36 Prof. Tyndall's Notes on Scientific History. 



IV. 



Having cleared his way through the powers of inorganic 

 nature,, he turns to vital phenomena, and at once fixes the 

 attention of his readers upon the sun. 



25. Measured by human standards, the sun is an inexhaust- 

 ible source of physical energy. This is the continually wound- 

 up spring which is the source of all terrestrial activity. The vast 

 amount of force sent by the earth into space in the form of wave 

 motion would soon bring its surface to the temperature of death. 

 But the light of the sun is an incessant compensation. It is the 

 sun's light, converted into heat, which sets our atmosphere in 

 motion, which raises the water into clouds, and thus causes the 

 rivers to flow*. The heat developed by friction in the wheels of 

 our wind- and water-mills was sent from the sun to the earth in 

 the form of vibratory motion. 



(The reader cannot fail to remark the insight implied in this 

 last utterance. But a still higher order of thought immediately 

 reveals itself.) 



26. Nature has proposed to herself the task of storing up the 

 light which streams earthward from the sun — of converting the 

 most volatile of all powers into a rigid form, and thus preserving 

 it for her purposes. To this end she has overspread the earth 

 with organisms, which, living, take into them the solar light, 

 and by the consumption of its energy generate incessantly che- 

 mical forces. 



27. These organisms are plants. The vegetable world con- 

 stitutes the reservoir in which the fugitive solar rays are fixed, 

 suitably deposited, and rendered ready for useful application. 

 With this prevision the existence of the human race is also inse- 

 parably connected. The reducing action of the sun's rays on 

 inorganic and organic substances is well known; this reduction 

 takes place most copiously in full sunlight, less copiously in the 

 shade, and is entirely absent in darkness, and even in candle- 

 light. The reduction is a conversion of one form of force into 

 another — of mechanical effect into chemical tension. 



28. The time does not lie far behind us when it was a sub- 

 ject of contention whether, during life, plants did not possess 

 the power of changing the chemical elements, and indeed 

 of creating them. Facts and experiments seemed to favour the 

 notion, but a more accurate examination has proved the con- 

 trary. We now know that the sum of the materials employed 

 and excreted is equal to the total quantity of matter taken 



* This, and much more, was stated by Sir John Herschel in 1833 (Out- 

 lines of Astronomy), but Mayer was the first to show the relation of all 

 these actions to the law of the conservation of energy. 



