Part 17 (2/2)

FOOTNOTES:

[A] The vibrations of the air of a room in which a musical instrument is sounded may be made manifest by the way in which fine sand arranges itself upon a thin stretched membrane over which it is strewn; and indeed Savart has thus rendered visible the vibrations of the tympanum itself. Every trace of sand was swept from a paper drum held in the clock-tower of Westminster when the Great Bell was sounded. Another way of showing the propagation of aerial pulses is to insert a small gas jet into a vertical gla.s.s tube about a foot in length, in which the flame may be caused to burn tranquilly. On pitching the voice to the note of an open tube a foot long, the little flame quivers, stretches itself, and responds by producing a clear melodious note of the same pitch as that which excited it. The flame will continue its song for hours without intermission.

[B] I am not aware whether in his own country, or in any other, a recognition at all commensurate with the value of the performance has followed Schwerd's admirable essay ent.i.tled 'The Phenomena of Diffraction deduced from the Theory of Undulation.'

[C] I think, however, that the strong irradiation from the glistening sides of the twigs and branches must also contribute to the result.

[Sidenote: RADIANT HEAT.]

(2.)

Thus, then, we have been led from Sound to Light, and light now in its turn will lead us to _Radiant Heat_; for in the order in which they are here mentioned the conviction arose that they are all three different kinds of motion. It has been said that the beams of the sun consist of rays of different colours, but this is not a complete statement of the case. The sun emits a mult.i.tude of rays which are perfectly non-luminous; and the same is true, in a still greater degree, of our artificial sources of illumination. Measured by the quant.i.ty of heat which they produce, 90 per cent. of the rays emanating from a flame of oil are obscure; while 99 out of every 100 of those which emanate from an alcohol flame are of the same description.[A]

[Sidenote: OBSCURE RAYS.]

In fact, the visible solar spectrum simply embraces an interval of rays of which the eye is formed to take cognizance, but it by no means marks the limits of solar action. Beyond the violet end of the spectrum we have obscure rays capable of producing chemical changes, and beyond the red we have rays possessing a high heating power, but incapable of exciting the impression of light. This latter fact was first established by Sir William Herschel, and it has been amply corroborated since.

The belief now universally prevalent is, that the rays of heat differ from the rays of light simply as one colour differs from another. As the waves which produce red are longer than those which produce yellow, so the waves which produce this obscure heat are longer than those which produce red. In fact, it may be shown that the longest waves never reach the retina at all; they are completely absorbed by the humours of the eye.

What is true of the sun's obscure rays is also true of calorific rays emanating from any obscure source,--from our own bodies, for example, or from the surface of a vessel containing boiling water. We must, in fact, figure a warm body also as having its particles in a state of vibration.

When these motions are communicated from particle to particle of the body the heat is said to be _conducted_; when, on the contrary, the particles transmit their vibrations through the surrounding ether, the heat is said to be _radiant_. This radiant heat, though obscure, exhibits a deportment exactly similar to light. It may be refracted and reflected, and collected in the focus of a mirror or of a suitable lens.

The principle of interference also applies to it, so that by adding heat to heat we can produce _cold_. The ident.i.ty indeed is complete throughout, and, recurring to the a.n.a.logy of sound, we might define this radiant heat to be light of too low a pitch to be visible.

I have thus far spoken of _obscure_ heat only; but the selfsame ray may excite both light and heat. The red rays of the spectrum possess a very high heating power. It was once supposed that the heat of the spectrum was an essence totally distinct from its light; but a profounder knowledge dispels this supposition, and leads us to infer that the selfsame ray, falling upon the nerves of feeling, excites heat, and falling upon the nerves of seeing, excites light. As the same electric current, if sent round a magnetic needle, along a wire, and across a conducting liquid, produces different physical effects, so also the same agent acting upon different organs of the body affects our consciousness differently.

FOOTNOTES:

[A] Melloni.

(3.)

[Sidenote: HEAT A KIND OF MOTION.]

Heat has been defined in the foregoing section as a motion of the molecules or atoms of a body; but though the evidence in favour of this view is at present overwhelming, I do not ask the reader to accept it as a certainty, if he feels sceptically disposed. In this case, I would only ask him to accept it as a symbol. Regarded as a mere physical image, a kind of paper-currency of the mind, convertible, in due time, into the gold of truth, the hypothesis will be found exceedingly useful.

All known bodies possess more or less of this molecular motion, and all bodies are communicating it to the ether in which they are immersed. Ice possesses it. Ice before it melts attains a temperature of 32 Fahr., but the substance in winter often possesses a temperature far below 32, so that in rising to 32 it is _warmed_. In experimenting with ice I have often had occasion to cool it to 100 and more below the freezing point, and to warm it afterwards up to 32.

If then we stand before a wall of ice, the wall radiates heat to us, and we also radiate heat to it; but the quant.i.ty which we radiate being greater than that which the ice radiates, we lose more than we gain, and are consequently chilled. If, on the contrary, we stand before a warm stove, a system of exchanges also takes place; but here the quant.i.ty we receive is in excess of the quant.i.ty lost, and we are warmed by the difference.

In like manner the earth radiates heat by day and by night into s.p.a.ce, and against the sun, moon, and stars. By day, however, the quant.i.ty received is greater than the quant.i.ty lost, and the earth is warmed; by night the conditions are reversed; the earth radiates more heat than is sent to her by the moon and stars, and she is consequently cooled.

But here an important point is to be noted:--the earth receives the heat of the sun, moon, and stars, in great part as _luminous_ heat, but she gives it out as _obscure_ heat. I do not now speak of the heat reflected by the earth into s.p.a.ce, as the light of the moon is to us; but of the heat which, after it has been absorbed by the earth, and has contributed to warm it, is radiated into s.p.a.ce, as if the earth itself were its independent source. Thus we may properly say that the heat radiated from the earth is _different in quality_ from that which the earth has received from the sun.

[Sidenote: QUALITIES OF HEAT.]

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