SECT. II. METHODS OF EXCITING AND COLLECTING HEAT. OF the different methods of exciting heat, the following are the most usual. 1. PRODUCTION OF HEAT BY PERCUSSION, OR COLLISION. This method of producing heat is the simplest, and therefore it is generally made use of in the common purposes of life for obtaining fire. When a piece of hardened steel is struck with a flint, some particles of the metal are scraped away from the mass, and so violent is the heat which follows the stroke, that it melts and vitrifies them. If the fragments of steel are caught upon paper and viewed with a microscope, most of them will be found perfect spherules, and very highly polished. Their sphericity demonstrates that they have been in a fluid state, and the polish upon their surface shows them to be vitrified. This evolution of heat by percussion, seems to be the consequence of a permanent or temporary condensation of the body struck. Mr. Dalton has proved that when air is suddenly condensed, a thermometer surrounded by it rises several degrees. Mr. Mollett was the first who observed that a small piece of linen, rolled up, takes fire when put into the narrow canal in which the lower extremity of a pump for condensing air generally terminates. Mr. Northmoore has inflamed phosphorus by condensed oxygen gas.‡ 2. PRODUCTION OF HEAT BY COMPRESSION OR FRICTION. Heat may likewise be excited by mere friction. Wood rubbed against wood, or against any hard body; metal * Manchester Memoirs, v. 515. Phil. Mag. xiv. 364. These experiments have produced the common tube and piston for firing spunk. rubbed against metal, or against any other body; in short, solid bodies rubbed against each other are thereby heated, often so far as to become red hot. The natives of New Holland are said to produce fire in this manner with great facility, and spread it in a wonderful manner. For that purpose they take two pieces of dry wood, one is a stick about eight or nine inches long, and the other piece is flat; the stick they shape into an obtuse point at one end, and pressing it upon the other piece, they turn it very nimbly by holding it between both hands as we do a chocolate mill, often shifting their hands up and then moving them down upon it, in order to increase the pressure as much as possible. By this method they get fire in a few minutes, and from the smallest spark they increase it with great speed and dexterity. If the irons at the axis of a coach-wheel are applied to each other without the interposition of some unctuous matter to keep them from immediate contact, they will become so hot when the carriage runs swiftly along, as to set the wood on fire; and the fore wheels being smallest and making most revolutions in a given time will be most in danger. The same will happen to mill-work or to any other machinery. It is no uncommon practice in this country for blacksmiths to use a plate of iron as an extemporaneous substitute for a tinder-box; for it may be hammered on an anvil till it becomes red hot, and will fire a brimstone match. A strong man who strikes quick, and keeps turning the iron so that both sides may be equally exposed to the force of the hammer, will perform this in less time than would be expected. If in the coldest season one dense iron plate be laid on another, and pressed together by a weight, and then rubbed upon each other by reciprocal motions, they will gradually grow so hot, as, in a short time, to emit sparks, and at last become ignited. It is not necessary that the substances should be very hard, a cord rubbed backwards and forwards swiftly against a post or a tree will take fire. Count Rumford* and Professor Pictett have made * Nicholson's Journal (4th edit.) vol. ii. p. 106. some very ingenious and valuable experiments concerning the heat evolved by friction. It is asserted, that heat may be produced by friction where there is absolutely no oxygen gas whatever; so that in these cases it cannot be derived from the decomposition of that gas. This has made several persons suspect that heat is not the effect of a peculiar substance, called caloric, but that it is only a peculiar motion or vibration of the particles of bodies. It must, however, be considered, that there is no friction which does not produce compression, viz. a contraction of the bulk of the bodies concerned, at least for a time, and therefore that the caloric is squeezed or forced out of the bodies themselves, and being communicated to the surrounding bodies, produces the usual signs of heat. What very much corroborates this assertion is, that substances which are not compressible, are not heated by mechanical force; thus a flint will only be broken, but a piece of metal will be heated by the strokes of a hammer. Thus, also, we may place any weight upon a quantity of water without altering its temperature, because the compressibility of that fluid is almost nothing; but if we place an additional weight upon a quantity of air, the bulk of that fluid will be contracted, and its temperature will be raised by the compression. 