ON THE EXTERNAL CAUSES OF DISEASE. 265 
brain, entirely suspended. The arterial blood is no less con¬ 
nected with the irritability of the muscles and the sensibility of 
the nerves, than with the performance of all the secretions. 
The atmospheric air is composed, by measure, of about twenty- 
one per cent, of oxygen, the remainder being azote or nitrogen ; 
about one per cent., in the driest weather, of aqueous vapour, 
and about the thousandth part of the whole being carbonic acid 
gas. And this proportion exists, except in places where a num¬ 
ber of living beings are confined, in which the air cannot be 
easily renewed, over the whole surface of the globe, whether on 
sea or land, on the tops of mountains, or on the lowest valleys: 
—whether at the equator or the poles, the proportion of oxygen 
and nitrogen remains the same. 
All other aerial mixtures and gases are, in a shorter or longer 
time, destructive of animal life. Some cannot be respired at all; 
others again can be respired for a short time; but life cannot be 
long sustained by breathing any air but the mixture which con¬ 
stitutes the atmosphere. 
“ This thin, this soft contexture of the air 
Shews the wise Author’s providential care, 
Who did the wondrous structure so contrive. 
That it might life to breathing creatures give; 
Might reinspire, and make the circling mass 
Through all its winding channels lit to pass. 
Else the tir’d heart had strove with fruitless pain 
To push the lazy tide along the vein.” 
Oxygen, azote, and carbonic acid gases, breathed without any 
admixture, are speedily fatal to life. Every one has heard of the 
destructive effects upon dogs, consequent upon these animals 
breathing the air of the Grotto del Cani; and that faintness or 
suspended animation is speedily produced by holding the head 
over the fermenting vats of brewers. 
We have often witnessed the effects of the deleterious gases 
that issue from the chimneys of smelting works on the fea¬ 
thered tribe of the creation, when they fly within the influence 
of its poisonous vapours: their little wings quiver like aspen 
leaves—their drooping heads shew that strength and energy 
