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When a substance takes fire in the atmosphere without being exposed to intense or high heat, the action is called " spontaneous combustion." Phosphorus is the only common substance which is subject, under all ordinary drevmstances, to this action. The fuels (wood and coal) employed to produce artificial heat, require exposure to a high temperature before they will burn, hence they are not subject to spontaneous combustion; that is, they will not take fire of themselves under ordinary circumstances. Were it otherwise, there would be no safety for the " dwellings of men," and it would be impossible to conduct any kind of manufacturing operations requiring fuel. Although these statements are positive facts, yet it is also true that disastrous fires sometimes do take place under such peculiar circumstances that no other theory of explanation as to their cause is left but that of " spontaneous combustion." We have a letter now before us, received fran F. Dunworth, of Dobbs' Ferry, N. Y., in which he relates two rather singular cases of this character, known personally to himself. One of these took place in the Britannia Metal Works of Jamos Dixon, Birmingham, England, and the other in an establishment in the same place, where the manufacture of German silverware was carried on. In the first manufactory, rottenstone in fine powder, rubbed up between the hands with oil, was ased for polishing the metal. A quantity of this, wrapped up in paper, was laid upon one of the iron beams in the shop by one of the workmen, j ust before quitting work in the evening. On his arrival next morning, he found it, to his surprise, in an incandescent state, glowing like molten brass—it had taken fire spontaneously. This circumstance threw light on the cause of a fire which had consumed a former factory of Mr. Dixon, and which had been considered the work of an unknown incendiary. In the German silverware establishment, lime in fine powder mixed with oil, like the rottonstone, was employed for polishing. A quantity of this was left one evening on a bench, as it had often been left before, and no thought of danger entertained. Next morning, howover, when the first workman arrived and opened the door of the shop, he was driven back, for a few minutes, by dense fumes rushing out; and when enabled to enter, his surprise was great to behold the prepared lime on fire, and luminous as molten metal in a crucible. As neither rottenstone nor slacked lime are combustible substances, they certainly could not have taken fire of themselves in the foregoing cases. The cause of spontaneous combustion in both of these instances was the oil spread thinly over very extended surfaces when mixed with the powders. Various fires have taken place spontaneously, by oil being mixed with cotton waste in factories ; but as cotton'is very combustible in itself, not so much surprise is excited by such instances, in comparison with combustion produced in lime and rottenstone. Oil has a great affinity for the oxygen of the atmosphere when spread minutely over an extensive surface. During the action of absorbing the oxygen, considerable heat is generated, which, if not conducted away, owing to confinement in a somewhat warm place, is liable to become so concentrated as to produce intense, or " spontaneous combustion." The oil does not take fire spontaneously, like phosphorus ; it is only liable to take fire spontaneously under certain circumstances, such as those related. On ihis very account, there is, perhaps, a greater necessity to be oautious and watohful in its use, as persons are apt to forget that it m——y tako fire. Any substance in a finely subdivided state which contains oil should never be left in an insecure place. As charcoal dust rubbed with oil is sometimes amployed to polish metal, it is as liable to spontaneous combustion as cotton waste. Great care should also be exercised iu preparing charcoal dust for other purposes, not. to allow oil to get amongst it, because of the danger stated. Bituminous coals in the holds of ships arc liable to spontaneous combustion under certain conditions, but not anthraeite coal or coke. There is a groat quantity of oil in rich bituminous coal, and this may he tho miiin cause of the ooals' liability to tako fire spontaneously. This oil is distilled at a comparatively low temperature; and if there is iron pyrites in it, a little moisture finding access will unite with the sulphur, and generate sufficient heat to decompose tho oil, which, as it is rather volatile, and has a great affinity for oxygen, may ultimately engender sufficient heat to produce intense combustion. This may bo the process by which spontaneous combustion takes place with bituminous coal in tho holds of ships; but be this explanation correct or not, a sure and accessible means of detecting incipient combustion in tho holds of ships is much needed, because a few buekets-ful of water, in the early stages of any fire, will prevent a conflagration. The late case of the burning of tho Sarah Sands—an iron steamer, well known in this city—by spontaneous combustion of coal while on a voyage to India with troops, has called forth a method of detecting such fires in an early stage, by Dr. Hay, the Admiralty Chemist, at Portsmouth, England. It consists of a small thin copper cylinder, like the air chamber of a water ram or force pump, capable of containing a quart of air, placed in tho coal bunker, and connected with a small iron gas pipe, bent down like a syphon, then carried to any suitable place up into the cabin. This pipe is terminated with a glass tube attached to a graduated scale, which tube is filled to zero with a solution of soda and water tinted with litmus. Tho slightest rise of temperature in the air of the metal chamber in the coal bunker will show a material rise of the fluid in the tube of the indicator, and thus the officer on duty can at onco detect when the temperature below is increased from any cause, and take prompt measures against danger. Such an apparatus is adapted for being placed in the holds of vessels carrying cotton, which sometimes take iiro, and it should be applied to the store-rooms and holds of all ships, as a protective indicator.