PERSPIRATION. 65 



is susceptible of direct proof, and is by no means a mere 

 matter of inference. We do not indeed see vapour flying off 

 from the surface of plants ; neither do we from that of animals, 

 except when the air is so cold as to condense the vapour ; yet 

 we know that in both cases perspiration is perpetually going 

 on, and it would appear that in plants it takes place more 

 abundantly than in animals. If a plant covered with leaves is 

 placed under a glass vessel, and exposed to the sun, the sides 

 of the vessel are speedily covered with dew, produced by the 

 condensation of the insensible perspiration of the plant. If 

 the branch of a plant is placed in a bottle of water, and the 

 neck of the bottle is luted to the branch, so that no evaporation 

 can take place, nevertheless the water will disappear; and 

 this can only happen from its having been abstracted by the 

 branch which lost it again by insensible perspiration. Hales, 

 an excellent observer, devised many experiments connected with 

 this subject ; * among others the following, which he relates 

 thus : — " August 13. In the very dry year 1723, I dug down 

 two and a half feet deep to the root of a thriving baking Pear- 

 tree, and laying bare a root half an inch in diameter (Fig. XIV.), 

 I cut off the end of the root at i, and put the remaining stump 

 (i n) into the glass tube d r, which was an inch in diameter, and 

 eight inches long, cementing it fast at r; the lower part of the 

 tube d z was eighteen inches long, and a quarter of an inch 

 diameter in bore. . . . Then I turned the lower end of the 

 tube {z) uppermost, and filled it full of water, and then imme- 

 diately immersed the small end z into the cistern of mercury 

 at the bottom, taking away my finger which stopped up the end 

 of the tube z. . . . The root imbibed the water with so much 

 vigour, that in six minutes' time the mercury was raised up the 

 tube d z B& high as z, namely, eight inches. . . . The next 

 morning at eight o'clock the mercury was fallen to two inches 

 in height, and two inches of the end of the root i were yet 

 immersed in water. As the root imbibed the water, innume? 

 rable air bubbles issued out at i, which occupied the upper 

 part of the tube at r as the water left it." On another occasion 



* See Vegetable Statichs, London, 1727. 



