ATMOSPHERE. 
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 refrac- 
tion than former writers. But neither Al- 
hazen, nor his follower Vitello, knew any 
thing of its just quantity, which was not 
known, to any tolerable degree of exact- 
ness, 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 moun- 
tains. But the true constitution of the 
density of the atmosphere, deduced after- 
wards from the Torricellian experiment, 
afforded a juster idea of these refractions, 
especially after it was found, that the re- 
fractive power of the air is proportional to 
its density. By this variation in the den- 
sity 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, lias also a reflec- 
tive power; and this power is the means 
by which objects are enlightened so uni- 
formly on all sides. The want of this 
power would occasion a strange alteration 
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 immoveable, must 
wait till the sun could come round upon 
them. Such a pellucid unreflective atmos- 
phere would indeed have been very com- 
modious 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 tiie sun, without any twilight, and 
even upon turning to or from the sun at 
noon day, would have been very inconve- 
nient and offensive to our eyes. However, 
though the atmosphere be greatly assistant 
in the illumination 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 discoveries 
of the moderns. The opinions of the earlier 
chemists were too vague to merit any parti- 
cular notice. Boyle, however, and his con- 
temporaries, put it beyond doubt that 
the atmosphere contained two distinct sub ■ 
stances, 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 contain- 
ed a great variety of other substances, 
which were continually mixing with it from 
the earth, and which often altered its pro- 
perties, and rendered it noxious or fatal. 
Since the 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 parts of the 
atmosphere are, according to Mr. Murray, 
By measure. 
By weight- 
Nitrogen gas 
77.5 ... 
... 75.55 
Oxygen gas 
... 23.32 
Aqueous vapour. 
... . 1.42 .., 
Carbonic acid gas... .08 ... 
100.00 
100.00 
It has been imagined that a portion of 
hydrogen may exist in the atmospheric air. 
But in the usual analysis of it oxygen is ab- 
stracted, and the residual air is found to be 
nitrogen. The nitrogen is probably not per- 
fectly 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 detected. 
The properties of atmospheric air appear 
to be merely the aggregated properties of the 
gases of which it consists. It is invisible, in- 
odorous, insipid, compressible, and perma- 
nently elastic. It supports combustion, and 
as it does so from the oxygen it contains, 
the combustion is less rapid and vivid, and 
continues for a shorter time. By the same 
agency it supports animal life ; a portion of 
its oxygen is consumed in respiration, and 
from some experiments of Mr. Davy, there 
appears to be a consumption of a very small 
portion of its nitrogen. Atmospheric air is 
very sparingly absorbed by water; and the 
absorpdon is unequal, more of the oxygen 
