The principal causes of the contamination of air in rooms, other than want of adequate change of air, are: (1) Deposits of animal and organic matter , including exhalations from human beings and animals, the products of respiration, etc.; (2) Condensation; '3) Evaporation; and (4) Absorption.

In every inhabited room, particularly those in which meals are partaken of, there must necessarily be deposited a certain amount of organic matter, such as crumbs, particles of skin, respiratory matter from human beings and domestic animals. This matter, if permitted to accumulate, will putrefy, and give off gases, which will pollute the air and make it unwholesome to breathe.

On all surfaces, which by not absorbing and retaining heat are Mow the normal temperature of the room, condensation will take place, particularly when change of air is not frequent. In addition to the moisture or vapour of water, which is ever present in the atmosphere, moisture is caused by respiration and exhalations from human beings and animal-, steam from cooked meats, the products of combustion (whether for heat or light), tobacco-smoke, and the volatilization of various organic substances; every one of these forms of moisture, with their several organic or other impurities, may be condensed upon the surfaces within the room, when these are Mow the general temperature of the atmosphere it contains and the change of air is not sufficiently rapid to at once carry them away.

This condensation can always be noticed upon large surfaces of glass, when the temperature outside is below that within, but although it is more noticeable upon the glass, it is, in fact, being deposited uponi all the surfaces in the room in proportion to their temperature; hence the importance of constructing and furnishing dwellings with materials which absorb and retain heat, and not with normally cold and impervious substances.

Much of the moisture, and of the more volatile portion of the substances condensed, will again be carried off by evaporation when the temperature of the surfaces is raised, or a freer circulation of air is permitted, but the less volatile organic matter remains and forms a film over the surfaces, which will either putrefy and so contaminate the air, or will dry and fall off as dust, probably so fine that it will be carried about by movements of the air. In this way, the germs of disease may contaminate the air, and enter the lungs of persons occupying the room.

Condensation and evaporation, therefore, play an important part in connec-tion with ventilation, and I am inclined to belive that the popular fancy for hard, impervious substances, which do not readily retain heat, for the construction and lining of walls, is more than questionable, if comfort and good ventilation are to be secured in our homes.1

1 It must not be forgotten that those materials which "absorb and retain heat". - such as wool ordinary piaster. unglazed wall- papers. - are the very ones to abaorb organic matter. Sir Douglas Gallon, in his book on Heathly Dwellings, says that "in 1862, in the French Academy of Medicine, a case was mentioned in which an analysis bad been made of the plaster of a hospital wall, and 46 per cent of organic matter was found in the plaster". This was Many substances and materials possess, in varying degree, the property of absorption, not alone of moisture but of volatilized substances and even of gases:for sample, clothes and draperies retain for a considerable time the odour of tobacco-smoke. Even to plan) surfaces, air itself somewhat clings, for it is a well-known fact that a stream of air, propelled at an angle against a flat surface, does not rebound at the same angle as would a solid, but goes off at a more obtuse angle, indicating that an attractive or retarding influence has been exerted.

Some, who have observed the stuffy effect produced by a superabundance of draperies, rush to the opposite extreme, denude their rooms of all such, strip the papers from the walls and paint them instead, or cover them with tiles and glazed wane, discard carpets, and varnish the floors. Yet, in doing so, they probably fall into greater error, for to secure change of air in such rooms, without causing draughts, becomes more difficult; the rooms have a chilling effect in cold weather, and unless the hard surfaces are constantly cleaned, they beome incrusted with dirt resulting from condensation thereon.1

Observation has demonstrated that the dust ever present in the air influences the deposition of moisture, every particle of which carries with it an atom of dust; in addition to which, the moistened surfaces will cause other particles to beome attached. Motilities.? most dust-particles are inert; some, however, are organic, and of these some are living microbes. Hence it must be a questionable proceeding to give resting-place for such, particularly in conjunction with moisture, which is supposed to retain their vitality, and under favourable circumstances to assist in their multiplication.

Frequent change of air too often brings with it a large amount of dust and soot, etc., and the careful housewife will often close windows and doors and ventilators to keep it out. In doing so, ventilation may be retarded. Except in very windy weather, when heavier particles are raised and blown about, it is a question whether, in badly-ventilated houses, dust is not deposited upon before the days of bacteriological examination, but undoubtedly this organic matter contained many micro-organisms, passably pathogenic; even ordinary bricks have proved capable of harbouring and multiplying this minute life. Besides, absorbent materials do not prevent condensation ; they merely hide it. The true remedy would appear to lie in the direction of warming the surfaces in a room, especially the walls, and of providing a freer circulation of air. An interesting example of a bouse constructed on this principle is given by Dr. Billings in his book on " Ventilation and The house is at Creil in France, and is of two stories, the external walls having an outer skin about 9 inches thick, sod an inner skin 4 inches thick, with a cavity about 9 inches wide between. The air circulating in the canty is warmed by mesne of hot-water pipes to a temperature of about 120' Fahr., and gives to the inner akin of the wall a temperature of about 90 Fahr. The air to the rooms is admitted cold, and is extracted by means of "chimneys". -The location of the boose is a damp one, but the interior is dry, and the house is said to be very comfortable." - Ed. On the other hand, so great is the absorption of organic and other impurities by ordinary wall-papers and plaster, that every sanitarian declares that old wall papers must be stripped off before new ones are laid, and some go so far as to advocate the removal of the outer skin of plaster every few years.-Ed.