ATMOSPHERE. 



And, by pursuing the calculations in this 

 table, it might be easily shewn, that a 

 cubic inch of the air we breathe would be 

 so much rarefied at the height of 500 

 miles, that it would h'll a sphere equal in 

 diameter to the orbit of Saturn. 



It has been observed above, that the 

 atmosphere has a refractive power, by 

 which the rays of light are bent from the 

 right lined direction, as in the case of the 

 twilight; and many other experiments 

 manifest the same virtue, which is the 

 cause of many phenomena. Alhazen, the 

 Arabian, who lived about the year 1100, 

 it seems, was more inquisitive into the 

 nature of refraction than former writers. 

 But neither Alhazen, nor his follower, 

 Vitello, knew any thing of its just quan- 

 tity, which was not known, to any tolera- 

 ble degree of exactness, till Tycho Brahe, 

 with great diligence, settled it. But 

 neither did Tycho nor Kepler discover 

 in what manner the rays of light were 

 refracted by the atmosphere. Tycho 

 thought the refraction was chiefly caused 

 by dense vapours, very near the earth's 

 surface : while Kepler placed the cause 

 wholly at the top of the atmosphere, 

 which he thought was uniformly dense ; 

 and thence he determined its altitude to 

 be little more than that of the highest 

 mountains. But the true constitution of 

 the density of the atmosphere, deduced 

 afterwards from the Torricellian experi- 

 ment, afforded a juster idea of these re- 

 fractions, especially after it was found 

 that the refractive power of the air is pro- 

 portional to its density. By this variation 

 in the density and refractive power of the 

 air, a ray of light, in passing through the 

 atmosphere, is continually refracted at 

 every point, and thereby made to describe 

 a curve, and not a straight line, as it would 

 have done, were there no atmosphere, or 

 were its density uniform. 



The atmosphere, or air, has also a re- 

 flective power; and this power is the 

 means by which objects are enlightened 

 so uniformly on all sides. The want of 

 this power would occasion a strange al- 

 teration in the appearance of things, the 

 shadows of which would be so very dark, 

 and their sides enlightened by the sun so 

 very bright, that probably we could see 

 no more of them than their bright halves ; 

 so that for a view of the other halves, we 

 must turn them half round, or, if immove- 

 able, must wait till the sun could come 

 round upon them. Such a pellucid un- 

 reflective atmosphere would indeed have 

 been very commodious for astronomical 

 observations on the course of the sun and 

 planets among the fixed stars, visible by 



day as well as by night ; but then such a 

 sudden transition from darkness to light, 

 and from light to darkness, immediately 

 upon the rising and setting of the sun, 

 without any twilight, and even upon turn- 

 ing to or from the sun at noon day, would 

 have been very inconvenient and offen- 

 sive to our eyes. However, though the 

 atmosphere be greatly assistant in the il- 

 lumination of objects, yet it must also be 

 observed that it stops a great deal of light. 

 The knowledge of the component parts 

 of the atmosphere is among the discove- 

 ries of the moderns. The opinions of the 

 earlier chemists were too vague to merit 

 any particular notice. Boyle, however, 

 and his contemporaries, put it beyond 

 doubt, that the atmosphere contained two 

 distinct substances, viz. an elastic fluid, 

 distinguished by the name of air, and 

 water in the state of vapour. Besides 

 these two bodies, it was supposed that 

 the atmosphere contained a great variety 

 of other substances, which were continu- 

 ally mixing with it from the earth, and 

 which often altered its properties, and 

 rendered it noxious or fatal. Since ihe 

 discovery of carbonic acid gas by Dr. 

 Black, it has been ascertained that this 

 elastic fluid always constitutes a part of 

 the atmosphere. The constituent pr.rts 

 of the atmosphere are, according to Mr. 

 Murray, 



By measure. By weight. 

 Nitrogen gas 77.5 /j55 



Oxygen g;ts 21.0 23 52 



Aqueous vapour 1.42 1 03 



Carbonic acid gas .08 .10 



100.00 



loj.oo 



It has been imagined, that a portion of 

 hydrogen may exist in the atmospheric 

 air. But in the usual analysis of it oxy- 

 gen is abstracted, and the residual air is 

 ibund to be nitrogen. The nitrogen is 

 probably not perfectly pure, and it is 

 possible a small portion of hydrogen is 

 mixed with it, which, from the quantity 

 being very trifling, is difficult to be de- 

 tected. 



The properties of atmospheric air ap- 

 pear to bo merely the aggregated proper- 

 ties of the gases of which it consists. It 

 is invisible, inodorous, insipid, compi t-ssi- 

 ble, and permanent!;,- elastic, [t si'| .r.rts 

 combustion, aad as it does so fr 

 oxygen it comuins, the combnstu.. ;? less 

 rapid and vivid, and continues tor a short- 

 er time. By the same agei.tv it supports 

 animal life ; a portion of its oxygen is 

 consumed in respiration, and from some 



