CHEMICAL INFLUENCE OF LIGHT. 275 



with chemical results in nature, the remark of this philos- 

 opher when it was made was considered extravagant, and 

 his light-pictures were looked upon by his friends as mere 

 pleasant curiosities, there is at the present time every day 

 more and more realization among chemists of the great 

 truth which he uttered. The solar ray is now regarded as 

 one of the grand chemical powers of our earth. 



390. Chemical Influence of Light on Vegetables. Light 

 produces chemical results in all of the three kingdoms of 

 nature, but they are perhaps the most observable in the 

 vegetable world. The green coloring substance called 

 chlorophyll, which, appearing in the leaves and other parts 

 of plants, makes the general face of nature so pleasant to 

 the eye, is entirely dependent upon the stimulus of light, 

 as may be seen in many common facts. The sprouts of 

 vegetables in our cellars, for example, are destitute of this 

 coloring substance exactly in proportion to the exclusion 

 of light. This explains the deep green of leaves in trop- 

 ical countries, where " the sun shines forever unchangeably 

 bright." Light has the same influence upon other colors, 

 and hence the rich deep colors of tropical fruits and flowers, 

 and the subdued tints of those of colder regions. But the 

 stimulus of light not only acts thus upon the colors of veg- 

 etables, but it is absolutely essential to the formation of 

 their substance. That chemistry of the leaves which, as 

 you learned in 128, furnishes to plants from the air so 

 large a part of their carbon, can not go on without the in- 

 fluence of light. Indeed, as stated in 129, the leaves rest 

 from this chemical work, this laying in of carbon, when the 

 light is withdrawn at night. 



391. Light and Locomotives. It was in relation to the in- 

 fluence of light upon vegetable growth that George Ste- 

 phenson, the great inventor of locomotives, said that light 

 was the power that moved them. The conversation in 



