4. The last effect of heat, which yet remains for us to consider, is ignition. Ignition, or, as it is sometimes called, incandescence, is the property which some bodies possess, after being exposed to a high temperature, of becoming luminous, without any chemical change taking place in their composition. In this respect, it differs essentially from combustion, where the body extricates light, but where it is found to have experienced a complete change in its chemical nature. Combustion also differs from ignition in another particular, that, in the former process, the co-ope ration of the atmosphere, or some other external substance, is necessary ; whereas the latter is capable of existing with out the intervention of any other body. It has been con ceived, that, by proper management, all solids, and even li quids, may be rendered luminous, or brought into the state of ignition ; and Mr \Vedgewood performed some experi ments which lead to the conclusion, that they all undergo this change at the same temperature. Many philosophers have attempted to find out the exact degree at which igni tion commences; and although their processes do not furnish precisely the same result, yet they correspond so far, as to render it probable, that it must be somewhere between the 800° or 1000° of Fahrenheit. Mr Wedge•ood supposes, that a body becomes just luminous in the dark at 947°; and that a full red heat, visible in open day-light, takes place at 1077°. According to the intensity of the temperature, the colour of the ignited body is altered. At first, it exhi bits what has been called a cherry red ; afterwards, the red acquires a yellowish tinge; and, lastly, all colour disap pears, and we have only a brilliant white light : (Phil. Trans. 1784, p. 370.) It is generally supposed, that aeriform fluids are not capable of being A quantity of air, heated m to such a degree that a piece of metal, when suspended in it, was rendered luminous, was itself quite invisible.
Respecting the cause of ignition, two opinions have pre vailed ; the first, that light is a modification of heat, and that, at a high temperature, the one becomes converted in to the other; the second, that light actually exists as a com ponent part of different bodies, and that, when the body is strongly heated, the light is propelled or disengaged from it. It is perhaps impossible to adduce any direct arguments in favour of either of these hypotheses ; but the general re lations of heat and light are in many respects so different from each other, that we are induced to adopt the opinion which supposes them to depend upon the agency of princi ples essentially dissimilar. This, however, we advance as a doctrine which must be confirmed or refuted by future observations and experiments.
Sources of Heat.
AFTER having described the properties and effects of After having described the properties and effects of heat, we must now give an account of the sources whence it is derived. Those that are usually enumerated are the following : the rays of the sun,—combustion,—a change in the capacity of bodies, and their chemical union,—the me chanical means of producing heat, as friction, percussion, and condensation,—and, lastly, the electric and galvanic discharge. The great source of heat to our world, and
probably to the rest of our solar system, is the radiant caloric which is projected from the sun. When a suffi cient number of rays are collected into one spot, either by a concave mirror, or by a convex lens, the most powerful heat is excited of which we have any conception; and we are capable of producing effects by it, which we cannot otherwise accomplish. The effect, however, is necessa rily confined to a small spot; and therefore it can only be employed in refined operations of analysis, or foi• the pur pose of delicate philosophical processes. It would appear, from an experiment of Rumford's, that the great heat ex cited in these cases, depends entirely upon the concentra tion of the rays, and not upon any change in their nature ; because, when he directed a certain portion of the sun's rays against a substance adapted for absorbing them, the total amount of heat communicated to it was the same, whether the rays were received on the surface in a diffus ed state; or brought into a small focus.
The most important supply of heat which we have it in our power to produce at pleasure, is that which depends upon combustion. Nearly all the heat which we employ for domestic and manufacturing purposes, proceeds from the burning of fuel ; an operation of a strictly chemical na ture, in which a union takes place between the fuel and the oxygen of the atmosphere, the result of which is a combination of the combustible matter with the oxygen, and the extrication of a quantity of heat and light. It has been a much controverted question, whence does the heat proceed in this operation ? The discussion is strictly chemi cal, and must therefore be dismissed with a few observa tion. A part of the heat liberated probably depends upon the less capacity kf the product of combustion, than of the substance supporting it, i. e. of carbonic acid, than of oxy gen and carbon separately ; but there are instances in which the quantity of heat extricated appears to bear no propor tion to the bulk of the materials consumed, or of carbonic acid formed; and hence we are induced to conjecture, that heat exists as an ingredient in combustible substances, and is disengaged from them in consequence of the changes of composition that take place. It has been supposed, that, besides what necessarily belongs to it as a gas, oxygen contains caloric as one of its constituents, which is liberat ed by a kind of precipitation ; and the phenomena which attend the detonation of gunpowder lead us to conclude, that caloric also enters into the composition of the nitric salts, and others that possess similar properties. Of the heat produced by a change of capacity, in consequence of an alteration in the state of a body, or of new combina tions, we have already spoken at sufficient length in the last Section.