64 Royal Institution : — 



But we cannot stop at vegetable life ; for this is the source, 

 mediate or immediate, of all animal life. The sun severs the carbon 

 from its oxygen ; the animal consumes the vegetable thus formed, 

 and in its arteries a reunion of the severed elements takes place, and 

 produces animal heat. Thus, strictly speaking, the process of building 

 a vegetable is one of winding up ; the process of building an animal 

 is one of running down. The warmth of our bodies, and every 

 mechanical energy which we exert, trace their lineage directly to 

 the sun. The tight of a pair of pugilists, the motion of an army, or 

 the lifting of his own body up mountain-slopes by an Alpine climber, 

 are all cases of mechanical energy drawn from the sun. Not, there- 

 fore, in a poetical, but in a purely mechanical sense, are we children 

 of the sun. Without food we should soon oxidize our own bodies. 

 A man weighing 150 lbs. has 64 lbs. of muscle ; but these, when 

 dried, reduce themselves to 15 lbs. Doing an ordinary day's work, 

 for eighty days, this mass of muscle would be wholly oxidized. 

 Special organs which do more work would be more quickly oxidized : 

 the heart, for example, if entirely unsustained, would be oxidized in 

 about a week. Take the amount of heat due to the direct oxidation 

 of a given amount of food ; a less amount of heat is developed by 

 this food in the working animal frame, and the missing quantity is 

 the exact equivalent of the mechanical work which the body accom- 

 plishes. 



I might extend these considerations ; the work, indeed, is done to 

 my hand ; but I am warned that I have kept you already too long. 

 To whom, then, are we indebted for the striking generalizations of 

 this evening's discourse? All that I have laid before you is the 

 work of a man of whom you have scarcely ever heard. All that I 

 have brought before you has been taken from the labours of a German 

 physician, named Mayer. Without external stimulus, and pursuing 

 his profession as town physician in Heilbronn, this man was the 

 first to raise the conception of the interaction of natural forces to 

 clearness in his own mind. And yet he is scarcely ever heard of 

 in scientific lectures, and even to scientific men his merits are but 

 partially known. Led by his own beautiful researches, and quite 

 independent of Mayer, Mr. Joule published his first Paper " On the 

 Mechanical Value of Heat" in 1843; but in 1842 Mayer had 

 actually calculated the mechanical equivalent of heat from data which 

 a man of rare originality alone could turn to account. From the 

 velocity of sound in air, Mayer determined the mechanical equivalent 

 of heat. In 1845 he published his Memoir on " Organic Motion," 

 and applied the mechanical theory of heat in the most fearless and 

 precise manner to vital processes. He also embraced the other 

 natural agents in his chain of conservation. In 1853 Mr. Waterston 

 proposed, independently, the meteoric theory of the sun's heat ; and 

 in 1854 Professor William Thomson applied his admirable mathe- 

 matical powers to the development of the theory ; but six years 

 previously the subject had been handled in a masterly manner by 

 Mayer, and all that I have said on the subject has been derived from 

 him. When we consider the circumstances of Mayer's life, and the 



