§ 126. There is one curious phenomenon in the crystallization of certain substances, which has not been explained by any of the hypotheses which have yet been framed, and which awaits some higher stage of inductive generalization than that at which we are at present arrived; and that is, they are capable of assuming two forms, which belong to different systems of crystallization, and cannot be derived the one from the other: these have been called dimorphous bodies, and they occur both among simple and compound substances. Sulphur deposited from solution in bi-sulphuret of carbon crystallizes in octohedrons, with rhombic bases, and also occurs in this form in native specimens; but when it is melted, and allowed to cool slowly, till part of the mass is consolidated, and when the remaining liquid is poured off, crystals are obtained in the form of an oblique rhombic prism. These two forms have incompatible geometrical relations to each other. Temperature seems to be the controlling agent of these changes; if the crystal be formed below 232° Fah. it assumes the first form; if above that point, the second. This is proved by the influence of temperature on a crystal of either system; a crystal of fusion, when first formed, is perfectly transparent; but kept at common temperatures it becomes opaque. The same change occurs when a native crystal is put into a solution of salt at 232°. The opacity is in both cases produced by a breaking up of the old crystal and a re-arrangement of the particles in a structure corresponding to the crystallization peculiar to the temperature. Pure carbon occurs also in nature in two states; as diamond, which crystallizes in regular octohedrons; and as graphite, in six-sided plates, which are also incompatible forms. Amongst compound bodies calcareous spar, as we have seen, crystallizes with the rhombohedral structure, and also as arragonite, in forms which have reference to a rectangular octohedron; and iron pyrites is also met with in the common form of the cube, and with an incompatible prismatic structure.

§ 127. There are some solids again, which are capable of slowly changing their internal structure, or of even rapidly passing from one state to another, when the unstable equilibrium of their particles is disturbed by slight mechanical force.

Sugar which has been rapidly boiled down to a solid consistence, in the well-known form of barley-sugar, is perfectly transparent, is difficultly broken, and presents a glassy fracture:

after a little time it becomes opaque, and almost friable. In the same way brass-wire will in the lapse of time become brittle, and unfit for mechanical purposes. If crystals of sulphate of nickel, or seleniate of zinc, in the form of prisms, be laid on paper, and exposed to the rays of the sun, in a few moments they will become opaque, and will be found when broken to be made up of minute octohedrons, with square bases. Similar crystals of sulphate of zinc undergo a like change, but more slowly. Fresh sublimed iodide of mercury also affords a beautiful example of this readjustment of the cohesive force. A slight scratch with a hard substance, as the point of a pin, is sufficient to set the particles of the crystals in motion, and changes their colour from a sulphur-yellow to a bright red.

VII.

HEAT-TEMPERATURE.

§ 128. We have already traced (§ 20) the origin of homogeneous repulsion, the antagonist force to homogeneous attraction, or cohesion, to that cause, whatever it may be, which produces in us the sensation of heat: it is necessary that we should now inquire with more precision into the nature of this subtle agent, and the laws which regulate its action.

And first, with regard to the active sources of its abundant supply: the principal of these is the sun. The most inattentive observer cannot have failed to remark that the direct rays of the sun, some communication that is from that splendid but distant globe, which we designate by rays,-impress him with the sensation of heat; and without this benign influence upon the animate and inanimate creation, all nature would be bound in the adamantine chains of cohesion; and could we imagine man to exist for a moment upon the face of the globe under its privation, vain would be all his puny efforts to remedy the defect from artificial sources of this life-preserving agent. The conflagration of every combustible upon the face of the earth would be insufficient to compensate for twenty-four hours the absence of the glorious orb of day.

§ 129. The second source to which we may advert, we may distinguish as mechanical, and consists in the friction, or rubbing together of solid substances. In this operation strong mechanical force is opposed to the forces of cohesion or adhesion, and heat is generated by the reaction of the two.

From some late experiments of M. Becquerel, it appears that when a rough body is rubbed against another with a smooth surface, the former becomes hotter than the latter. When polished glass is rubbed against cork the first becomes the hotter in the proportion of 34 to 5: ground glass becomes hotter than cork in the proportion of 40 to 7. Silver and cork -the first becomes hotter than the second, in the proportion of 50 to 12. No change in the state of qualities of bodies, thus acting upon each other, necessarily accompanies the evolution of heat; neither is it necessary that they should be of different natures, or possessed of any opposition of qualities. Many familiar instances of high degrees of heat, derived from this source, will readily occur to a reflecting mind.

