502. No physiologist pretends that less than seven cubic feet of air are adequate for a man to breathe each minute, while Dr. Reid allows ten feet. The necessity of fifteen or twenty times the amount of air actually taken into the lungs, arises from the circumstance, that the expired air mixes with and vitiates the surrounding element that has not been inhaled. 503. _The quantity of air which different persons actually need, varies._ The demand is modified by the size, age, habits, and condition of the body. A person of great size who has a large quantity of blood, requires more air than a small man with a less amount of circulating fluid. Individuals whose labor is active, require more air than sedentary or idle persons, because the waste of the system is greater. On the same principle, the gormandizer needs more of this element than the person of abstemious habits. So does the growing lad require more air than an adult of the same weight, for the reason that he consumes more food than a person of mature years. Habit also exerts a controlling influence. A man who works in the open air suffers more when placed in a small, unventilated room, than one who is accustomed to breathe the confined air of workshops. -=-=-=-=-=-=-=-=-=-=-=-= Observation 3d. 501. What questions may be asked respecting the inspired air? Give the remark of Birnan. 502. How many cubic feet of air are adequate for a man to breathe each minute? How much does Dr. Reid allow? 503. Mention some reasons why different persons do not require the same amount of air. -=-=-=-=-=-=-=-=-=-=-=-= 504. _Air, in which lamps will not burn with brilliancy, is unfitted for respiration._ In crowded rooms, which are not ventilated, the air is vitiated, not only by the abstraction of oxygen and the deposition of carbonic acid, but by the excretions from the skin and lungs of the audience. The lamps, under such circumstances, emit but a feeble light. Let the oxygen gas be more and more expended, and the lamps will burn more and more feebly, until they are extinguished. _Illustrations._ 1st. The effects of breathing the same air again and again, are well illustrated by an incident that occurred in one of our halls of learning. A large audience had assembled in an ill-ventilated room, to listen to a lecture; soon the lamps burned so dimly that the speaker and audience were nearly enveloped in darkness. The oppression, dizziness, and faintness experienced by many of the audience induced them to leave, and in a few minutes after, the lamps were observed to rekindle, owing to the exchange of pure air on opening the door. -=-=-=-=-=-=-=-=-=-=-=-= How is it with the laborer? With the gormandizer? With the person that works in the open air? 504. What effect has impure air on a burning lamp? Give the illustration of the effects of impure air on lighted lamps. -=-=-=-=-=-=-=-=-=-=-=-=