3. PRODUCTION OF HEAT BY CHEMICAL ACTION. To this belongs the heat produced by combustion. There are besides this many chemical processes wherein rapid chemical action takes place accompanied with a development of heat, or fire and flame. Instances of this. kind have been noticed already, page 51. More striking experiments will be given in the progress of this work. It has been remarked, that whenever a mixture of two or more bodies is attended with heat, the bulk of that mixture is less than the sum of the bulk of the separate ingredients, viz. a compression or concentration takes place, which is accompanied with a development of caloric; viz. if 100 grains of alcohol, of 0,825 specific gravity, occupy a space equal to 100, at a temperature of 60° Fahr. and 100 grains of water, at the same temperature, occupy a volume equal to 82,5 the volume of these two fluids, after being mingled and reduced to the original temperature, viz. 60°, will be 177,41; a diminution of bulk equal to 5,09 has therefore taken place. The specific weight of the mixture is 0,93,002. 4. SOLAR HEAT. It is well known that every lens, whether convex or plano-convex, will collect by refraction the rays of the sun dispersed over its surface into one point or focus, and produce the most astonishing effects. Dr. Herschell has discovered, that there are rays emitted from the sun which have not the power of illuminating or producing vision; and that these are the rays which produce the heat of the solar light, besides others which have a deoxydating power. Consequently heat is emitted from the sun in rays; but these rays are not the same with the rays of light, as shall be more fully proved hereafter. The direct rays of the sun on the same spot of the surface of the earth are more or less hot according to the time of the year, clearness of the atmosphere, state of the wind, and the colour, together with the quality of the spot. On this island, and in the hottest time of the summer, the direct rays of the sun seldom raise the thermometer so high as 110°o. In other climates, especially within the tropics, they raise the thermometer considerably higher, viz. 20 or 30, or as it is said, even 40° higher than 110. But we must not believe the idle stories of their melting lead, or even of their setting fire to gunpowder. No sensible heat is known to be derived from any other celestial body. The moon, indeed, on account of the great light it reflects on the earth, might be expected to communicate some degree of heat; but though that light, concentrated by a large concave mirror, has been thrown upon the most sensible thermometers, yet I am not certain that it ever affected them. A great many calculations have been made concerning the proportion between the light which we receive directly from the sun, and that which is reflected to us from the moon ; from which it appears that the latter is several hundred times less dense than the former; and the heat of both is supposed to be in the same proportion. 5. PRODUCTION OF HEAT BY THE ELECTRIC SPARK, AND BY GALVANISM. The effects of electricity are too well known in this point of view to need any description. Galvanism has of late become a powerful instrument for the purpose of exciting heat. Not only easily inflammable substances, such as phosphorus, sulphur, &c. have been fired, but likewise gold, silver, copper, tin, and the rest of the metals, have been burnt by means of galvanism, as was shown by professor Davy in his lectures at the theatre of the Royal Institution. Such are the principal methods of exciting heat. We shall now endeavour to examine experimentally the effects which heat produces upon bodies in general. SECT. III. GENERAL EFFECTS OF HEAT. EXPANSIVE PROPERTY OF HEAT. This is the first and most obvious effect which heat produces on bodies. Experience has taught us, that at all times when bodies become hot they increase in bulk. The bodies experience a dilatation which is greater in proportion to the accumulation of caloric, or, in other words, to the intensity of the heat. This is a general law which holds good as long as the bodies have suffered no change either in their combination or the quantity of their chemical principles. This power which heat possesses consists therefore in a constant tendency to separate the particles of bodies. Hence philosophers consider heat as the repulsive power |