AND PHYSICAL FORCES 11 



the air. The cause of this diiference in temperature between the 

 animal and its medium has been variously explained at different 

 times. The true theories on this subject, however, may be said to 

 date as far back as the close of the eighteenth century, and to have 

 commenced with the brilliant discoveries of Lavoisier. He showed 

 that the oxygen introduced by the respiratory passages attacks 

 the organic substances furnished by digestion, burns them, com- 

 bining with their carbon and their hydrogen to form carbonic acid 

 and water. He showed that this slow combustion of the organic 

 materials of the blood is an incessant source of heat. Lavoisier 

 then instituted experiments to determine the quantity of heat 

 abstracted from the animal by radiation, by contact with air, 

 and by evaporation of fluids from the surface of the body. On the 

 other hand, he measured the quantity of oxygen consumed, cal- 

 culated the proportions of carbonic acid and of water produced by 

 the combination of this oxygen with the materials of the blood, 

 and then estimated the quantity of heat disengaged during these 

 reactions. From a comparison of the results thus obtained in 

 these two series of observations, he came to the conclusion that 

 the chemical reactions carried on within the body would furnish 

 enough heat to maintain the animal at its proper temperature. 

 This conclusion was afterwards confirmed by many other experi- 

 ments and observations. The researches of Lavoisier still left us 

 in doubt, however, as to whether the combustion of the materials 

 of the blood took place in the capillaries of the body generally, or 

 in those of the pulmonary circulation. This doubt was removed 

 by Spallanzani; and the subsequent experiments of Magnus and of 

 Claude- Bernard only tended to confirm his conclusion, that the 

 heat-producing chemical changes were carried on in the capillaries 

 of the body generally. Thus the heat evolved in animals is some 

 of that solar heat which had previously impinged upon plants, and 

 which was gradually locked up in the form of potential energy 

 during the growth of the plant-tissue, subsequently taken as food 

 by animals. 



2. Turning now to the next dynamic manifestation of animals — 

 to their power of movement — we may, for the sake of brevity, con- 

 sider this as it presents itself in the higher animals only — in those 

 in which the movements depend upon the contractility of definite 

 structures known as ' muscles.' Contractility is the essential attribute 

 of the muscle, and, being one of the peculiarly vital endowments, 

 we may now enquire how far this vital property is one which is 



