In the open air the effects of respiration on the atmosphere cannot be appreciated, but in enclosed spaces, such as houses, rooms, etc., which are occupied by many persons, the air soon becomes appreciably changed by their breathing.

The most important changes are (1) removal of oxygen, (2) increase in carbonic acid, and (3) the appearance of some poisonous materials which, though highly injurious, cannot be determined. The deficiency in oxygen never causes any inconvenience, as it is never reduced below what is sufficient for the saturation of the haemoglobin. The excess of C02 seldom gives any inconvenience, since the air becomes disagreeably fusty or stuffy long before the amount of C02 from breathing has reached o. 1 per cent., which amount of pure C02 can be inspired without any unpleasantness. It is, then, the exhalations coming from the lungs, and probably skin, some of which must have a poisonous character, that render the proper supply of fresh air imperative.

The difficulty of determining the presence of the poisonous organic materials makes it convenient to use the amount of C02 present in the air as the means of measuring its general purity. For this we must suppose that the relation between the poisonous organic ingredients and the C02 is constant.

Air which is rendered impure by breathing becomes disagreeable to the sense of smell when the C02 has reached the low standard of .06 or .08 per cent., that is to say, scarcely twice as much C02 as is contained in the pure atmosphere. Supposing that air is unwholesome when its impurities are appreciable by the senses, then, if the animal body be the source of the C0.2,.06 per cent, of this gas makes the air unfit for use.

An adult man disengages more than half a cubic foot of C02 in one hour (.6, Parkes), and consequently in that time he renders quite unfit for use more than 1000 cubic feet of air, by raising the percentage of C02 to.1 (0.4 being initial, and.06 respiratory).

It is obvious that the smaller the space and the more confined, the more rapidly will the air become vitiated by respiration. It becomes necessary for health, therefore, to have not only a certain cubic space and a certain change of air for each individual, but the cubic space and the change of air should bear to each other a certain proportion, in order that the air may remain sufficiently pure.

The space allowed in public institutions varies from 500 to 1500 cubic feet per head, in such apartments as are occupied by the individuals day and night. As a fair average 1000 cubic feet may be fixed as the necessary space in a perfect hygienic arrangement. In order to keep this perfectly wholesome and free from a stuffy smell, and the C02 below.06 per cent., it is necessary to supply some 2000 cubic feet of air per head per hour.

To give the necessary supply of fresh air without introducing draughts or greatly reducing the temperature of the room is no easy matter, and forms the special study of the hygienic engineer.