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 aërial pulses is to insert a small gas jet into a vertical glass 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 entitled '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.
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 multitude 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 quantity 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]
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 identity indeed is complete throughout, and, recurring to the analogy 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.