Upon the subject of ventilation in general, a great misunderstanding exists. Be it far from me to say anything that will cause either my readers or his chickens to sleep less in the fresh air, yet for the love of truth and for the simplification of the problem of incubation, the real facts about ventilation must be given. In breathing, oxygen is absorbed and carbon dioxide and water vapor are given off. It is popularly held that abundance of fresh air is necessary to supply the oxygen for breathing and that carbon dioxide is a poison. Both are mistakes. The amount of oxygen normally in the air is about 20 per cent. Of carbon dioxide there is normally three hundredths of one per cent. During breathing these gasses are exchanged in about equal volume. A doubling or tripling of carbon dioxide was formerly thought to be "very dangerous." Now, if the carbon dioxide were increased 100 times, we would have only three per cent., and have seventeen per cent. of oxygen remaining. This oxygen would still be of sufficient pressure to readily pass into the blood. We might breathe a little faster to make up for the lessened oxygen pressure. In fact such a condition of the air would not be unlike the effects of higher altitudes. Some investigations recently conducted at the U.S. Experiment Station for human nutrition, have shown the utter misconception of the old idea of ventilation. The respiratory calorimeter is an air-tight compartment in which men are confined for a week or more at a time while studies are being made concerning heat and energy yielded by food products. It being inconvenient to analyze such an immense volume of air as would be necessary to keep the room freshened according to conventional ventilation standards, experiments were made to see how vitiated the air could be made without causing ill effects to the subject. This led to a remarkable series of experiments in which it was repeatedly demonstrated that a man could live and work for a week at a time without experiencing any ill effects whatever in an atmosphere of his own breath containing as high as 1.86 per cent. of carbon dioxide, or, in other words, the air had its impurity increased 62 times. This agrees with what every chemist and physiologist has long known, and that is that carbon dioxide is not poisonous, but is a harmless dilutant just as nitrogen. This does not mean that a man or animal may not die of suffocation, but that these are smothered, as they are drowned, by a real absence of oxygen, not poisoned by a fraction of 1 per cent. of carbon dioxide.