rooms, but even here it very rarely reaches four tenths of one per cent., or ten times its usual amount, and this is still well below the harmful limit. It thus appears that carbon dioxide, like oxygen, may be eliminated from the problem of fresh air, except under the rarest and most extreme circumstances.
The amount of carbon dioxide present is often regarded as a convenient and proper index of the degree of vitiation of air by human beings, and a limit to this is sometimes established by law for factories where many employees work together. Our country unfortunately has not reached that stage of governmental control of its industries in which legal standards of ventilation are established and maintained. We ought to do this in the interests of the health of the workman, but when we are prepared for it we should select some other index of the air's impurity than the amount of carbon dioxide present in it.
There has long existed a belief—and it has been strengthened by the advocacy of competent men of science—that air that has once been breathed by human beings is poisonous apart from its content in carbon dioxide, and this belief has fixed upon a hypothetical unknown organic constituent, a toxic protein, which is supposed to be produced within the body, volatilized and then cast out with the outgoing breath. Various attempts have been made during the past twenty-five years to support this belief experimentally. The most of such experiments have consisted in condensing expired air, injecting it into animals and obtaining symptoms of intoxication. Notwithstanding their seeming conclusiveness one by one these apparently positive results have been explained on other grounds than as due to the presence of an expired organic poison coming from the lungs and, moreover, they have been offset by more conclusive experiments terminating negatively. The latest of these researches finds no evidence whatever to support the theory of an organic poison, a "crowd poison," as it is sometimes called; and we must believe that the theory represents one of those erroneous conclusions which science frequently draws from incomplete evidence and then proceeds to utilize in discovering the truth.
Another feature of vitiated air is odor. Odor is always due to the existence of material, in the form of either gas or very finely divided solid particles, which has the power of stimulating the delicate terminals of the olfactory nerves in the walls of the nasal passages. Pure air contains nothing that can stimulate these nerve terminals and therefore is wholly free from odor. Odor may be introduced into air through decaying organic matter, through illuminating gas or the products of its combustion, through various foreign substances used in industrial procedures, and through emanations from the human body. These last are many and varied, both in quality and origin, and together they give to the air of a crowded assembly which lacks adequate ventilation the
