220 
ON THE EXTERNAL CAUSES OF DISEASE. 
polluted atmosphere is the great cause which gradually and im¬ 
perceptibly undermines the health of a large portion of its inha¬ 
bitants, and, as far as the evil admits of remedy, it ought to be 
applied. This is, happily, in a great measure within our controul; 
and to accomplish this, we have only to imitate nature and em¬ 
ploy ventilation on a small scale, as she does it on a large one. 
As an axiom to be governed by in every instance in the ventila¬ 
ting of apartments, there should be a regular and renovated sup¬ 
ply of atmospheric air . 
Of the vital importance of pure air no further proof need be 
required. We have likewise shewn that this vital importance 
depends on its capability of assisting to withdraw from the body, 
chiefly through the agency of the lungs, portions of that peculiar 
principle called carbon, the permanent retention of which would 
be incompatible with the continuance of life. Some idea of the 
quantity of carbonic acid that is discharged from the blood by 
means of respiration, may be formed by the experiments that 
have been instituted for this purpose ; from which it appears that, 
during the process of respiration in an animal of ordinary size, 
and in health, about twenty-seven cubic inches and a half of 
carbonic acid gas are given off from the lungs in the course of one 
minute, which, at the end of twenty-four hours, would amount 
to 39,600 cubic inches, or in round numbers, 40,000; and, as 
100 cubic inches weigh 46J grains, 40,000 would weigh 18,532 
grains. A volume of carbonic acid gas weighing 100 grains con¬ 
tains 28 grains of carbon; a quantity, therefore, weighing 18,532 
grains would contain 5190 grains, or nearly eleven ounces ; so 
that a quantity of carbon equalling two-thirds of a pound in 
weight is daily discharged from the blood by means of the 
simple process of respiration. 
Every exhalation, with the exception of that aqueous vapour 
which is constantly rising from the surface of the earth as well 
as of the ocean and rivers, must be considered as foreign to the 
constitution of the air. Speaking according to the language of 
the atomic theory, air is composed of one equivalent of oxygen, 
and two of nitrogen : the best experiments are found to agree 
very nearly with this, when the air is perfectly pure. The 
quantity of carbonic acid gas found in air varies from three to 
eight parts out of a thousand in weight, and the quantity of 
water is very variable, but is not generally more than one and a 
half per cent, of the whole weight. These proportions of oxy¬ 
gen and nitrogen are found, as we before stated, in a free at¬ 
mosphere, not to vary sensibly throughout the whole globe; 
but when a number of animals are collected in a confined place, 
where the air cannot be easily renewed, the quantity of oxygen 
diminishes rapidly, and that of carbonic acid gas increases. 
