The following short summary of the principles of ventilation is taken from Messrs. Notter and Firth's "Practical Domestic Hygiene." The composition of pure dry air may be taken to be as follows. Nitrogen, 79-02 by volume, 76.84 by weight; oxygen, 20.94 b.v., 2310 b.w.; carbon dioxide (carbonic acid), 004 b.v., 0.06 b.w. There are also present in the atmosphere, which is free from colour, taste, or smell, a certain quantity of watery vapour, with various impurities; and LordRayleigh and Prof. Ramsay have recently shown that about 1 per cent, of what was considered to be nitrogen is an elementary gas called argon. The nitrogen in the air is incombustible, and incapable of supporting life, and evidently acts as a diluent of the oxygen, which is necessary to life, combustion, and light. Carbon dioxide, or carbonic acid, is produced in all processes of combustion, and by the breathing of men and animals, as well as by the process of putrefaction. The watery vapour in the air prevents undue evaporation from the body and from plant life. The physical properties of the air are weight, expansion and contraction, and diffusion. The pressure of the air at sea-level is equal to 14-75 lb. per square inch of surface.

The pressure on the atmosphere is never constant, but varies with the temperature and with the presence of moisture. The ventilation of ordinary dwellings is rendered necessary by the fact mentioned above - that when air is breathed or used up in combustion, its oxygen, which supports life, is exhausted, and is replaced by carbon dioxide, which, as already seen, is incapable of supporting life or light. Hence the necessity tor ventilation, which is defined, in the book mentioned above, as " the dilution or removal, by a supply of pure air, of the products of respiration and combustion in ordinary dwell-ings." The average amount of carbonic acid given oil' by adults is O'O cub. ft. per hour, besides about 550 grains of watery vanour. A cubic foot of coal gas yields, on combustion, 0'52 cub. ft. of carbonic acid and 1.3 cub. ft. of watery vapour; while an ordinary gas burner may be reckoned as equal to at least three adults in its effect on the atmosphere. The atmosphere of the home, to he of the standard degree of purity, should not, contain more than 0.6 part of carbon dioxide in 1,000, and in order to maintain this standard it is necessary to supply at least 3,000 cub. ft. of fresh air per head for healthy persons, whilst the sick need at least 4,500 cub. ft. of fresh air per hour.

In actual practice, however, it is found that, in England, the air of a room cannot, be changed more than three times an hour without giving rise to draught. Air at a temperature of 60 F and moving at the rate of more than 3 it. per second, becomes a perceptible draught; but if the temperature be, say, 70° F., the velocity of the air may be greater than 3 ft. per second without causing an unpleasant sensation of draught. Each adult in a room should have an air space of at least 1,000 cub. ft.; hut in lodging-houses the allowance is only 300 cub. ft. In Board schools the regulation minimum allowance is 100 cub. ft. per head; in factories and workshops, 250 cub. ft. per head in the daytime, and 400 cub. ft. at night; for military barracks, 000 cub. ft. per head; while in hospitals the' allowance ought to be quite 1,500 cub. ft., if not nearly 2,000 cub. ft., and the minimum floor since 100 sq. it. The question of floor space is of considerable importance, and it is recommended that the lowest limit of floor space should be not less than one-twelfth of the cubic space. " It cannot be too well understood,' say the authors of the above-mentioned excellent manual, "that cubic space is of no value when it is principally obtained by means of lofty ceilings.

The space at the bottom of a well, if crowded, would speedily become unwholesome, although the air space above is unlimited; similarly, people have been known to die of suffocation in a crowd, though in the open air." A room, therefore, need not exceed 14ft. in height, and 12ft. is sufficient. Minimum floor areas prescribed are for soldiers in barracks, 50 sq. ft. each; for children in schools, 8 sq. ft. (but in newer schools the allowance is sometimes extended to 15 sq. ft.); patients in hospitals, 100 sq. ft. to 150 sq. ft. and more. From the foregoing facts it is deducible that proper ventilation is a means of renewing the air in au apartment without creating a draught; the inside air being constantly kept up to the standard of purity previously stated. An agreeable atmosphere for a room has a humidity of 60 per cent, and a tern-perature of 60° F.