beyond, or even as far as, making up the deficiency caused by heating, since the human system is used to considerable changes without any real discomfort. Indeed, dry air, if pure, is probably more beneficial to normal adults than moist air. The principal reason why the demand for moisture in heated rooms has arisen is the irritating effect of floating dust which has been set in motion by the heating system, directly or indirectly. In the worst form this may be noticed with hot-air heating through floor registers, which invite all sorts of rubbish to fall into the flue, only to be dried and sent up again, often directly into one's nose. Radiators also, especially those with inaccessible surfaces, will gather dust. When cold, it will lay there and molest no more than that on furniture, but as soon as heat is turned on, the tiny drops of moisture, which always cling to these solid particles, will evaporate. Free of this weight, the dust is easily set in motion by the currents of warm air rising from the radiator, as may often be seen by the tell-tale shadows on the wall above. Heating apparatus thus contaminates the air with dust and bacteria which otherwise would lay undisturbed and out of harm's way. Moistening of the air will not prevent this to any extent. It increases, in fact, another source of contamination, still too common with modern heating systems—the dry distillation of the organic matter on hot surfaces. This phenomenon has recently been studied by the noted hygienists Professors Esmarch and Nussbaum, who have independently reached the conclusion, that organic dust begins to distil or singe when a radiator reaches a temperature of about 165° F., and that this process is rather encouraged by moisture, probably because the hygroscopic matter clings longer to the heated surfaces and is therefore decomposed before it rises up in the air. To reduce' the vitiating effect of heating apparatus, we must insist on the most accessible and simple styles of radiators, on which any dust can readily be seen and is apt to be removed, and on ample heating surfaces of moderate temperature which will tend to avoid the decomposition of organic matter.
Overheating by itself must be considered as vitiating the air; at least in so far as it makes it unfit, or less wholesome, according to some noted hygienists who have thoroughly investigated its effect. It seems, at any rate, to give the air a lifeless quality, which soon imparts itself to the victim of our wasteful modes of heating.
Apparatus for artificial moistening, which is now often installed in connection with heating and ventilating systems, aside from the liability of exceeding the desirable humidity, also gives opportunity for contamination of the air supply. Unless the devices are designed on sanitary principles and intelligently attended to, they are very liable to become foul and malodorous, if not unhealthy.
Like the heating of buildings, artificial cooling may also have un-