Part 27 (1/2)
[Sidenote: CONNEXION OF NATURAL FORCES.]
Great scientific principles, though usually announced by individuals, are often merely the distinct expression of thoughts and convictions which had long been entertained by all advanced investigators. Thus the more profound philosophic thinkers had long suspected a certain equivalence and connexion between the various forces of nature; experiment had shown the direct connexion and mutual convertibility of many of them, and the spiritual insight, which, in the case of the true experimenter, always surrounds and often precedes the work of his hands, revealed more or less plainly that natural forces either had a common root, or that they formed a circle, whose links were so connected that by starting from any one of them we could go through the circuit, and arrive at the point from which we set out. For the last eighteen years this subject has occupied the attention of some of the ablest natural philosophers, both in this country and on the Continent. The connexion, however, which has most occupied their minds is that between _heat_ and _work_; the absolute numerical equivalence of the two having, I believe, been first announced by a German physician named Mayer, and experimentally proved in this country by Mr. Joule.
[Sidenote: MECHANICAL EQUIVALENT OF HEAT.]
A lead bullet may be made hot enough to burn the hand, by striking it with a hammer, or by rubbing it against a board; a clever blacksmith can make a nail red-hot by hammering it; Count Rumford boiled water by the heat developed in the boring of cannon, and inferred from the experiment that heat was not what it was generally supposed to be, an imponderable fluid, but a kind of motion generated by the friction. Now Mr. Joule's experiments enable us to state the exact amount of heat which a definite expenditure of mechanical force can originate. I say _originate_, not drag from any hiding-place in which it had concealed itself, but actually bring into existence, so that the total amount of heat in the universe is thereby augmented. If a ma.s.s of iron fall from a tower 770 feet in height, we can state the precise amount of heat developed by its collision with the earth. Supposing all the heat thus generated to be concentrated in the iron itself, its temperature would thereby be raised nearly 10 Fahr. Gravity in this case has expended a certain amount of force in pulling the iron to the earth, and this force is the _mechanical equivalent_ of the heat generated. Furthermore, if we had a machine so perfect as to enable us to apply all the heat thus produced to the raising of a weight, we should be able, by it, to lift the ma.s.s of iron to the precise point from which it fell.
But the heat cannot lift the weight and still continue heat; this is the peculiarity of the modern view of the matter. The heat is consumed, used up, it is no longer heat; but instead of it we have a certain amount of gravitating force stored up, which is ready to act again, and to regenerate the heat when the weight is let loose. In fact, when the falling weight is stopped by the earth, the motion of its ma.s.s is converted into a motion of its molecules; when the weight is lifted by heat, molecular motion is converted into ordinary mechanical motion, but for every portion of either of them brought into existence an equivalent portion of the other must be consumed.
What is true for ma.s.ses is also true for atoms. As the earth and the piece of iron mutually attract each other, and produce heat by their collision, so the carbon of a burning candle and the oxygen of the surrounding air mutually attract each other; they rush together, and on collision the arrested motion becomes heat. In the former case we have the conversion of gravity into heat, in the latter the conversion of chemical affinity into heat; but in each case the process consists in the generation of motion by attraction, and the subsequent change of that motion into motion of another kind. Mechanically considered, the attraction of the atoms and its results is precisely the same as the attraction of the earth and weight and _its_ results.
[Sidenote: HEAT PRODUCED IF THE EARTH STRUCK THE SUN.]
But what is true for an atom is also true for a planet or a sun.
Supposing our earth to be brought to rest in her orbit by a sudden shock, we are able to state the exact amount of heat which would be thereby generated. The consequence of the earth's being thus brought to rest would be that it would fall into the sun, and the amount of heat which would be generated by this second collision is also calculable.
Helmholtz has calculated that in the former case the heat generated would be equal to that produced by the combustion of fourteen earths of solid coal, and in the latter case the amount would be 400 times greater.
[Sidenote: s.h.i.+FTING OF ATOMS.]
Whenever a weight is lifted by a steam-engine in opposition to the force of gravity an amount of heat is consumed equivalent to the work done; and whenever the molecules of a body are s.h.i.+fted in opposition to their mutual attractions work is also performed, and an equivalent amount of heat is consumed. Indeed the amount of work done in the s.h.i.+fting of the molecules of a body by heat, when expressed in ordinary mechanical work, is perfectly enormous. The lifting of a heavy weight to the height of 1000 feet may be as nothing compared with the s.h.i.+fting of the atoms of a body by an amount so small that our finest means of measurement hardly enable us to determine it. Different bodies give heat different degrees of trouble, if I may use the term, in s.h.i.+fting their atoms and putting them in new places. Iron gives more trouble than lead; and water gives far more trouble than either. The heat expended in this molecular work is lost as heat; it does not show itself as temperature. Suppose the heat produced by the combustion of an ounce of candle to be concentrated in a pound of iron, a certain portion of that heat would go to perform the molecular work to which I have referred, and the remainder would be expended in raising the temperature of the body; and if the same amount of heat were communicated to a pound of iron and to a pound of lead, the balance in favour of temperature would be greater in the latter case than in the former, because the heat would have less molecular work to do; the lead would become more heated than the iron. To raise a pound of iron a certain number of degrees in temperature would, in fact, require more than three times the absolute quant.i.ty of heat which would be required to raise a pound of lead the same number of degrees.
