472 ON THE ORIGIN OF FORCE. 



their axes. The latter amount fluctuates to and fro 

 according to laws easily calculable ; but the former we 

 have no means whatever of computing, and to what ex- 

 tent, or -within what limits, it may be variable, we are 

 altogether ignorant. 



(ii.) In what is here said, it is by no means intended 

 to call in question the validity or to underrate the impor- 

 tance of those remarkable physical investigations which 

 have resulted in exhibiting heat as one of the forms in 

 which vis viva reappears in the apparent destruction of 

 motion. That all heat consists in molecular tremor (or 

 circulation), and is therefore accompanied with the 

 alternate development and disappearance of vis viva 

 within a limited space and quantity of matter according 

 to the dynaniical laws of such t7'eniulous or rotating move- 

 ments, may very readily be granted. But that there are 

 no forms of internal molecular movement other than 

 heat, and what we now speak of as its " correlated 

 forces " in which vis viva may be temporarily stored up, 

 to make its appearance ultimately in a form cognizable 

 to our senses, is what can by no means be so readily ad- 

 mitted. Nor (while accepting with all due admiration 

 as approximate truths these great revelations as to the 

 mutual convertibility of these correlatives according to 

 the measure of vis viva appropriate to eacli) shall we 

 advance any nearer to a rational theory of any one of 

 them, till it shall be shown with much more distinct- 

 ness than at present appears, in what these molecular 

 movements themselves consist ; by what forces (in the 

 dynamical acceptation of the term) they are controlled ; 



