THE VEGETABLE KINGDOM. 421 



whole circumference of the root of a white poppy, so as 

 to prevent the vapour of the earth from interfering with 

 his expei'iment. The plant was then covered with a bell- 

 glass cemented to the lead. After that each morning 

 when the naturalist came to visit the imprisoned plant he 

 observed, that even during the driest nights its leaves 

 were covered with an innumerable quantity of those 

 drops of water to which the name of dew is given, and 

 that the sides of the glass themselves were quite ob- 

 scured with it. It is not then from the air that the dew 

 of the meadow and the leaf comes, but, as the Dutch 

 naturalist learned, from the transpiration of the plant; 

 dew is only their perspiration condensed. 



This fact being thoroughly established, it only remained 

 to decide the amount which vegetable transpiration pro- 

 duces. Mariotte tried a very elementary experiment on 

 this head. Having cut off a branch and covered the sec- 

 tion with impermeable cement, he observed that the leaves, 

 while withering, had lost two tea-spoonfuls of water in two 

 hours, at a time when the air was tolerably warm. The 

 naturalist therefore conclued that in twelve hours the 

 branch would lose a dozen tea-spoonfuls. 



But such an estimate was far from being exact. 

 Guettard managed better; he conceived the idea of not 

 se]3arating the branch from the plant, but of inclosing it 

 in a globe of glass, terminating outwardly in a neck which 

 was inserted into a flask. When all was hermetically 

 sealed, the moisture transpired, condensing itself little by 

 little on the sides of the globe, fell drop by drop into the 

 bottle situated beneath it, and could be collected without 

 the slightest loss, so that nature Avas left to herself 



Inclosed in this apparatus, a branch of a cornel-tree 

 weighing only five drachms and a half distilled each day 



