The great objection to this form of stove is that the metal is apt to become overheated, which not only gives rise to accidents, but has a hurtful effect upon the air. The exact nature of the change that highly heated metal produces upon air is not very well under stood. It cannot be said to burn it, in the proper sense of the word, for none of its oxy is abstracted, but it gives it a peculiar odor, which is both unpleasant and unwhole some. This is thought to arise in some measure at least from the hot iron burning the particles of dust that light on it, which particles consist of organic matter, such as wool, wood, etc.
Part at least of the unwholesomeness of air so heated arises from its excessive dry ness; it parches and withers everything it touches, like the African simoom, It must not, however, be supposed that this is peculiar to air heated by contact with metal; air suddenly heated is always unwholesomely dry. This is an importantpoint in regard to the subject of warming, and requires consideration. A cubic foot of air, say at 32°, can contain a certain quantity of moisture and no more; but if heated to 80°, it is capable of coutainingfire times as much, and has thus become thirsty, and drinks up moisture from everything that contains 'any. The heating of air, therefore, does not dry it, in the sense of taking moisture from it, it only renders it greedier of more; and this is equally true whether it is heated by a stove or an open fire. The chief difference is that in the latter case the warming is more gradual, and no part of the air becomes very highly heated; while the air that touches a metal plate near redness is all at once rendered intensely thirsty, and, before its fierceness is tempered by thoroughly mixing with the rest of the atmosphere of the room, must he highly pernicious. But whenever the tem perature within doors is much higher than without, the air is in a too thirsty state, and parches the skin and lungs, unless means be taken to supply the necessary moisture. An evaporating pan or other contrivance is an essential part of warming apparatus; it is specially necessary to attend to this during e. winds, which are generally too dry even at their natural temperature.
All improvements on this simple and rude form of stove aim at avoiding a high heat in the warming surface, and this chiefly by lining the fire-box with brick, and inclosing it in several casings, so as to enlarge the heated surface. In the kind of stove called a cockle, the fire is burned in a small furnace within the inner case, and the air is warmed by circulating between the inner and outer cases. When placed in the apartment or hall to be warmed, the outer casing has perforations about the top for the issue of the warm air. For heating churches and similar buildings, the stove is placed in a separate furnace-room, and the warm air is conveyed to the different parts of the building in pipes or flues, while fresh air is driiwn to the stove through a channel or culvert leading from outside the building to the openings in the outer casing.
The stove invented by Dr. Arnott is upon the same principle of an extensive and
moderately warm heating surface. Under a sense of professional honor, Dr. Aiwa did not take out a patent for his stove; it was therefore made by many furnishing ironmon gers in the metropolis and elsewhere, some of whom took out patents for what they considered as improvements upon it. No fewer than 12 patents were taken out in one year for modifications of this stove, all of which Dr. Arnott considered to be upon false principles. The consequence has been that many Arnott stoves, which had been intro duced into houses, have been given up on account of the inconvenience felt from the species of beat which they generated. It is also, however, to be observed that the stove.
made even upon the most approved principles, requires certain adjuncts and conditions in order to operate healthfully and agreeably.
The accompanying figure represents the Arnott stove in the most improved form given to it by the inventor. We give the description in his own words: " The'complete self-regulating stove may indeed be considered as a close stove, Nall an external case, and certain additions and modifications now to be described. The dotted lines and small let ters mark the internal stove, and the entire lines, the external case or covering. The letters A13CD mark the external case, which prers.its the intense heat of the inner stove, abed, Ire ,a damaging the air of the room, F is the regulating-valve, for admitting the air to feed the fire. It may be placed near the ash-pit door, or wherever more convenient. The letters if mark the fire-brick lining of the fire-box or grate, which prevents such cooling of the ignited mass as might interfere with the steady combustion. II is a hopper, or receptacle with open mouth below, sus pended above the fire like a bell, to hold a sufficient charge of coal for 24 hours or more, which coal always falls down of itself, as that below it in the fire-box is consumed. The hopper may at any time be refilled with coal from above, through the lid, le„ of the hopper, and the other lid, K, of the outer case. These lids are rendered nearly air-tight by sand-joints; that is, by their outer edges or circumference turned down, and made to dip into grooves filled with sand, as at e, e. The burned air or smoke from the fire, M, rises up in the space between the hopper and the inner stove-case, to pass away by the internal flue, x, into the other flue, X, of the outer case. L is the ash-pit under the fire-bars. G is the ash-pit door, which must be carefully fitted to shut in an air-tight manner, by grinding its face or otherwise. M is the coal intensely ignited below where the fresh air maintains combus tion, but colder gradually as it is further up Only the coal in the fire grate below, where the fresh air has access to it through the fire-bars, can be in a state of active com bustion." The self-regulating valve above mentioned is an ingenious contrivance by which the passage for the air is rendered narrower according to the force of the draught..