ASSIMILATION AND NUTRITION, IQ^ 



The quantity of this fluid discharged, either in a state of quiescence or 

 of increased action, has not been determined with any great degree of 

 exactness. According to M. de Sauvages,* a man of middle stature and 

 age weighing 146 lbs., takes daily of food and drink about 66 ounces (cir- 

 citer quinquaginta sex uncias,) his dinner being about twice as much as 

 his supper. In the same period he perspires about 28 ounces ; viz. about 

 twelve during the third part of his time in which he sleeps, and sixteen 

 during the two-thirds in which he is awake. It appears certain from the 

 experiments of Gorter, that the weight of the body is more dimmished by 

 the same quantity of sweat than of mere perspiration. 



Sanctorius, whose experiments of measuring the weight of the body 

 I 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 insen- 

 sible secretions, reduced to three ; making the proportion of insensible 

 i exhalation as five to eight. In cold climates, however, it has been de- 

 ! termined, that it does not amount to more than two-thirds of this pro- 

 portion ; and of either quantity, it has lately been very satisfactorily es- 

 tablished, that more than half this secretion 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 carbone or charcoal in every four-and- 

 twenty hours.! 



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

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

 part of their cuticle, may, as I have observed on a former occasion,J be 

 also contemplated 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 

 j down its sides. He collected these on a piece of muslin, carefully weigh- 

 I ed, and repeated the experiment for several days between twelve and 

 three o'clock ; and estimated, as the result of his experiment, that an acre 

 I of grass land transpires in twenty-four hours not less than 6,400 quarts of 

 I water. Dalron, for dew and rain together, 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 

 i alone, gives 73| millions in the present day.i! 



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

 I secretes delicate fluids, largely also imbibes them by the corresponding 

 system of absorbent vessels, opening with their spongy mouths or ducts 

 in every direction. Hales ascertained that the above sun-flower, which 

 threw ofl* not less than twenty ounces of fluid in twelve hours, suspended 

 its evaporation as soon as the dew fell, and absorbed tW9 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 va- 

 pours, as those of mercury, turpentine, and saffron ; of various baths, as 



* Nosol. MethoJ. ii. 369. t Ser. I Lect. XIII. | Ser. I. Lect. IX. 



S Phi!. Trans, for 1811, p. 265, M Phil. Mag. xix. 197. Youaai's Nat, Phi!, ii. S6P 



