288 PERSPIEATION. t BOOK 



If, again, we take three similar plants, and, preventing 

 their absorption by the roots, after weighing them carefully, 

 place them in three similar situations, we shall find that the 

 plant exposed to the sun has lost a great quantity of water, 

 that in common daylight a less amount, and that which was 

 in total darkness almost nothing." 



It is, however, to be supposed, that light is, to a certain 

 extent, in these cases, a remote, as well as immediate, cause of 

 perspiration : for we cannot apply solar light to plants without 

 heating and rarefying their atmosphere. It is a well-known 

 fact, that plants perspire in a sitting-room the air of which is 

 constantly dry, but which is but imperfectly illuminated, so 

 much more than in the open air exposed to the direct rays of 

 the sun, that it is impossible to keep many kinds of plants 

 alive in such a situation. 



The admirable experiments of Hales, recorded in his 

 Vegetable Staticks, demonstrate not only the fact of per- 

 spiration taking place, but its amount, and with what external 

 influences it is connected. 



"July 3, 1724, in order to find out the quantity imbibed 

 and perspired by the Sunflower, I tot>k a garden-pot, with a 

 large Sunflower, 3- feet high, which was purposely planted 

 in it when young. I covered the pot with a plate of thin 

 milled lead, and cemented all the joints fast, so as no vapour 

 could pass, but only air, through a small glass tube, nine 

 inches long, which was fixed purposely near the stem of the 

 plant, to make a free communication with the outward air, 

 and that under the leaden plate. I cemented also another 

 short glass tube into the plate, two inches long and one inch 

 in diameter. Through this tube I watered the plant, and 

 then stopped it up with a cork ; I stopped up also the holes 

 at the bottom of the pot with corks. I weighed this pot and 

 plant morning and evening, for fifteen several days, from 

 July 3 to August 8, after which I cut off the plant close to 

 the leaden plate, and then covered the stump well with 

 cement ; and upon weighing found there perspired through 

 the unglazed porous pot, two ounces every twelve hours' day, 

 which being allowed in the daily weighing of the plant and 



