ASSIMILATION AND NUTRITION. 



157 



the weight of the body is more diminished by the same quantity of sweat 

 than of mere perspiration. 



Sanctorius, whose experiments of measuring the weight of the body were 

 made in the warm climate of Italy, ascertained that in that region eight pounds 

 of food received by the mouth were, by the different insensible secretions, 

 reduced to three ; making the proportion of insensible exhalation as five to 

 eight. In cold climates, however, it has been determined that it does not 

 amount to more than two-thirds of this proportion; and of either quantity it 

 has lately been very satisfactorily established, that more than half this secre- 

 tion has "been thrown forth from the surface of the lungs ; which I estimated 

 in a previous lecture, and from the experiments and calculations of Lavoisier, 

 as discharging not less than eleven ounces of solid carbon or charcoal in 

 every four-and-twenty hours.* 



Plants transpire precisely in the same way, and to a much greater extent, 

 through the medium of their leaves ; which, while they form a great part of 

 their cuticle, may, as 1 have observed on a former occasion,! be also contem- 

 plated as their lungs. Hales calculated that a sun-flower, three feet high, 

 transmits in twelve hours one pound four ounces of fluid by avoirdupois 

 weight. Bishop Watson put an inverted glass vessel, of the capacity of twenty 

 cubic inches, on grass which had been cut during a very intense heat of the 

 sun, and after many weeks had passed without rain ; in two minutes it was 

 filled with vapour, which trickled with drops down its sides. He collected 

 these on a piece of muslin, carefully weighed, and repeated the experiment 

 for several days between twelve and three o'clock ; and estimated, as the 

 result of his experiment, that an acre of grass land transpires in twenty-four 

 hours not less than 6,400 quarts of water. Dalton, for dew and rain toge- 

 ther, makes the mean of England and Wales 36 inches, thus amounting, in a 

 year, to 28 cubic miles of water. Grew, in 1711, calculated the number of 

 acres in South Britain at 46,800,000, and allowed a million to Holland.:|: 

 Smith, for England alone, gives 73{ millions in the present day.^ 



But the same general surface in animals and vegetables that thus largely 

 secretes delicate fluids, largely also imbibes them by the corresponding sys- 

 tem of absorbent vessels, opening with their spongy mouths or ducts in every 

 direction. Hales ascertained that the above sun-flower, which threw off not 

 less than twenty ounces of fluid in twelve hours, suspended its evaporation 

 as soon as the dew fell, and absorbed two or three ounces of the dew instead. 

 And among animals, and especially among mankind, the manifest operations 

 of medicines and other foreign substances, merely diffused through the air, 

 or simply applied to the skin ; of various vapours, as those of mercury, tur- 

 pentine, and saflfron ; of various baths, as of tobacco, bitter-apple, opium, 

 cantharides, arsenic, and other poisons, producing the most fatal effects, and 

 altogether absorbed by the skin, are decisive and incontrovertible proofs of 

 such an action. It is hence the bradypus, or sloth, supports itself without 

 drinking, perhaps, at any time, and the ostrich and camel for very long pe- 

 riods, though the latter is also possessed of a natural reservoir. And hence 

 the chief impletion of the human body, in many cases of abdominal dropsy ; 

 since persons labouring under this disease have often been observed to fill 

 with rapidity during the most rigid abstinence from drinks of every kind. 



Along with the common odour of insensible perspiration, discharged from 

 the human surface, we often meet with other odours of a much stronger 

 kind, produced by particular diseases or particular modes of life, and which 

 are distinctly perceptible. Thus the food of garlic yields a perspiration pos- 

 sessing a garlic smell; that of pease a leguminous smell; coarse oils and fat 

 a rancid smell, which is the cause of this peculiar odour among the inhabit- 

 ants of Greenland; and acids a smell of acidity. Among glass-blowers, 

 from the large quantity of sea-salt that enters into the materials of their 

 manufacture, the sweat is sometimes so highly impregnated, that the salt thev 



• Series i. Lecture xiii. 



J Phil Trails for 1811, p. 2G6 



t Series i. Lecture ix. 



^ Phil. Mag. xix. 197. Young's Nat. PhU. ii. 369 



