THE BEGINNINGS OF LIFE. 31 



may have been locked up for ages in the bowels of the 

 earth, but which is now set free by a process of com- 

 bustion in the engine fire the result is the same, and 

 in the muscle, as much as in the steam-engine, we have 

 to do with a machine in which the transference of heat 

 into mechanical energy is capable of being effected. 

 The muscle, it is true, is a much more subtle kind of 

 machine, and the precise mode of its action is as yet 

 hidden from us ; we know not ho<w it is through what 

 precise molecular changes taking place in the substance 

 of the muscle that the heat which disappears as heat, 

 is, through the property of contractility, enabled to re- 

 appear in the form of mechanical energy whilst 

 the animal performs its manifold muscular move- 

 ments. That this is so, however, we know; and we 

 know, moreover, that as a mere machine for the con- 

 version of heat into mechanical energy, the muscle far 

 excels the best steam-engine which has ever been 

 constructed. The merits of such a machine must of 

 course be judged, other things equal, according to the 

 greater or less proportion of the total heat liberated 

 therein which is capable of being converted into me- 

 chanical energy available 1 for the execution of actual 

 work. Now, the investigations of Helmholtz and 



1 Available muscular energy must be distinguished from total muscular 

 energy, some of which whilst a man is performing work is ex- 

 pended in balancing the body or in other ways not directly effective. 

 It is the amount of loss of effective or available energy in this and in 

 other ways (such as from friction, radiation, &c.) which regulates the 

 value of muscular and all other machines. 



