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of electricity discharged, and is due to its excessive velocity and the violent compression of the air in its transit through the atmosphere. Prodigious quantities of heat are constantly absorbed or disengaged by the changes to which substances are liable in passing from the solid to the liquid and from the liquid to the gaseous form and the contrary, causing endless vicissitudes of temperature over the globe, and endless expansions and contractions, which are correlative terms for heat and cold, while radiation of heat is merely a transfer of motion from the particles on the surface of bodies to the adjacent particles of the atmosphere.
By the continual application of heat, that is of the expansive force, liquids are converted into steam or vapour, which is invisible and highly elastic. Under the mean pressure of the atmosphere, that is when the barometer stands at 30 inches, water in a boiler absorbs heat continually till it attains the temperature of the boiling point, which is 212° Fahrenheit. After that it ceases to show any increase of sensible heat; but when it has absorbed an additional 1000° of heat or expansive energy, that energy converts it into steam, and a condensing force equivalent to 1000° of heat reduces it again to water. Water boils at different temperatures under different degrees of pressure. It boils at a lower temperature on the top of a mountain than on the plain below, because the weight of the atmosphere is less at the higher station. There is no limit to the temperature to which water might be raised: it might even be made red hot, could a vessel be found strong enough to resist the pressure, for the intensity of the expansive force prevented from having effect by the extreme pressure of the boiler would be converted into sensible heat which might eventually render the water red hot. Thus, since the force of steam is in proportion to the temperature at which the water boils, or to the pressure, it is under control, and, perhaps with the exception of electricity, it is the greatest power that has been made subservient to the wants of man.
It is found that the absolute quantity of heat consumed in the process of converting water into steam is the same at whatever temperature water may boil, but that the absolute heat of the steam is greater exactly in proportion as its sensible heat is less. Thus, steam raised at 212° Fahrenheit under the mean
pressure of the atmosphere, and steam raised at 180° under half the pressure, contain the same quantity of heat, with this difference, that the one has more absorbed heat and less sensible heat than the other. It is evident that, as the same quantity of heat is requisite for converting a given weight of water into steam, at whatever temperature or under whatever pressure the water may be boiled, therefore, in the steam engine, equal weights of steam at a high pressure and a low pressure are produced by the same quantity of fuel; and whatever the pressure of the steam may be, the consumption of fuel is proportional to the quantity of water converted into vapour. Steam of whatever tension expands on being set free, but the expansion of high pressure steam at the expense of its sensible heat is so great, that the hand may be plunged into it without injury the instant it issues from the orifice of a boiler. The steam becomes hotter by friction in issuing through the orifice which maintains it in its dry form, for there is no doubt that high-pressure steam is dry.
The elasticity or tension of steam, like that of common air, varies inversely as its volume—that is, when the space it occupies is doubled, its elastic force is reduced to one half. The expansion of steam is indefinite; the smallest quantity of water expanded into vapour will occupy many millions of cubic feet; a wonderful illustration of the minuteness of the ultimate particles of matter.
The force of steam, tremendous as the lightning itself when uncontrolled, is merely the result of chemical affinity: it is the chemical attraction between the particles of carbon, of coal or wood, and the oxygen of the atmosphere. Mr. Joule has ascertained that a pound of the best coal when burnt gives sufficient heat to raise the temperature of 8086 pounds of water one degree of the Centigrade thermometer, whence it has been computed by M. Helmholtz that the chemical force arising from the combustion of that pound of coal is capable of lifting a body of one hundred pounds weight to the height of twenty miles. That is the work performed by the heat arising from the combustion of a pound of coal. In all cases where work is produced by heat, a quantity of heat proportional to the work done is expended; and conversely, by the expenditure of a like quantity of work, the same amount of heat may be produced. The equivalence of heat and work is a law of nature. The mechanical force exerted by
the steam engine for example is exactly proportional to the consumption of heat, nor more nor less; if we could produce a greater quantity than its equivalent we should have perpetual motion, which is impossible. Mechanical engines generate no force. We cannot create force; we can only avail ourselves of the inexhaustible stores of nature, the lightning, fire, water, wind, chemical action, &c. The quantity of mechanical power in nature is ever the same; it is never increased, it is never diminished, throughout the whole circuit of natural powers. The conservation of force is as permanent and unchangeable as matter. It may be dormant for a time, but it ever exists. We are unconscious of the enormous dynamic power that is either active or latent throughout the globe, because we do not attend to it. By the ebb and flow of the tide alone a power is exerted by which 25,000 cubic miles of water is moved over a quarter of the globe every twelve hours; and Professor W. Thomson has computed, by means of Pouillet's data of solar radiation and Mr. Joule's mechanical equivalent of heat, that the mechanical value of the whole energy active and potential of the disturbances kept up in the ethereal medium by the vibrations of the solar light within a cubic mile of our atmosphere is equal to 12,050 times the unit of mechanical force, that is to say, 12,050 times the force that would raise a pound of matter to the height of one foot, whence some idea may be formed of the vast amount of force exerted by the sun's light within the limits of the whole terrestrial atmosphere. (N. 223.)
