At the perfectly invisible focus where these effects are produced, the air may be as cold as ice. Air, as already stated, does not absorb radiant heat, and is therefore not warmed by it. Nothing could more forcibly illustrate the isolation, if I may use the term, of the luminiferous ether from the air. The wave-motion of the one is heaped up to an extraordinary degree of intensity, without producing any sensible effect upon the other. I may add that, with suitable precautions, the eye may be placed in a focus competent to heat platinum to vivid redness, without experiencing any damage, or the slightest sensation either of light or heat.
The important part played by these ultra-red rays in Nature may be thus illustrated: I remove the iodine filter, and concentrate the total beam upon a test tube containing water. It immediately begins to splutter, and in a minute or two it boils. What boils it? Placing the alum solution in front of the lamp, the boiling instantly ceases. Now, the alum is pervious to all the luminous rays; hence it cannot be these rays that caused the boiling. I now introduce the iodine, and remove the alum: vigorous ebullition immediately recommences at the invisible focus. So that we here fix upon the invisible ultra-red rays the heating of the water.
We are thus enabled to understand the momentous part played by these rays in Nature. It is to them that we owe the warming and the consequent evaporation of the tropical ocean; it is to them, therefore, that we owe our rains and snows. They are absorbed close to the surface of the ocean, and warm the superficial water, while the luminous rays plunge to great depths without producing any sensible effect. But we can proceed further than this. Here is a large flask containing a freezing mixture, which has so chilled the flask, that the aqueous vapour of the air of this room has been condensed and frozen upon it to a white fur. Introducing the alum-cell, and placing the coating of hoar-frost at the intensely luminous focus of the electric lamp, not a spicula of the dazzling frost is melted. Introducing the iodine-cell, and removing the alum, a broad space of the frozen coating is instantly melted away. Hence we infer that the snow and ice, which feed the Rhone, the Rhine, and other rivers with glaciers for their sources, are released from their imprisonment upon the mountains by the invisible ultra-red rays of the sun.
§ 6. Identity of Light and Radiant Heat. Reflection from Plane and Curved Surfaces. Total Reflection of Heat.
The growth of science is organic. That which today is an end becomes to-morrow a means to a remoter end. Every new discovery in science is immediately made the basis of other discoveries, or of new methods of investigation. Thus about fifty years ago Œrsted, of Copenhagen, discovered the deflection of a magnetic needle by an electric current; and about the same time Thomas Seebeck, of Berlin, discovered thermoelectricity. These great discoveries were soon afterwards turned to account, by Nobili and Melloni, in the construction of an instrument which has vastly augmented our knowledge of radiant heat. This instrument, which is called a thermo-electric pile, or more briefly a thermo-pile, consists of thin bars of bismuth and antimony, soldered alternately together at their ends, but separated from each other elsewhere. From the ends of this 'thermo-pile' wires pass to a galvanometer, which consists of a coil of covered wire, within and above which are suspended two magnetic needles, joined to a rigid system, and carefully defended from currents of air.
The action of the arrangement is this: the heat, falling on the pile, produces an electric current; the current, passing through the coil, deflects the needles, and the magnitude of the deflection may be made a measure of the heat. The upper needle moves over a graduated dial far too small to be directly seen. It is now, however, strongly illuminated; and above it is a lens which, if permitted, would form an image of the needle and dial upon the ceiling. There, however, it could not be conveniently viewed. The beam is therefore received upon a looking-glass, placed at the proper angle, which throws the image upon a screen. In this way the motions of this small needle may be made visible to you all.
The delicacy of this apparatus is such that in a room filled, as this room now is, with an audience physically warm, it is exceedingly difficult to work with it. My assistant stands several feet off. I turn the pile towards him: the heat radiated from his face, even at this distance, produces a deflection of 90°. I turn the instrument towards a distant wall, a little below the average temperature of the room. The needle descends and passes to the other side of zero, declaring by this negative deflection that the pile has lost its warmth by radiation against the cold wall. Possessed of this instrument, of our ray-filter, and of our large Nicol prisms, we are in a condition to investigate a subject of great philosophical interest; one which long engaged the attention of some of our foremost scientific workers—the substantial identity of light and radiant heat.
That they are identical in all respects cannot of course be the case, for if they were they would act in the same manner upon all instruments, the eye included. The identity meant is such as subsists between one colour and another, causing them to behave alike as regards reflection, refraction, double refraction, and polarization. Let us here run rapidly over the resemblances of light and heat. As regards reflection from plane surfaces, we may employ a looking-glass to reflect the light. Marking any point in the track of the reflected beam, cutting off the light by the dissolved iodine, and placing the pile at the marked point, the needle immediately starts aside, showing that the heat is reflected in the same direction as the light. This is true for every position of the mirror. Recurring, for example, to the simple apparatus employed in our first lecture (fig. 3, p. 11); moving the index attached to the mirror along the divisions of our graduated arc (m n), and determining by the pile the positions of the invisible reflected beam, we prove that the angular velocity of the heat-beam, like that of the light-beam, is twice that of the mirror.
FIG 49.