534 Professor Tyndall [June G, 



produced by its combustion was sacrificed by the sun to form that 

 bit of cotton. 



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

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

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

 in its arteries a reunion of tlie severed elements take place, and produce 

 animal heat. Thus, strictly speaking, the process of building a vege- 

 table 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 tlie sun. The 

 fight 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, therefore, in a poetical, 

 but in a purely mechanical sense, are we children of the sun. With- 

 out food we should soon oxidize our own bodies. A man weighing 

 ] 50 lbs. has 64 lbs. of muscle ; but these, when dried, reduce themselves 

 to 15 lbs. Doing an ordinary day's work, for 80 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 accomplishes. 



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

 my hand — but 1 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 independeni of Mayer, Mr. Joule 

 published his first Paper on the ' Mechanical Yalue 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 me- 

 chanical 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 emb aced 

 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 Tiiomson applied his admirable 

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

 previously the subject had been handled in a masterly manner by Mayer, 