Conversely, if we place the pound of iron and the pound of lead, heated to the same temperature, into ice, we shall find that the quant.i.ty of ice melted by the iron will be more than three times that melted by the lead. In fact, the greater amount of molecular work invested in the iron now comes into play, the atoms again obey their own powerful forces, and an amount of heat corresponding to the energy of these forces is generated.
This molecular work is that which has usually been called _specific heat_, or _capacity for heat_. According to the _materialistic_ view of heat, bodies are figured as sponges, and heat as a kind of fluid absorbed by them, different bodies possessing different powers of absorption. According to the _dynamic_ view, as already explained, heat is regarded as a motion, and capacity for heat indicates the quant.i.ty of that motion consumed in internal changes.
The greatest of these changes occurs when a body pa.s.ses from one state of aggregation to another, from the solid to the liquid, or from the liquid to the aeriform state; and the quant.i.ty of heat required for such changes is often enormous. To convert a pound of ice at 32 Fahr. into water _at the same temperature_ would require an amount of heat competent, if applied as mechanical force, to lift the same pound of ice to a height of 110,000 feet; it would raise a ton of ice nearly 50 feet, or it would lift between 49 and 50 tons to a height of one foot above the earth's surface. To convert a pound of water at 212 into a pound of steam at the same temperature would require an amount of heat which would perform nearly seven times the amount of mechanical work just mentioned.
[Sidenote: HEAT CONSUMED IN MOLECULAR WORK.]
This heat is entirely expended in _interior work_,[A] and does nothing towards augmenting the temperature; the water is at the temperature of the ice which produced it, both are 32; and the steam is at the temperature of the water which produced it, both are 212. The whole of the heat is consumed in producing the change of aggregation; I say ”_consumed_,” not hidden or ”latent” in either the water or the steam, but absolutely non-existent as heat. The molecular forces, however, which the heat has sacrificed itself to overcome are able to reproduce it; the water in freezing and the steam in condensing give out the exact amount of heat which they consumed when the change of aggregation was in the opposite direction.
At a temperature of several degrees below its freezing point ice is much harder than at 32. I have more than once cooled a sphere of the substance in a bath of solid carbonic acid and ether to a temperature of 100 below the freezing point. During the time of cooling the ice crackled audibly from its contraction, and afterwards it quite resisted the edge of a knife; while at 32 it may be cut or crushed with extreme facility. The cold sphere was subjected to pressure; it broke with the detonation of a vitreous body, and was taken from the press a white opaque powder; which, on being subsequently raised to 32 and again compressed, was converted into a pellucid slab of ice.
[Sidenote: ICE NEAR THE MELTING POINT.]
But before the temperature of 32 is quite attained, ice gives evidence of a loosening of its crystalline texture. Indeed the unsoundness of ice at and near its melting point has been long known. Sir John Leslie, for example, states that ice at 32 is _friable_; and every skater knows how rotten ice becomes before it thaws. M. Person has further shown that the latent heat of ice, that is to say, the quant.i.ty of heat necessary for its liquefaction, is not quite expressed by the quant.i.ty consumed in reducing ice at 32 to the liquid state. The heat begins to be rendered latent, or in other words the change of aggregation commences, a little before the substance reaches 32,--a conclusion which is ill.u.s.trated and confirmed by the deportment of melting ice under pressure.
[Sidenote: ROTTEN ICE AND SOFTENED WAX.]
In reference to the above result Professor Forbes writes as follows:--”I have now to refer to a fact ... established by a French experimenter, M.
Person, who appears not to have had even remotely in his mind the theory of glaciers, when he announced the following facts, viz.--'That ice does not pa.s.s abruptly from the solid to the fluid state; that it begins to _soften_ at a temperature of 2 Centigrade below its thawing point; that, consequently, between 28 4' and 32 of Fahr. ice is actually pa.s.sing through various degrees of plasticity within narrower limits, but in the same manner that wax, for example, softens before it melts.'”
The ”_softening_” here referred to is the ”friability,” of Sir J.
Leslie, and what I have called a ”loosening of the texture.” Let us suppose the Serpentine covered by a sheet of pitch so smooth and hard as to enable a skater to glide over it; and which is afterwards gradually warmed until it begins to bend under his weight, and finally lets him through. A comparison of this deportment with that of a sheet of ice under the same circ.u.mstances enables us to decide whether ice ”pa.s.ses through various degrees of plasticity in the same manner as wax softens before it melts.” M. Person concerned himself solely with the heat absorbed, and no doubt in both wax and ice that heat is expended in ”interior work.” In the one case, however, the body is so const.i.tuted that the absorbed heat is expended in rendering the substance viscous; and the question simply is, whether the heat absorbed by the ice gives its molecules a freedom of play which would ent.i.tle it also to be called viscous; whether, in short, ”rotten ice” and softened wax present the same physical qualities?