Professor TyndaVs Lecture on Force. 249- 



bonic acid gas — which is formed by the falling together of carbon 

 and oxygen. These atoms thus in close union resemble our lead 

 weight while resting on the earth ; but I can wind up the weight 

 and prepare it for another fall, and so these atoms can be wound 

 up, separated from each other, and thus enabled to repeat the 

 process of combination. In the building of plants carbonic acid 

 is the material from 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 forest, 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 man- 

 ner indicated. "Without the sun the reduction of the carbonic 

 acid cannot be eff'ected, and an amount of sunlight is consumed 

 exactly equivalent to the molecular work done. Thus trees are 

 formed; thus the cotton, on which Mr. Bazley discoursed last 

 Friday, 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 sa- 

 crificed by the sun to form that bit of cotton. 



But we cannot stop at vegetable hfe, 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 pro^ 

 cess 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 fight of a pair of pugilists, 

 the motion of an army, or the lifting of his own body up moun- 

 tain slopes by an Alpine climber, are all cases of mechanical en- 

 ergy 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 oxidise our own bodies. A man weighing 

 150 lbs. has sixty-four lbs. of muscle; but these, when dried, 

 reduce themselves to fifteen lbs. During an ordinary day's work, 

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

 Special organs which do more work would be more quickly oxi- 

 dised : the heart, for example, if entirely unsustained, would be 



