volume of carbonic acid. The oppression and 
other injurious effects of such air must therefore 
be ascribed more to the temperature and to the 
presence of other noxious substances, as carbonic 
acid and exhaled animal substances, than to a 
want of oxygen. ! 
It has already been stated that, in abstracting 
the oxygen from the atmosphere by any substance 
which will combine with it, the remaining gas 
i! consists almost entirely of nitrogen, but mixed 
with a minute quantity of carbonic acid and 
watery vapour. 
Nitrogen. — The principal characteristic of 
| nitrogen is its indifference to oxygen, as well as 
| to other substances, and its consequent apparent 
'| reluctance to enter directly into combination 
with them. Its use seems, therefore, principally 
to be to dilute the oxygen; for, from the power- 
ful affinity of this latter substance, it is obvious, 
| that if it alone constituted our air, it would soon, 
with the present arrangement of our earth and 
I its inhabitants, produce universal destruction, 
| for all animals would perish from the excessive 
influence of the oxygen on the blood and the res- 
| | piratory organs, and the greatest part of all com- 
| bustible matters would soon be involved in one 
general conflagration. It may, however, be re- 
marked that, although the nitrogen of the air 
| does not seem necessary by itself to the continu- 
| ance of life, nor any direct reaction between it 
| and the living creation has yet. been found to 
exist, yet it forms an invariable and essential 
| constituent of both animals and plants. 
From the fact of the invariable proportion of 
| the oxygen to the nitrogen, and this ratio being 
very nearly as 1 to 4, some chemists have ad- 
vanced the idea that the air is a chemical com- 
pound of 1 volume of oxygen with 4 volumes of 
nitrogen, forming a compound with half as much 
oxygen as in nitrous oxide; but, if this were the 
|| ease, it would afford the only instance of a me- 
| chanical mixture having exactly the same pro- 
perties as a chemical compound. For an artifi- 
| cial mixture of oxygen and nitrogen in the same 
proportion does not differ in any respect from the 
| air, and yet that they enter into no chemical 
combination by their mixture is evident from 
the absence of any change in temperature or 
volume. Besides the fact that nitric oxide ab- 
stracts oxygen from the air, being thereby con- 
verted into hyponitrous acid, would thus afford 
|) an instance of one oxide reducing another of the 
| same radical and liberating the latter, which 
would be both improbable and without a parallel. 
| The objection that the oxygen, as the heavier 
| gas, ought in still weather to settle down at the 
|| bottom in the lower strata is unfounded, since 
| gases mix together as liquids of different specific 
gravity, without any tendency to subsequent se- 
paration by mere gravitation. On the contrary, 
the law of diffusion of gases would make them 
mix and penetrate each other perfectly without 
any mechanical agitation. 
ATMOSPHERE. 
in the air may be easily proved by exposing lime 
or barytic water in an open vessel to the air, or 
passing the latter through their solution, when 
a precipitate will be formed, which effervesces by 
the addition of an acid, and consists of carbonate 
of lime or baryta. 
the carbonic acid is diffused through the whole 
Like the two former cases, 
atmosphere ; its proportion is, however, very 
small. By a series of experiments, carefully per- 
formed by Th. de Saussure, it has been ascer- 
tained that the mean proportion of carbonic acid 
in the air does not amount to more than 0:0004, 
or 4 volumes in 10,000 volumes of air, but that 
this amount varies from 0:00057 as a maximum 
to 0:00032 asa minimum. During the night its 
proportion is greater than during the day, and in 
populous cities greater than in the country, and 
also in winter and in high situations greater than 
in summer, or in low situations, owing probably 
to the effect of vegetation. Dry weather and 
frost seem also to increase its amount, probably 
by preventing absorption by the soil, while over 
lakes and other waters, and after rain, it is some- 
what less. 
Notwithstanding the small proportion which 
the carbonic acid bears to the whole atmosphere, | 
its presence is of the highest importance in an 
economical point of view. We have seen that 
the animal kingdom consumes annually an im- 
mense quantity of oxygen which it converts into 
carbonic acid, while, on the other hand, the | 
| 
plants again absorb the carbonic acid and return 
283 
Carbonic acid.—The existence of carbonic acid 
the liberated oxygen to the air; for, although 
during the night they absorb oxygen and exhale | 
carbonic acid, the reversed action during the day | 
predominates greatly. The very existence and | 
continual growth of plants seem, therefore, to | 
depend on the presence of the carbonic acid. 
How far animals and plants counterbalance each 
other we have not yet been able to ascertain by 
direct experiments, but considering the continual 
increase of carbonaceous matter in the soil, and 
the immense deposits of carbon as coal and lignite, 
which owe their origin to a previous vegetation, 
it would seem, indeed, that the amount of car- 
bonic acid in the atmosphere must have been 
formerly greater, and that its absorption by plants 
so far prevails over the causes which reproduce | 
it as to cause its slow but steady diminution. | 
Carbonic acid acts as a poison to man when 
inhaled. Its accumulation, by respiration, and 
the burning of lights in crowded and lighted | 
rooms, constitutes, therefore, a frequent cause of | 
the vitiation of the air in such places, and its in- | 
jurious effects are even felt before it reaches the 
amount of one per cent. | 
Vapour of Water.—A continual evaporation | 
taking place from seas, lakes, rivers, and other 
waters, as well as from the soil in general, these | 
vapours diffuse themselves through the air, and | 
form an important constituent of it. Vapour of 
water not being a permanent gas, as the other 
veils 
i ceeeetomes | 
