Dr Neil Arnott has addressed a letter on this subject to the 'Times' newspaper. Any expression of opinion by him on such a subject, and more particularly with reference to the prevailing epidemics, must be deemed of so much importance, that we are anxious, as far as in our power, to keep it before the world. He commences by assuming, what will readily be granted, that fresh air for breathing is one of the essentials to life, and that the respiration of air poisoned by impure matter is highly detrimental to health, insomuch that it will sometimes produce the immediate destruction of life. The air acquires impurities from two sources in chief--solid and liquid filth, and the human breath. Persons exposed to these agencies in open places, as the manufacturers of manure in Paris, will suffer little. It is chiefly when the poison is caught and retained under cover, as in close rooms, that it becomes notedly active, its power, however, being always chiefly shown upon those whose tone of health has been reduced by intemperance, by improper food or drink, by great fatigue and anxiety, and, above all, by a habitual want of fresh air. Dr Arnott regards ventilation not only as a ready means of rendering harmless the breath of the inmates of houses, as well as those living in hospitals and other crowded places, but as a good interim-substitute for a more perfect kind of draining than that which exists. 'To illustrate,' he says, 'the efficacy of ventilation, or dilution with fresh air, in rendering quite harmless any aërial poison, I may adduce the explanation given in a report of mine on fevers, furnished at the request of the Poor-Law Commissioners in 1840, of the fact, that the malaria or infection of marsh fevers, such as occur in the Pontine marshes near Rome, and of all the deadly tropical fevers, affects persons almost only in the night. Yet the malaria or poison from decomposing organic matters which causes these fevers is formed during the day, under the influence of the hot sun, still more abundantly than during the colder night; but in the day the direct beams of the sun warm the surface of the earth so intensely, that any air touching that surface is similarly heated, and rises away like a fire balloon, carrying up with it of course, and much diluting, all poisonous malaria formed there. During the night, on the contrary, the surface of the earth, no longer receiving the sun's rays, soon radiates away its heat, so that a thermometer lying on the ground is found to be several degrees colder than one hanging in the air a few feet above. The poison formed near the ground, therefore, at night, instead of being heated and lifted, and quickly dissipated, as during the day, is rendered cold, and comparatively dense, and lies on the earth a concentrated mass, which it may be death to inspire. Hence the value in such situations of sleeping apartments near the top of a house, or of apartments below, which shut out the night air, and are large enough to