10 
POPULAR SCIENCE REVIEW. 
weight, of the whole atmosphere. In the neighbourhood of 
towns and districts where this gas is produced, either artifi- 
cially or naturally, a larger proportion of the gas is found. 
The vapour of water is constantly present in the atmosphere. 
It is present in small quantities in the driest atmospheres, and 
during rain the atmosphere is saturated with it. In its largest 
quantities it is not an impurity. It nevertheless exercises a 
most important influence. The quantity of heat that falls 
upon the surface of the earth is regulated by the quantity of 
moisture in the ah*. Heat is conducted much more rapidly 
from the body in a moist than a dry atmosphere. It is, how- 
ever, in the power that the particles of moisture possess in 
taking up and retaining organic matter and various gases, 
that its influence is seen in rendering the air impure. It is in 
damp states of the atmosphere that poisons most readily 
traverse its currents, and that all the destructive agents which 
render ah* impure are most rife. It is the prevailing moisture 
of the atmosphere of the British Islands which renders their 
inhabitants more liable to the injurious influences of impu- 
rities than in countries where the temperature of the air is 
greater, but where the prevailing moisture is less. The atmo- 
sphere, however, is not rendered impure by the less or greater 
quantity of moisture it contains. 
Having surmised thus much of pure air, we are now in 
a position to judge of the nature of those impurities which 
render it injurious to animal life, and are more especially 
dangerous to human beings. We may divide these impurities 
into those which are gaseous and those which are solid, and 
speak first of gaseous impurities. 
The first of these which I shall refer to, and which is by 
far the most commonly injurious, is carbonic acid gas. We 
have seen what are the sources of this gas, and that in small 
quantities it exists naturally in the atmosphere. It cannot, 
however, be greatly increased without danger to health. The 
most common source of its increase is the interior of houses 
and buildings where human beings are gathered together. 
Human beings, when placed in rooms, are constantly con- 
suming the oxygen of the atmosphere and throwing into it 
carbonic acid gas ; thus, if means are not taken to get rid of 
it, it accumulates and takes the place of the oxygen con- 
sumed. The system is thus exposed to a diminished supply 
of oxygen and an increased supply of carbonic acid. Although 
carbonic acid can be imbibed with impunity in the form of 
effervescing beverages, as soda-water, ginger-beer, or cham- 
pagne, there is no doubt of its deleterious influence when 
inhaled by the lungs. The destruction of English prisoners 
in the Black Hole at Calcutta is an eminent example. Other 
