34 EVOLUTION AND GllOWTH. 



mere concurrence of water and the gases, or vapors which are dlf 

 fused through the atmosphere. This fact is demonstrated by the 

 following experiment : — 



In a sufficient quantity of properly moistened roughly pounded 

 brick-dust, (which had been heated to redness in order to destroy 

 every trace of organic matter,) a few peas were sown on the 9th 

 of May, and the pot was transferred to a green-house in order to 

 protect the plants from the dust and impurities which always fiy 

 about in the open air. 



On the 16th of July, the peas, which looked extremely well and 

 healthy, were in flower. Each seed had sent forth one stem, and 

 each stem, abundantly covered with leaves, bore a flower. 



On the 15th of August the pods were ripe ; no more water was 

 given, and by the end of the month the plants were dry. 



The length of the stalks varied from about three feet three inches 

 to five feet ; but they were extremely slender, and the leaves not 

 more than one third the ordinary size. The pods were 1.3 inch, 

 by from 0.3 to 0.4 of an inch broad. They generally contained two 

 peas each ; one contained a single pea only, hut this was almost 

 twice the size of any of the others. 



In the course of three months, therefore, these peas came to per- 

 fect maturity — ripe seeds were gathered. The analysis of the crop 

 which I shall give by and by, in connection with another question 

 which we shall have to discuss, showed that the harvest obtained 

 under the conditions indicated, contained a considerably larger pro- 

 portion of each of the elements found than was originally contained 

 in the seed from which it sprung. 



Carbon being the predominating principle in plants, it is our first 

 duty to inquire into the origin of so much of this element as is as- 

 similated in the course of vegetation. 



Carbon is met with in very small quantity in the atmosphere in 

 the state of carbonic acid, and as this is one of the most soluble of 

 the gases which enter into the constitution of the air, water always 

 contains a considerable quantity of it in solution. Carbonic acid 

 may therefore be in relation with plants by the medium of the air 

 amidst which they live, and of the water which is no less indispen- 

 sable to their existence. We have now to ascertain in what way 

 thit gas evolves and sets free its carbon in favor of living vegetables. 



Bonnet, having put some fresh leaves at the bottom of a jar con- 

 taining spring water, observed that when exposed to the rays of • 

 the sun, they gave off bubbles of air. He sought to ascertain 

 whether this disengagement of gas was due to the leaves, or to 

 the liquid in which they were contained. For the spring water, he 

 therefore substituted water deprived of its air by boiling, and he 

 found that the leaves exposed to the sun's light in this water, no 

 longer gave oflT any bubbles of air. Bonnet, therefore, concluded 

 that the gas which he collected in his first experiment, proceeded 

 from the water. 



In 1771, Priestley discovered, that by emitting oxygen, plants had 

 the property of ameliorating atmospherical air, which had been 



