AIM AND PROGRESS OF PHYSICAL SCIENCE. 383 



'power of nature^ the faculty to build up the body accord- 

 ing to system, and to suitably accommodate it to external 

 circumstances, remained the most essential attribute of 

 this hypothetically controlling principle of the vitalistic 

 theory with which, therefore, by reason of its attributes, 

 only the name of soul fully harmonised. 



It is apparent, however, that this notion runs directly 

 counter to the law of the conservation of force. If vital 

 force were for a time to annul the gravity of a weight, 

 it could be raised without labour to any desired height, 

 and subsequently, if the action of gravity were again 

 restored, could perform work of any desired magnitude. 

 And thus work could be obtained out of nothing without 

 expense. If vital force could for a time suspend the 

 chemical affinity of carbon for oxygen, carbonic acid 

 could be decomposed without work being employed for 

 that purpose, and the liberated carbon and oxygen 

 could perform new work. 



In reality, however, no trace of such an action is to 

 be met with as that of the living organism being able 

 to generate an amount of work without an equivalent 

 expenditure. When we consider the work done by 

 animals, we find the operation comparable in every 

 respect with that of the steam-engine. Animals, like 

 machines, can only move and accomplish work by being 

 continuously supplied with fuel (that is to say, food) and 

 air containing oxygen ; both give off again this material 

 in a burnt state, and at the same time produce heat and 

 work. All investigation, thus far, respecting the amoimt 

 of heat which an animal produces when at rest is in 

 no way at variance with the assumption that this heat 

 exactly corresponds to the equivalent, expressed as work, 

 of the forces of chemical affinity then in action. 



As regards the work done by plants, a source of power 

 in every way sufficient, exists in the solar rays which they 