Two pieces of wood, rubbed together, speedily become hot; and when the force and the velocity are great so much heat may be evolved as to occasion combustion. The axles of the wheels of machinery, or of carriages, not unfrequently take fire from this cause; and it is well known that certain tribes of Indians, with a dexterity and address which we do not possess, have so far applied observation to practical purposes, that they are in the habit of kindling their fires by this means.

The sparks of the common flint and steel consist of small particles of the metal struck off by the stone, and burning under the influence of the heat elicited by the blow; and every one has had an opportunity of observing the more copious display of fire from the same source at the knife-grinder's wheel.

In North America, where the mechanical force of falling water is abundant, the surplus power of certain mills has been most ingeniously turned to good account, by causing large plates of iron to rub against each other, and applying the heat thus evolved to the warming of the building. The most remarkable circumstance attending this source of heat is, that it is absolutely unlimited: so long as solid bodies can be made thus to act mechanically upon each other, heat is given out.

Count Rumford endeavoured to ascertain how much heat was actually generated by friction. When a blunt steel borer, three inches and a half in diameter, was driven against the bottom of a brass cannon, seven inches and a half in diameter, with a pressure which was equal to the weight of 10,000lbs., and made to revolve thirty-two times in a minute, in forty-one minutes 837 grains of dust were produced; and the heat gene

rated was sufficient to raise 113lbs. of the metal 70° of Fah.,a quantity of heat which is capable of melting six pounds and a half of ice, or of raising five pounds of water from the freezing to the boiling-point. When the experiment was repeated under water, two gallons and a half water, at 60°, were made to boil in two hours and a half.

§ 130. The third source is chemical, or the action of particles of dissimilar or heterogeous substances upon each other, accompanied by complete change of properties. All cases of common combustion, all our artificial processes for obtaining light and heat, are familiar examples of this action. The changes which take place in the active bodies during this evolution of heat, it will be a principal part of our business to inquire into it hereafter: we have only at present to remark, that the heat evolved in each case, however copious and intense, is limited and proportionate to the quantities of the reacting substances.

§ 131. The fourth source of heat, but probably nearly allied to the last, is electrical. That force which we have recognised as arising from the friction of glass or resins, by operating upon larger surfaces, may be accumulated in the form of a spark, which will ignite combustible substances. This action we must also refer to future investigation.

§ 132. A fifth source is physiological, and exists in ourselves. Heat is a product of animal life: we are conscious of this in ourselves—we can feel it, and we can judge of it by our sensations, when communicated from other persons and other animals; we can increase it in ourselves by muscular exertion; our hands and our faces burn, and in this state we can communicate the sensation of heat to others.

Heat and cold, in fact, when referred to our sensations, are but comparative terms, and depend upon the temperature of our bodies at the time of experience. If we raise the temperature of one hand by plunging it in hot water, and at the same time cool the other in cold water, upon removing both into a basin of tepid water, the same water will feel warm to one hand and cold to the other. Any estimation, therefore, of temperature by sensation must necessarily be extremely

vague.

§ 133. Now, in none of these various actions of material bodies, which we have thus recognised as sources of heat, is there any loss of material substance. Notwithstanding the copious evolution of this wonderful agent, the quantity of the active matter-its weight-remains the same, whether combination ensue or not.

This something, then, which is generated in ourselves, or which may be communicated to us from any of the sources which have just been named,-which we recognise by our sensations, and denominate heat, may be communicated also to every form of matter without increase of weight; and these can again impart it to us without loss of ponderable matter.

Thus a piece of iron placed in the direct rays of the sun, will soon feel warm to our hands; or when made red hot in the fire, will communicate heat to us even at a distance; but its quantity of ponderable matter will be neither increased nor diminished.

The accession of heat, and consequent increase of repulsive force amongst the homogeneous particles of matter, not only determines the changes of state from solid to liquid, and from liquid to aëriform, which we have already noticed (§ 63), but produces expansion of bulk in bodies in all these states; and we must now inquire into the laws of this expansion, and the steps of the progress of a body from one physical state to

another.

§ 134. The increase of bulk, for the same increase of heat, differs very much in the different classes of substances. In solids it is small; in liquids greater; and in aëriform bodies the greatest of all. It also differs in bodies of the same class. By accurate measurement it has been ascertained that in raising the temperature of the following solids from the freezing to the boiling-point of water,

that is to say, 350 cubic inches of Lead become.

351

The subjoined table includes the most interesting results of

the best experiments upon the subject.