 sound or light as indestructible than to so designate heat In the latter portion of the 

 succeeding lecture the transformation of heat into work which isnot heat, is substan- 

 tially recognized. As Prof. J. Clerk Maxwell has correctly stated in his excellent 

 Treatise on Heat, — "heat may he generated or destroyed by certain processes, and this 

 shows that heat is not a substance." (Theory of Heat, chap, iii, p. 57.) And again: 

 "The hypothesis of caloric, or the theory that heat is a kind of matter, is rendered 

 untenable — first, by the proof given by Rumford, that heal can he generated at the 

 expense of mechanical work; and secondly, by the measurements of Him, which 

 show that when heat does work in au engine, a portion of the heat disappears." (Same 

 work, chap, viii, p. 147.) — Ed.~\ 



