gible than them. It must, therefore, consist of longer oscillations than those which we shall see constitute red light, and which are one forty thousandth of an inch in length. Such undulations at a certain intensity will expand the ponderable matter of bodies which they actuate, so as to impair and finally to subdue its cohesive force; whence the phenomena of softness, brittleness, fluidity, and vaporisation will arise. All the undulations included between the extremes of one sixty thousandth of an inch for the violet rays, and one forty thousandth of an inch for the red, are visible, that is, they are capable of exciting vibrations in the optic nerve; those which are less than one sixty thousandth can excite chemical effects, and those which are greater than one forty thousandth can excite heating effects, though they cause no sensation of light in the human eye.

It is probable, says Dr. Young, that light and heat occur to us, each in two predicaments, the vibratory or permanent, and the undulatory or tran sient state; vibratory light being the minute motion of ignited bodies, or of solar phosphori, and undulatory or radiant light, the motion of the ethereal medium excited by these vibrations. Vibratory heat is a motion to which all material substances are liable, and which is more or less permanent; and undulatory heat is that motion of the same ethereal medium which has been shown by Pictet and Herschel to be capable of reflection and separate refraction, like common light. Newton entertained the same sentiments. He regarded heat as

consisting in a minute vibratory motion of the particles of bodies; a motion communicable through an apparent vacuum, by the undulations of an elastic medium, which is also concerned in the phenomena of light. Such vibrations may be excited among the molecules of bodies, by percussion, friction, and the internal motions of matter which accompany, and probably constitute, its chemical changes. But the particles of fluids which cannot be heated by the most violent mechanical friction or percussion, seem to possess hardly any power of imparting heat to one another; showing apparently some analogy between the communication of heat, and its mechanical excitation.

The syllogistic division of the sunbeam into three orders of particles, the calorific, colorific, and the chemical, is incompatible with the beautiful optical experiment of M. Arago, related at the end of the next chapter, where we shall see that by doubling the rays of light, their usual chemical effect on muriate of silver becomes null. Surely our chemical compilers who introduced that verbal distinction, will not venture to maintain that chemical rays, added to chemical rays, produce unchemical rays; which is, however, the necessary consequence of their hypothesis.

That heat consists in such vibrations, seems to be demonstrated by a fine experiment made long ago, by Sir H. Davy; in which two pieces of ice were converted into water, by their mutual attrition, in an atmosphere at the freezing temperature. Now, since the specific heat of water is much supe

UNDULATORY THEORY PROVED.

5

rior to that of ice, no such cavil can be offered to that experiment, as was urged (however unjustly) against Count Rumford's similar deduction from the heat evolved in the boring of cannon.

Mont

golfier furnished a fact very favourable to-the undulatory theory, when he showed that a piece of ordnance, whether old or new, becomes much more speedily heated by discharges, than if it were filled with burning coals, for ten times the period of the explosions of the gun-powder. The intense and durable ignition of a platinum wire, placed under the electric influence, seems explicable also only on the same principles.

In Sir H. Davy's experiment, since the heat requisite for converting ice into water, could not be derived from the surrounding cold medium, nor from the body itself, whose capacity is low, there is no alternative, but to conclude, that heat must be actually generated by friction; and since it was in that case generated out of nothing, it cannot be any thing material; nor even an entity, immaterial or semi-material, if such language may be used. It must, therefore, be a quality; and this quality can only be motion; a deduction quite conformable to the sentiment of Newton. We may hence understand why both heat and light come to possess analogies with sound. Thus a magnetic steel bar set a ringing for some time, will be deprived of its magnetism as perfectly as if it had been heated red hot; and a charged electrical jar may be discharged equally by heat and by causing it to sound like a musical glass.

The great physico-geometers of France, MM. Fresnel, Poisson, Arago, and Fourier, now contemplate light and heat as modifications of the same power. A skilful mathematical analysis has satisfied them, that the equations of the propagation of heat in solid bodies may be conciliated, with the equations of the undulatory movements of an eminently elastic fluid. The propagation of the temperature of the molecules of a body cannot indeed be assimilated to a fluid current, or to the regular propagation of waves in an elastic medium. These molecules acquire, on the contrary, their proper temperature, that is to say, vibrations which continue after the passage of the calorific waves, only in virtue of the portion of their movement which is not regularly propagated.

When the brute quiescent mass is pervaded by this vibratory motion, its particles necessarily renounce their contact, and being at liberty to move through a greater or less space, assume such forms as the equilibrium of the attractive and calorific power demands. Fluidity, or absolute incoherence of the particles, is not indispensable for their changing the position of their attractive poles, and being grouped in new arrangements. Thus, if a mass of basalt be exposed to a high temperature, it will melt into a liquid glass, which quickly cooled remains a transparent and uniform vitreous body. Now if this body be heated again for some time, but so moderately as not even to have its substance softened, it will become throughout its whole interior a congeries of regular crystals.

ORIGINAL SOLIDITY OF THE GLOBE.

7

When first the calorific energy was made to actuate the body of the earth a mighty change would ensue. The central mass composed, most probably, of the metallic bases of the earths and alkalis, as volcanic phenomena seem to attest, would fuse, the exterior parts would oxidize into the crust of mineral strata, and the outermost coat of all, the fixed ice, would melt into the moveable waters.

The infusion of this quickening energy, seems distinctly indicated by the inspired historian of the earth. "In the beginning, God created the heaven and the earth. And the earth was without form and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters." This last idea, has been, perhaps, more truly rendered by Milton, in the expression, "dove-like sate brooding on the vast abyss, and made it pregnant." In this sublime conception, thus finely paraphrased, may we not recognise the impregnation of the torpid sphere, with elementary fire, that principle of all material activity; that power which loosens the bands of primordial cohesion, and communicates the essence of plastic mobility to a refractory solid? But for this marvellous constitution, as displayed especially in water, the face of nature would have for ever exhibited a death-like silence and a dread repose. The globe would have been cased in an unchanging and waveless ocean crust.

Many theorists have supposed the pre-existence or pre-creation of a chaos, of which the actual sun and planetary globes, were afterwards formed, either