PLANTS AND THE ATMOSPHKRE. ' 429 



Torous profit by these stores and complete t!)e return to the atmosphere of that 

 -which plants have extracted from it, and herbivorous animals have preserved of it, 

 and every animal, whatever may be the class to \rhrch it belongs, rejects, by na- 

 tural channels, an abundant supply of nitrogenous matter, -which it distributes over 

 the soil. It is precisely this matter which plants take up again, without which 

 they could not live, which they are able to elaborate, change, store away, and re- 

 turn to animals after having restored the nutritive qualities it had lost. Thus is 

 completed this admirable circle of opposite transformation and mutual services, in 

 which we see animal and vegetable unceasingly exchanging the same matter, the 

 latter collecting it in a gaseous state, deoxydising and solidifying it, the former 

 receivino; it as a combustible, and giving it forth again after consumption. Priest- 

 ley saw in plants predestined servants whose duty it is to purify the atmosphere; 

 they have another function far more direct, and render a service which affects us 

 far more closely, that of extracting and preparing our food. Their action upon 

 the air would only be felt after a long series of ages ; but if a single year of 

 drought should annihilate the fruits of the soil, a frightful famine would, in a few 

 months, destroy all the animals that the earth supports. 



?rom the sun come daily food, life, strength and all our powers. Liglit, the 

 chemical emanations, all the rays which that orb sends us, are extremely rapid 

 vibrations analogous to those which produce sound : here is motion, and therefore 

 force : as soon as this force strikes upon plants, it is absorbed, disappears and is 

 destroyed. But no force is destroyed, except under the condition of having pro- 

 duced an effect, and of having executed' some work which is an equivalent for it. 

 Ifow the work which the light absorbed by leaves accomplishes is that of decom- 

 posing carbonic acid. Thus, remembering that it requires a given amount offeree 

 to disunite a given quantity of oxygen and carbon, this required force is furnished 

 hourly and gratuitously by the sun. 



If now, we place before ourselves this oxygen and carbon, and, by an inverse 

 operation, combine tliem by burning the carbon, they will in reuniting produce all 

 the force which was expended in order to separate them, that is to say, all that 

 the sun had furnished. This will be, as experience shows, heat and light, and it 

 will also be the force which can be collected by engines worked by fire, and em- 

 ployed for man's service. And, let this be well reflected upon, it is the sun which 

 has prepared for us this heat, this light, and this force ; that which it furnished to 

 the forests of the coal measures at an epoch when man was yet uncreated, man 

 discovers and makes use of to-day. 



And, what is true of inanimate furnaces appears also, and may be repeated in 

 these living furnaces called animals. They also consume organic matter, produce 

 heat, raising their temperature, and develops force and motion : a force which 

 they do not create, which they owe to this same combustion, and hold by the same 

 right as do steam engines : a force previously pcured int© plants by the sun, ab- 

 sorbed by them, virtually preserved ia their products, which constitute our food, 

 which we set free by respiration, and which our muscles apply at will, according 

 to our varied requirements. The whole of this great generalization of the world's 

 phenomena is the work of chemists and modern physicists. Messrs. Dumas and 

 Boussingault were the first to detect it ; the mechanical theory of heat completed 



