With every thought and feeling some actual particles of brain and nerve are dissolved, and sent floating on this crimson current.  With every motion of a muscle, whether great or small, with every process that can take place in the body, this ceaseless change of particles is going on.  Wherever oxygen finds admission, its union with carbon to form carbonic acid, or with hydrogen to form water, produces heat.  The waste of the body is literally burned up by the oxygen; and it is this burning which means the warmth of a living body, its absence giving the stony cold of the dead.  “Who shall deliver me from the body of this death?” may well be the literal question for each day of our lives; and “pure air” alone can secure genuine life.  Breathing bad air reduces all the processes of the body, lessens vitality; and thus, one in poor health will suffer more from bad air than those who have become thoroughly accustomed to it.  If weakened vitality were the only result, it would not be so serious a matter; but scrofula is soon fixed upon such constitutions, beginning with its milder form as in consumption, but ending in the absolute rottenness of bone and tissue.  The invalid may live in the healthiest climate, pass hours each day in the open air, and yet undo or neutralize much of the good of this by sleeping in an unventilated room at night.  Diseased joints, horrible affections of the eye or ear or skin, are inevitable.  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.  From both lungs and skin, matter is constantly thrown off, and floats in the form of germs in all impure air.  To a person who by long confinement to close rooms has become so sensitive that any sudden current of air gives a cold, ventilation seems an impossibility and a cruelty;