MESSRS. EDITORS—Everybody knows that there is always a current of hot air ascending close to the sides of a heated stove. A friend, who professes to be well posted in the philosophy of such matters, asserts that this hot air passing off rapidly and unobstructedly upon a smooth surface, and mingling with the cooler air of a room, is, comparatively, healthful and innoxious; but that upon coming in contact with, and being delayed (if but for an instant), by an heated iron projection, a chemical change in the particles is produced, rendering it more or less odorous, unwholesome, and unfit for respiration. He contends, therefore, that, having regard to health, the exterior of stoves should be entirely smooth, without horizontal projections of any kind, for ornament or otherwise, and particularly that the top of a stove should not project over its sides. What says the SCIENTIFIC AMERICAN to the above? I. New York, December, 1857. [No chemical change takes place in the particles of air brought into contact with a stove, unless its plates are red hot. Iron at a, red heat decomposes the air brought into contact with it, by the affinity which the oxygen has for the metal in this condition. The oxygen leaves the nitrogen of the air and unites with the iron. However, although burned air, or that which has come in contact with the red hot plates of stoves and heaters, is very unhealthy to inhale, owing to its snper-dryness, we are of opinion that it is not deprived of all its oxygen, as some writers have asserted, for if the quantity of air which is brought into contact with a red hot stove plate in one day were decomposed, that plate would be entirely reduced to an oxyd in that period. Now we know that cast iron cylinder stoves, are maintained at a red heat for months without reducing the metal to an oxyd. In regard to the healthfullness of smooth and rough stoves, there can be no difference ; but the former should be preferred, because they are much easier to keep clean and polish, The profusion of figures, projections, and intended ornaments with which most stoves are disfigured, exhibits a prevailing bad taste in those who get them up.—EDS.