The dynamic energy of the undulations of the solar light gives the leaves of plants the power of decomposing carbonic acid, and of separating the particles of carbon and hydrogen from the oxygen for which they have so strong an affinity. In this operation the undulations of the sunbeam are extinguished as light and heat, and Professor W. Thomson has proved that the quantity of these undulations thus extinguished is precisely equal to the potential or quiescent energy thus created, and that precisely that very quantity of light and heat is restored when the plants are burned, whatever state they may be in; and that thus, as Mr. George Stephenson* has truly and beautifully observed, our coal fires
*To this remarkable man the world is indebted for the locomotive railway system, which is rapidly advancing the civilization of mankind. Britain may well be proud of its working classes, which can produce such men; and Mr. George Stephenson is not the only one; there are many others; but no man has ever had greater influence by his labours and discoveries on human affairs.
and gas lamps restore to our use the light and heat of the sun of the early geological epochs which have rested as dormant powers under the seas and mountains for unnumbered ages. The sun
is therefore the source of the mechanical energy of all the heat and motion of inanimate things, of all the motions of the heat and light of fires and artificial flames, and of the heat of all living creatures. For animal heat, and weights raised or resistance overcome, are mechanical effects of the chemical combination of food with oxygen; and food is either directly or indirectly vegetable, consequently dependent upon the sun.
Professor Helmholtz of Bonn has put in a strong point of view the enormous store of force possessed by our system by comparing it with its equivalent of heat. The force with which the earth moves in its orbit is such, that if brought to rest by a sudden shock, a quantity of heat would be generated by the blow equal to that produced by the combustion of fourteen such earths of solid coal; and supposing the capacity of the earth for heat as low as that of water, the globe would be heated to 11,200° Cent. It would be quite fused and for the most part reduced to vapour. If it should fall to the sun, which it would certainly do, the quantity of heat developed by the shock would be four hundred times as great.
The application of heat to the various branches of the mechanical and chemical arts has within the present century effected a greater change in the condition of man than had been accomplished in any equal period of his existence. Armed by the expansion and condensation of fluids with a power equal to that of the lightning itself, conquering time and space, he flies over plains, and travels on paths cut by human industry even through mountains with a velocity and smoothness more like planetary than terrestrial motion; he crosses the deep in opposition to wind and tide; by releasing the strain on the cable, he rides at anchor fearless of the storm; he makes the lightning his messenger; and like a magician he raises from the gloomy abyss of the mine the sunbeam of former ages to dispel the midnight darkness.
The principal phenomena of heat may be illustrated by a comparison with those of sound. Their excitation is not only similar but identical, as in friction and percussion; they are both communicated by contact and radiation; and Dr. Young observes that the effect of radiant heat in raising the temperature of a
body upon which it falls, resembles the sympathetic agitation of a string when the sound of another string which is in unison with it is transmitted through the air. Light, heat, sound, and the waves of fluids are all subject to the same laws; their undulatory theories are perfectly similar: hence the interference of two hot rays must produce cold, that is, they must extinguish one another: darkness results from the interference of two undulations of light, silence ensues from the interference of two undulations of sound, and still water or no tide is the consequence of the interference of two tides. The propagation of sound, however, requires a much denser medium than that of light and heat; its intensity diminishes as the rarity of the air increases: so that, at a very small height above the surface of the earth, the noise of the tempest ceases, and the thunder is heard no more in those boundless regions where the heavenly bodies accomplish their periods in eternal and sublime silence.
A consciousness of the fallacy of our senses is one of the most important consequences of the study of nature. This study teaches us that no object is seen by us in its true place, owing to aberration; that the colours of substances are solely the effects of the action of matter upon light; and that light itself as well as heat and sound are not real beings, but mere motions communicated to our perceptions by the nerves. The human frame may therefore be regarded as an elastic system, the different parts of which are capable of receiving the tremors of elastic media, and of vibrating in unison with any number of superimposed undulations, all of which have their perfect and independent effect. Here our knowledge ends: the mysterious influence of matter on mind will in all probability be for ever hid from man.