292 HALES ON PERSPIRATION". [BOOK u. 



shortened roots occupy ; and on account of the loose position 

 of the new-turned earth, which touches the roots at first but 

 in few points. This (as well as experience) strongly evinces 

 the great necessity of well-watering new plantations. 



" From July 29th to August 25th I weighed for twelve seve- 

 ral mornings and evenings, a Paradise stock Apple-tree, which 

 grew in a garden-pot, covered with lead, as the Sunflower : 

 it had not a bushy head full of leaves, but thin spread, being 

 in all but 163 leaves; whose surface was equal to 1589 square 

 inches, or 11 square feet + 5 square inches. The greatest 

 quantity it perspired in twelve hours' day, was 1 1 ounces, its 

 middle quantity 9 ounces, or 15^ cubic inches. The 15-^ 

 cubic inches perspired, divided by the surface, 1589 square 

 inches, gives the depth perspired off the surface in twelve 

 hours' day, viz., -y-^- of an inch. The area of a transverse cut 

 of its stem, 4 of an inch square, whence the sap's velocity 

 here, will be to its velocity on the surface of the leaves as 

 1589 x 4 = 6356 : 1. 



" July 27th. I fixed an Apple-branch, 3 feet long, - inch 

 diameter, full of leaves and lateral shoots, to a tube, 7 feet 

 long, -f- diameter. I filled the tube with water, and then 

 immersed the whole branch as far as over the lower end of 

 the tube, into a vessel full of water. The water subsided 

 six inches the first two hours (being the first filling of the 

 sap vessels), and 6 inches the following night, 4 inches the 

 next day, and 2 + \ the following night. The third day in 

 the morning, I took the branch out of the water, and hung 

 it, with the tube affixed to it, in the open air ; it imbibed this 

 day 27 + ^ inches in twelve hours. This experiment shows 

 the great power of perspiration ; since, when the branch was 

 immersed in the vessel of water, the seven feet column of 

 water in the tube, above the surface of the water, could drive 

 very little through the leaves, till the branch was exposed to 

 the open air. This also proves that the perspiring matter of 

 trees is rather actuated by warmth and so exhaled, than 

 protruded by the force of the sap upwards." 



Light is, however, to all appearance, the exclusive cause of 

 the decomposition of carbonic acid. It was long since re- 

 marked by Priestley, that, if leaves are immersed in water and 



