398 SCIENTIFIC THOUGHT. 



plants, so as to embrace likewise the imponderables 

 heat, light, electricity, &c. We find Mohr treating of 

 heat and animal energy as substances which must be 

 counted among the elements or prime materials known 

 to chemists just as the French chemists of Lavoi- 

 sier's school enumerated the imponderable along with 

 the ponderable elements of nature : even Liebig's first 

 edition of the ' Chemical Letters ' is not quite averse to 

 such an interpretation. The ideas on this matter were, 

 however, vague, and needed defining. When Mayer 

 .attempted a first step in this direction, Liebig did not 

 see the value of it. The subject was only cleared up 

 when Helmholtz, in 1847, showed that all so-called 

 living forces were the different manifestations of a 

 certain quantity of power to do work later termed 

 -energy and that this power could show itself in 

 actual change and motion, or be stored up in tensions 

 in the system, later called " potential energy." After 

 this, " Stoffwechsel " appeared not only as an exchange 

 of material, but also as a change in the form of energy, 

 whereby potential or latent energy could be accumulated 

 in the organism and let loose, as the latent power of 

 an explosive substance is let loose by the pulling of a 

 trigger. 



One of the immediate consequences of these varied 

 researches all tending to show how the conception 

 formerly established in chemistry, physics, and dynamics 

 could be utilised in the description of the phenomena 

 of living matter, how the complex phenomenon of life 

 could be split up into a number of separate chemical 

 and physical processes, which could be imitated in 



