Ordinary air contains about 0.04 per cent of carbon dioxid; that is, four parts in ten thousand parts of air, the other nine thousand nine hundred ninety-six being made up of oxygen and nitrogen. Of course, it is not possible to express any definite value for the amount of carbon dioxid which is objectionable in air, because, in the first place, it is not certain that the carbon dioxid in itself is the cause of diminished vitality due to insufficient ventilation, and, in the next place, insufficient ventilation affects different people in different ways. But it is known that in the lungs the life-giving oxygen is changed to carbon dioxid, and that just as carbon dioxid gas will prevent the combustion of a candle flame, so carbon dioxid gas will destroy the life of man. When a deep well is to be cleaned out, the decomposition of organic matter in the bottom of the well will have, in all probability, caused the formation of this same carbon dioxid gas, and it is not uncommon for a man descending into such a well to be overcome by the gas, which, in some cases, even causes death. For this reason, it is common to lower into a well, before it is entered by a man, a candle or lantern, on the probability that if the lantern can stand it, certainly the man can, while if the lantern goes out, it is wise to avoid the risk of having a man's life put out in the same way. _Organic matter in air._ The stuffy and close feeling perceived in an ill-ventilated room is, however, due to the organic matter from the lungs, which is expired along with the carbon dioxid, and some chemists have argued that this amount of organic vapor ought to be measured instead of the carbon dioxid. At the present time there is no simple and direct method of measuring organic vapor, and because this vapor increases in the atmosphere proportionately to the carbon dioxid gas, it is much simpler to measure the latter. Then it is impossible to fix a standard of carbon dioxid because a person whose lungs are well developed and whose blood is well oxygenated, or, as we say, one who has good red blood can stand, even if uncomfortable, a few hours of a bad atmosphere without suffering serious discomfort, while an anaemic or poor-blooded person would be affected to a greater degree. It is for this reason that in any house no living room, especially one heated by a coal stove, should be shut up tight against fresh air. This is the reason why the women of the family, who have to breathe the same air over and over all day, are pale and weak and easily susceptible to disease, while the men, who are out of doors most of the time, and when indoors are made restless by the bad air, suffer much less from the ill effects.