56 THE MECHANISM OF LIFE 



the temperature of the circulating water there, it has parted 

 with all the energy which is available under the circumstances 

 in which the steam engine works. The condensed steam is 

 then returned to the boiler, reheated, and the cycle of operations 

 recommences. Note that there is a source of energy contained 

 in the coal, and that this energy is converted into heat, which is 

 the kinetic energy of the molecules of steam. With each quantity 

 of steam that leaves the boiler and enters the cylinders a certain 

 quantity of energy is taken from the source (or coal) by the 

 working substance (or steam), and some of this is given up to the 

 condenser (for the water circulating in the latter becomes heated). 

 But much more energy is taken from the source than is imparted 

 to the condenser, and the difference is represented by the me- 

 chanical work done by the engine. Thus, of the total heat 

 generated in the furnace a certain fraction becomes transformed 

 into the kinetic energy (or mobility) of the engine. 



The Animate Engine.— Life in general — that is, plant and 

 animal life — presents a series of events similar to that just 

 described. There is a working substance which is represented 

 by the very simple chemical compounds, water, carbonic acid, 

 and certain mineral nitrogenous salts (which we shall call 

 " nitrate " for short). These things correspond to the water 

 employed in the steam engine. There is a source of energy 

 corresponding to the heat generated in the steam boiler; this 

 is the energy radiated by the sun (solar radiation we shall call 

 it). Just as the burning coal imparts energy to the working 

 substance of the steam engine, so the solar radiation enables 

 green plants to manufacture carbohydrates, fats, and proteids 

 from the water, carbonic acid, and nitrate supplied to them 

 by the atmosphere and soil. Solar radiation acting through 

 the green plants imparts energy to the working substance of life. 



The latter, in the form of carbohydrates, fats, and proteid.s, 

 all of which substances contain large quantities of chemical 

 energy, is then eaten by the animal organism, and, after the 

 digestive changes, incorporated in the tissues of its body. In the 

 course of the metabolic changes which it undergoes its energy 

 becomes transformed. Steam at a high temperature (and with 

 high intensity of energy) enters the cylinders of the inanimate 

 engine, and leaves the condenser at a relatively low temperature 

 (and thus with low intensity of energy). In much the same way 

 the life working substance enters the animal body while con- 



