Prof. Tyndall's Notes on Scientific History. 37 



up by the plant. The tree, for example, which weighs several 

 thousand pounds, has taken every grain of its substance from 

 its neighbourhood. In plants a conversion only, and not a 

 generation of matter, takes place. 



29. Plants consume the force of light, and produce in its 

 place chemical tensions. Since the time of Saussure, the action 

 of light has been known to be necessary to the reduction. In 

 the first place we must inquire whether the light which falls 

 upon living plants finds a different application from that which 

 falls upon dead matter ; that is to say, whether, ceteris paribus, 

 plants are less warmed by solar light than other bodies equally 

 dark-coloured. The results of the observations hitherto made on 

 a small scale seem to lie within the limits of possible error. On 

 the other hand, every-day experience teaches us that the heating 

 action of the sun's rays on large areas of land is moderated by 

 nothing more powerfully than by a rich vegetation, although 

 plants, on account of the darkness of their leaves, must be able 

 to absorb a greater quantity of heat than the bare earth. If, to 

 account for this cooling action, the evaporation from the plants 

 be not sufficient, then the question above proposed must be 

 answered in the affirmative. 



30. The second question refers to the cause of the chemical ten- 

 sion produced in the plant. This tension is a physical force. It is 

 equivalent to the heat obtained from the combustion of the plant. 

 Does this force, then, come from the vital processes, and without 

 the expenditure of some other form of force ? The creation of 

 a physical force, of itself hardly thinkable, seems all the more 

 paradoxical when we consider that it is only by the help of the 

 sun's rays that plants can perform their work. By the assump- 

 tion of such a hypothetical action of the " vital force " all further 

 investigation is cut off, and the application of the methods of 

 exact science to the phenomena of vitality is rendered impos- 

 sible. Those who hold a notion so opposed to the spirit of 

 science would be thereby carried into the chaos of unbridled 

 phantasy. I therefore hope that I may reckon on the reader's 

 assent when I state, as an axiomatic truth, that during vital pro- 

 cesses only a conversion of matter, as well as of force, occurs, and 

 that a creation of either the one or the other never takes place. 



(To the philosophy of vegetable life here so firmly sketched, 

 nothing to my knowledge has been added since. It will be 

 immediately seen that Mayer's power does not relax when he 

 treats of animal life and energy.) 



V. 



31. The physical force collected by plants becomes the pro- 

 perty of another class of creatures — of animals. The living 



