170 



of temperature (Reaumur's scale;) solid bodies themselves might be- 

 come fluid. In a word, a considerable diminution in the weight of the 

 atmosphere would have absolutely the same effect, as raising its tempera- 

 ture to a very great height, which, changing the face of the universe, 

 would convert all liquids into elastic fluids, and would, doubtless, melt 

 all solid bodies. 



The variations in the weight of the atmosphere, distinguishable by the 

 barometer, are of very little importance to the physiologist, and, I might 

 even add, to the physician, notwithstanding the minute attention with 

 which some writers note the state of the barometer, of the thermometer, 

 and hygrometer, and of the electrical state of the atmosphere, in giving 

 an account of a disease, or of an experiment, on which the above circum- 

 stances have no apparent or certain influence. The atmosphere, like 

 every other fluid, has a perpetual tendency to a state of equilibrium, 

 hence the rush of air into the lungs, or into other situations in which its 

 quantity is diminished, by the combinations which it forms, or by the ef- 

 fects of heat which renders it lighter, by rarefaction : the same principle 

 explains the formation of the trade and other winds. 



The atmospherical air combines with water and dissolves it, as the lat- 

 ter dissolves saline substances. In this consists the process of evapora- 

 tion. The air becomes saturated with water, in the same manner as wa- 

 ter becomes saturated with salt, to such a degree, as to be incapable of 

 holding a greater quantity in solution. As its temperature rises, its sol- 

 vent power increases, and the latter diminishes, as it grows cold ; varia- 

 tions of temperature produce the same effect on solutions of salts in li- 

 quids. The formation of all the aqueous meteors, depends on the differ- 

 ent conditions of the solvent powers of the atmosphere; when c msicler- 

 able, the atmosphere is warm and dry, and the air serene; clouds form, 

 when it is saturated; dews, fogs, and rain, are the consequence of a di- 

 minution of its solvent power, as snow and hail, of a degree of cold which 

 precipitates the fluid. The different degrees of dryness or moisture, 

 marked by the hygrometer, only sensibly affect the human body, when it 

 has been exposed for a considerable time to its influence. 



Chemically considered, the atmospherical air, which was long regard- 

 ed as a simple body, is composed of about 0,27 of oxygen, 0,73 of azote, 

 and of 0,01 or 0,02 of carbonic acid. The proportions of oxygen, ac- 

 cording to Humboldt, vary from 0,23 to 0,29; that of azote, is almost al- 

 ways the same; carbonic acid is the more abundant, as the air is less 

 pure*. This part of natural philosophy, which is called eudiometry, or 



the elaboration of chyle ; or, if the disposition to sleep be counteracted, the senses, at 

 least, become more dull, and the understanding less acute and energetic. 



The production of sleep, is favoured, too, as has been proved, by external warmth, 

 which lessens the supply of oxygen. 



It is from the combination of these causes, that among the inhabitants of hot climates, 

 the custom of sleeping during the day, and especially after eating 1 , unviersally prevails. 

 We must acknowledge that the outline of this theory was derived from the Lectures of 

 Mr. Allen, of Edinburgh, on Physiology.- Chapman. 



* Th s is pretty nearly the original estimate of Lavoisier, whose experiments have 

 since been very frequently repeated, and with no material difference in the results. It 

 is, however, proper to recollect, in speaking of the relative proportion of the ingredi- 

 ents of the atmosphere, that this estimate must be considered as having reference to 

 weight and not to measure. On this point Lavoisier is silent, as well as most other che- 

 mical writers. It is nevertheless a fact, as has been more particularly shxnvn by Ber- 



