AMOUNT OP FLUID EXHALED. 173 



262. It has been ascertained, that the young leaves and 

 shoots of the Wild Cornel exhale twice their own weight of 

 water daily. A common-sized Cabbage, in the twelve hours of 

 daylight, was ascertained by Hales (one of the best experi- 

 menters upon this interesting subject), to exhale from 15 to 25 

 ounces daily, according to the light and warmth to which it was 

 exposed. This quantity, in proportion to the amount of sur- 

 face exposed by the leaves, is probably as great as most Plants 

 furnish ; and is more than is given off from the skin of Man in 

 the same time. This has been reckoned to amount in twenty- 

 four hours to about 25 ounces ; which, as there is no great dif- 

 ference between day and night, would make 12J ounces in twelve 

 hours; admitting, therefore, that the surface of his body is about 

 one-fourth less than that of the leaves of the Cabbage, and reck- 

 oning the perspiration of the latter at the mean, between the 

 greatest and the least, it is still much greater than that of man. 



263. The transpiration of a Sun-flower in full growth, during 

 fifteen days and nights, was carefully observed by Hales. This 

 plant was 3^ feet high, its weight 3 pounds, and the surface of 

 its leaves was estimated at 5616 square inches, or about 2^ 

 times that of the human body. The average transpiration 

 during the whole period, was found to be 20 ounces per day ; 

 but in one warm dry day, it was as much as 30 ounces. During 

 a dry warm night, it lost 3 ounces probably by simple evapo- 

 ration ; when the dew was sensible though small, it neither lost 

 nor gained ; and by heavy rain or dew, it gained 2 or 3 ounces. 

 When this amount is compared with that perspired by Man, it 

 may be shown that, if their surfaces were equal, the man would 

 perspire 50, and the plant 15 ; but that, for equal weights, the 

 plant exhales 17, while the man perspires 1. Experiments upon 

 single leaves, when not too long separated from the plant so as 

 to lose their vitality, yield fully as striking results. Thus a 

 leaf of the Sun-flower, weighing 31^ grains, absorbed in four 

 hours, by its petiole immersed in water, 25 grains of that fluid ; 

 the leaf had increased in weight only 4 1 grains; so that 20^ 

 grains had disappeared by exhalation. Thus a quantity equal 

 in weight to the leaf itself, would have been exhaled in about 

 six hours. 



