12 VENTILATION AND LIGHT. 
be so great as to starve the animal when he is turned 
out, for though it is said to economise corn and lay on flesh 
to have the animals kept warm, still it may be carried to an 
improper excess. There is not generally sufficient discrimi- 
nation exercised in the distinction between hot air and pure 
air. Hot air is not always impure, nor is cold innocuous. 
The fact is, other things being equal, a cubic yard of hot air 
is certainly less nourishing than an equal quantity of cold air ; 
because being more rarefied the former will of necessity 
have less of the vivifying principle—the oxygen. Pure 
atmospheric air ought to contain in one hundred measures, 
seventy-three of nitrogen and twenty-seven of oxygen, nearly. 
There is usually more or less carbonic acid gas mixed with 
it, seldom, however, as much as one per cent. Now the 
warmer the air in any confined place, the thinner it becomes 
—it will be rarefied. Hence a stable filled with hot air, will 
contain less oxygen than cne maintained in cold, though 
neither the one nor the other may be absolutely impure. 
The operation of breathing destroys the oxygen and leaves 
in its place a poisonous atmosphere, carbonic acid gas. 
Now this is a heavy gas. It will remain at the bottom of 
the stable unless forced upwards ; whereas the vapour and 
hot air will always tend upwards whether there is an aperture 
or not. So long therefore as the air in the stable and that 
of the atmosphere are the same warmth, there will be little 
ventilation. It is this interchange from hot to cold, which 
causes the principle to be set in motion to its fullest extent ; 
and the most perfect ventilation of air is when the cold and 
pure air is admitted at the bottom of the building and the 
hot air allowed to escape at the top. It isa mistake also to 
imagine that the foul air from a stable can possibly escape 
unless fresh and pure air is allowed free access,” 
On such excellent and clearly defined principles comment 
is needless. Having in a measure shown their basis I will 
pass to discuss the effect of inattention to these essential 
rules. The effect, indeed, may be readily discovered in a 
