880 
ON TIIE EXTERNAL CAUSES OF DISEASE. 
most dreadful in their effects upon the property and even lives 
of the inhabitants, yet we may rest assured, on general principles 
of reasoning, that, in the main, they are beneficial. We, who 
are rarely oppressed for more than a few hours in a whole sum¬ 
mer by such a state of the atmosphere as occasionally precedes a 
thunder-storm, when no friendly breeze interposes to remove the 
close and humid stratum of air which envelopes our bodies, how 
grateful to our feelings is the refreshing coolness which is expe¬ 
rienced by the agitation of a storm ! The plants give out more 
oxygen, and this in some measure accounts for the delightful 
and life-giving freshness of the air. Persons who have delicate 
lungs owe, in a great degree, the difficulty and oppression which 
they feel to the smallness of their apartments, a difficulty which 
decreases on going into a large room or into open air. 
Whatever difference of purity may be attributed to the air of 
cities and of the country, the degree of agitation of the air has a 
most marked influence on the extent to which the chest dilates 
itself: the slightest agitation of the atmosphere, when its hygro- 
metric state and temperature are adapted to the system, produces 
such a feeling of well-being, that the chest dilates in consequence, 
and admits a large proportion of air. 
From the facts we have mentioned, we may safely infer that, 
in ventilating an apartment, we have only to imitate nature, to 
cause the motion in the air which will carry off’ the atmosphere 
heated by the warmth of the body. 
To explain, in a more familiar manner, the circulation of the 
air, and why, in spite of the architect, in spite of the mistaken 
notion of grooms and stable-boys, who are over anxious in keep¬ 
ing out the air from a stable, by accurately stopping by list, 
hay, and horse-dung, all the smallest openings that admit fresh 
air, the following simple experiment will suffice. If the air of a 
room is heated by fire, whilst the air of a contiguous room is 
cold , then let the door between these two rooms be opened; in 
which case the hot air of one room will pass through the upper 
part of the opening of the door into the cold room : and, on the 
contrary, the cold air of the other room will pass into the former 
through the lower part of the opening. This may be proved by 
applying a candle at the top and lower parts of the opening be¬ 
tween the two rooms. The direction of the flame of the candle 
will point out the contrary currents of air. 
Unless this were the case, and unless the air was constantly 
renewed, stables, and even dwelling-houses, would be rendered 
guided to an antidote beneficently adapted for bis use, which lias stript 
malaria of half its terrors. Dy bis industry the marsb lias been converted 
into fertile land, and disease has given place to salubrity.” 
