360 THE POPULAR SCIENCE MONTHLY. 



and are ultimately traceable to the " molecular " movements of the 

 conservative system of which our planet is a part. It would be a 

 work of supererogation to attempt to prove this, or to illustrate it in 

 detail ; it is sufficient to refer the reader to the eloquent exposition in 

 TyndalPs " Heat as a Mode of Motion," p. 447, et seq. (Appletons' 

 edition). 



I need not say that I use the term " molecular force " simply be- 

 cause it is a generally-received term, and becaiise I am constrained to 

 use it in order to be intelligible, but that I do not intend to commit 

 myself by this use to the theory of the constitution of matter which 

 it implies. In like manner I use the term " force " with the reserva- 

 tion that it rightfully denotes, not a substantive entity distinct from 

 matter, but the relation of at least two particular states of matter at 

 a given moment. 



The molecular character of molar motion is evinced in numerous 

 ways, which are almost wholly neglected and ignored by the modern 

 physicist. To take the simplest instance : when two solids impinge, 

 so that an exchange or distribution of their motions takes place, they 

 contract and immediately expand again, according to the degree of 

 their elasticity. It is unnecessary to inquire whether or not a com- 

 munication of motion between two absolutely rigid bodies is possible ; 

 all bodies, of which we know any thing, are more or less elastic, and 

 therefore contract and expand at the instant of impact. And their 

 contraction is accompanied by the evolution of heat, by the conversion 

 of molar into molecular motion, while in the expansion we have a 

 reconversion of molecular into molar motion. No transfer of molar 

 motion ever takes place without this momentary transition through the 

 molecular phase. 



Since the establishment of the doctrine of the conservation of en- 

 ergy and the correlation and mutual convertibility of forces, physicists 

 have repeatedly called attention to the fact that the old interpretation 

 of the phenomenon of an apparent destruction of force is inaccurate, 

 and that the true interpretation of this phenomenon consists in the 

 tracing of the evanescent molar motion into resulting molecular mo- 

 tion. But they fail to observe that the old notions respecting the 

 transfer of molar motion, when there is no loss, are in similar need of 

 rectification. 



Now, what is this molecular motion, in the light of the insight 

 which, as I hope, has been gained in the foregoing discussion ? Sim- 

 ply an exhibition of the struggle involved in the formation or consti- 

 tution of a body as a distinct conservative system. All molecular en- 

 ergy is in its nature constitutive, formative, or structural. All kinetic 

 energy, or actual motion, represents the progress of morphological 

 action in periodical alternations of advancing and retrograde meta- 

 morphosis. And the main problem of physical science is, not to cal- 

 culate the play of atomic motions, under the sway of their constant 



