SECT. II. GASES. 55 



taining three-fourths or two-thirds of its volume of 

 carbonic acid gas, they withered also ; but in an 

 atmosphere containing only one half of its volume 

 of carbonic acid they lived seven days. And in an 

 atmosphere containing but one fourth of its volume 

 of the same gas, they lived ten days and augmented 

 their weight by five grains. Lastly, the mean aug- 

 mentation in weight of such as were placed in an 

 atmosphere of common air, containing one-twelfth 

 part of carbonic acid gas, was eleven grains. This 

 experiment was repeated frequently, and was found 

 to yield a uniform result; the plants always suc- 

 ceeding better than in pure atmospheric air. Car- 

 bonic acid gas, therefore, is of great utility to the 

 growth of plants vegetating in the sun, as applied 

 to the leaves and branches ; and whatever increases 

 the proportion of this gas in their atmosphere, at 

 least within a given degpee, forwards vegetation.* 



But the result was not the same when the plant But j>re- 

 was placed in the shade; the smallest dose of car-i^lhe 

 bonic acid gas, in addition to that of the atmospheric shade> 

 air, being then prejudicial to vegetation. This 

 appears from the following experiments : Plants 

 kept in the shade, and placed in an atmosphere con- 

 taining one-fourth of its volume of carbonic acid 

 gas, died on the sixth day ; and when the atmos- 

 phere contained only one-twelfth of its volume of 

 this gas, they lived indeed ten days, but weighed 

 only three grains ; while those in pure atmospheric 

 * Sur la Veg. chap, ii, sect. v. 



