286 
diffusedly as dew, or precipitating itself more 
vigorously as rain, &c., and thus again returning 
to its first origin. Without the presence of the 
moisture in the atmosphere, neither plants nor 
animals could live. To it isalso due the constant 
decomposition and decay of animal and vegetable 
substances; and even the rocks and mountains 
wear away and decay under its all-levelling influ- 
ence, for besides its own dissolving power, it 
seems to be that which lends to the other de- 
stroying agencies of the atmosphere their force 
and eflicacy, for without it they seem inert, at 
least at ordinary temperatures. In dry air, steel 
does not rust, nor does wood decay. 
Other ingredients in the Atmosphere—Besides 
the above-named constant ingredients of the at- 
mosphere, a number of other gases and vapours 
are known to enter it in certain localities, but 
they are too inconsiderable in quantity to be de- 
tected afterwards in its general composition. As 
such may be mentioned sulphuretted hydrogen, 
chlorohydric acid gas, carburetted hydrogen, &c. 
Boussingault has proved the constant presence 
of a minute trace of a carburetted hydrogen, or 
some other similar compound, by passing atmo- 
_ spheric air over ignited oxide of copper, whereby 
| he obtained carbonic acid gas and water, from 
which he had carefully freed it previously. The 
air had also previously passed through concen- 
trated sulphuric acid, so that they could not 
derive from dust or other mechanically suspended 
organic particles. The final products of the decay 
of animal and vegetable matters being carbonic 
acid, water, and ammonia, which escape into the 
atmosphere, Liebig infers that the latter gas 
ought never to be absent, although in too minute 
quantity to be detected directly by ordinary ex- 
periment, but according to him, it always exists 
in rain water. Rain is also the principal mean 
of carrying down again most other accidental 
| substances, either dissolved or suspended in the 
atmosphere, among which may be included the 
different miasmata and -matter of contagion, 
which are perceptible only by the senses or by 
their effect upon the human system, but cannot 
otherwise be detected by chemical tests. although 
we may destroy them, or at least their injuricus 
effects, by chemical means, such as by muriatic, 
nitric, or acetic acids, or chlorine gas. The ac- 
tion of the atmosphere itself tends necessarily to 
destroy or oxidize them by the combined influ- 
ence of oxygen with light and heat. 
Physical properties of the Atmosphere—We have 
thus seen that the atmosphere consists of a me- 
chanical mixture of three permanent gases, and 
one condensable. Its physical and mechanical 
| properties are therefore such as might be calcu- 
lated from a mixture of them. 100 cubic inches 
of atmospheric air, deprived of its carbonic acid 
and aqueous vapour, weighs, according to Dr. 
Prout, 31:0117 grains, at 60° temperature, and 
30 inches barometric pressure, and its specific 
ATMOSPHERE. 
the standard for comparison with other gases, 
and therefore set equal to 1 or 1,000. Compared 
with water at 62° it is 815 times lighter than 
water, and 11,065 times lighter than mercury, 
At 32° Fahrenheit, it is 770 times lighter than 
water or its specific gravity = 0001299 that of © 
water at 39° =1. The weight of the atmosphere 
pressing upon the surface of the earth, is equiva- 
lent to about fifteen pounds on each square inch 
of surface, or equal to the weight of a column of 
mercury of 30 inches, or a column of water of 
nearly 34 feet in height. But this pressure varies | 
continually from the different changes in its vari- 
able constituents, from its commotion by currents 
of air, and from other causes. 
are indicated by the rise or fall of the mercurial 
column in the barometer (see this). Its mean 
pressure being taken at thirty inches of mercury, 
the contribution to this pressure of its different 
constituents, which may be considered as inde- 
pendent atmospheres, will be as follows, the 
amount of vapour being calculated from the ca- 
pacity of the air at 50°. 
Pressure of the Oxygen, 6°828 inches. 
a Nitrogen, . 228450, 
Ae Watery vapour, O;309.%5; 
.; Carbonic acid, 0-018 ,, 
30°000 © 
The weight and pressure of the atmosphere may 
be ascertained by very simple experiments. If 
we immerse in water a glass tube open at both 
ends, the water included in the tube will be on | 
the same level with the fluid which surrounds it. 
When we apply our mouth to the upper end of 
the tube, and draw out the air, the included water 
instantly ascends till the weight of the elevated | 
column added to the elasticity of the remaining 
air, exactly balances the pressure of the atmo- 
sphere on the surrounding fluid. If we now take 
a long tube, forty feet long for example, shut at 
one end, and having filled it with water, plunge | 
the open end into a vessel of water, the fluid will | 
then descend in the tube till the weight of the 
column exactly equals the pressure of the atino- 
sphere; for the air is now excluded from the 
upper part of the tube, and the weight of the 
column of fluid is the only force which is left to 
balance the weight of the atmospherical column. 
By making this experiment, it will be found that | 
the water stands at from 34 to 35 feet above the 
general level of the surrounding fluid, and there- 
fore the weight of a column of air reaching to 
the top of the atmosphere, is equal to the weight 
of a column of water, of the same hase, with the 
altitude of 34 feet, or about 21,5645 pounds on a 
square foot, or 15 pounds on every square inch. 
This experiment may be more easily made hy 
using quicksilver instead of water. The quick- 
silver will rise to the height of 29 inches in the | 
tube, and will thus measure the pressure of the 
atmosphere. Hence it follows, that the whole 
atmosphere exerts the same pressure on the 
gravity at this temperature is generally taken as , surface of the earth, as if the surface of the 
These differences | 
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