The greatest living authorities on lung-diseases pronounce deficient ventilation the chief cause of consumption, and more fatal _than all other causes put together_; and, even where food and clothing are both unwholesome, free air has been found able to counteract their effect.
In the country the balance ordained in nature has its compensating power.
The poisonous carbonic acid thrown off by lungs and body is absorbed by vegetation whose food it is, and which in every waving leaf or blade of grass returns to us the oxygen we demand.

Shut in a close room all day, or even in a tolerably ventilated one, there may be no sense of closeness; but go to the open air for a moment, and, if the nose has not been hopelessly ruined by want of education, it will tell unerringly the degree of oxygen wanting and required.
It is ordinarily supposed that carbonic-acid gas, being heavier, sinks to the bottom of the room, and that thus trundle-beds, for instance, are especially unwholesome.

This would be so, were the gas pure.

As a matter of fact, however, being warmed in the body, and thus made lighter, it rises into the common air, so that usually more will be found at the top than at the bottom of a room.

This gas is, however, not the sole cause of disease.