NATURAL PHILOSOPHY. 81 



which the carbon of the plant is derived ; and the solar beam is the 

 agent which tears the atoms asunder, setting the oxygen free, and 

 allowing the carbon to aggregate in woody fibre. Let the solar rays 

 fall upon a surface of sand ; the sand is heated, and finally radiates 

 away as much heat as it receives ; let the same beams fall upon a for- 

 est, the quantity of heat given back is less than the forest receives, for 

 the energy of a portion of the sunbeams is invested in building up the 

 trees in the manner indicated. "\Yithout the sun the reduction of the 

 carbonic acid cannot be effected, and an amount of sunlight is con- 

 sumed exactly equivalent to the molecular work done. Thus trees 

 are formed ; thus cotton is formed. I ignite this cotton and it flames ; 

 the oxygen again unites with its beloved carbon ; but an amount of 

 heat equal to "that which you see produced by its combustion was sacri- 

 ficed 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 sun severs the carbon from its 

 oxygen ; the animal consumes the vegetable thus formed, and in its ar- 

 teries a reunion of the several elements takes place, and produces ani- 

 mal heat. Thus, strictly speaking, the process of building a vegetable 

 is one of winding up ; the process of building an animal is one of run- 

 ning down. The warmth of our bodies, and every mechanical energy 

 which we exert, trace their lineage directly to the 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. Without food, 

 we should soon oxidize our own bodies. A man weighing 150 pounds 

 has sixty-four pounds of muscle ; but these, when dried, reduce them- 

 selves to fifteen pounds. 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 equiv- 

 alent of the mechanical work which the body accomplishes. 



" 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 

 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 labors 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 con- 

 ception 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 



