of matter; it teaches that light consists of vibrations or waves excited in the ether by the luminous body, and travelling from it to the eye, just as sound arises from undulations in the air. The differences in colour of the different rays are shown to arise from differences in the length of their waves, the least refrangible having the longest waves, the most refrangible the shortest. The position of the heat-rays in the spectrum, then, shows that they differ from the coloured rays in having longer waves. So far as their cause is concerned, heat-rays differ from red rays, only as red rays differ from blue.

The intimate connection of light and heat with electricity and magnetism, and their mutual convertibility (see pars. 68 and 75), suggest that the same cause may be at the foundation of all; and the theory of Prof. J. C. Maxwell, which is considered to be all but demonstrated, makes electrical and magnetical phenomena to consist in modifications of this same invisible medium, this elastic ‘jelly' (for it is conceived to be solid rather than fluid), the quivering of which causes the sensation of light.

91. Loss of Heat by Radiation.-Hot bodies exposed to the air lose their heat partly by convection and partly by radiation. The rate of cooling of a hot solid body, so far as radiation is concerned, is remarkably influenced by the state of its surface, and, in the case of liquids and gases, by the state of the surface of the vessels containing them. Thus, hot water placed in a tin vessel coated externally with lampblack, cools twice as fast as it does in a bright tin vessel. Hence, a kettle covered with soot is much less suited for retaining water warm, than if it had a polished metallic surface. So, also, metallic tea-pots and coffee-pots are preferable to those of porcelain and stoneware.

92. Absorption and Reflection of Rays of Heat.— When heat-rays fall on the surface of a body, they either enter it or are reflected. Those, again, that enter are either transmitted, like light through glass, or are retained and absorbed. It is only the rays that are absorbed that warm the body; those that are reflected off, as well as those that are transmitted, produce no effect on the temperature.

Surfaces that radiate heat best, are found also to imbibe it most readily. If a table, then, is formed of substances according to their power of radiating heat, the same table will serve for their power of absorption. The following is such a table, the radiating and absorbing power of a surface of lampblack being expressed by 100:

Since all the rays not absorbed by a surface must be reflected, this table, read from the bottom upwards, will give the same surfaces in the order of their reflecting powers. It thus appears that the best protection for the head from the burning rays of the sun is a polished metal helmet.

93. Effect of Colour on Radiation.-Colour has comparatively little influence on the radiation or absorption of heat so far as obscure or non-luminous rays are concerned. In the above table, lampblack and white-lead rank equal; and two garments of the same material, the one white and the other black, dissipate the heat of the body at the same rate, and afford the same protection

from the rays of a hot stove. heat, the case is different. both invisible and visible rays. Now, when such a beam falls on two pieces of cloth, one black and the other white, the effect of the obscure rays is the same on both; but the black surface absorbs also the luminous rays, while the white rejects them. The black is thus more heated than the white. As a protection against the direct rays of the sun, light-coloured clothing is thus better than dark.

But in regard to luminous A beam of the sun contains

94. All Bodies radiate Heat.-All bodies, even the coldest, are constantly radiating off more or less heat, according to their temperature; all are therefore both giving and receiving rays; but the warmer give more than they receive, the colder receive more than they give. A surface presented towards the open sky, with nothing to radiate or throw back the rays it is emitting, soon becomes cold; but the slightest curtain, such as a net, hung up before it, sensibly arrests the dissipation of heat. It is in this way that clouds act as warm curtains to the earth, and often prevent frosts in spring and autumn nights. The different radiating powers of bodies explain why dew is sooner deposited on some substances than on others. Those that are good radiators lose their heat most quickly, and thus condense the vapour of the atmosphere on their surfaces.

95. Transmission of Thermal Rays.-Some substances, it has been observed, allow heat to pass through them, as light passes through glass; such substances are called diathermanous. Bodies are not diathermanous and transparent in the same degree; for black glass transmits heat well, and water, which is highly transparent, is the least diathermanous of liquids. Of solid bodies, rocksalt is the most diathermanous, alum the least so. The

powers of gases and vapours to transmit radiant heat have been investigated by Professor Tyndall, with very striking results. The most important are those concerning the component elements of our atmosphere. Oxygen, nitrogen, and hydrogen transmit almost perfectly, and so does pure air, which is a mixture of oxygen and nitrogen. All the other gases absorb more or less of the rays. If the absorption of air is stated as 1, that of carbonic acid is 90, and of ammonia is 1195. But this is nothing to the absorbing power of the vapour of water. When the atmosphere is in an average state of humidity, the invisible aqueous vapour contained in it absorbs 72 times as much radiant heat as the air itself, although the quantity of the latter is 200 times that of the former. The consequences of this are of vital importance to the inhabitants of our globe (see par. 221).

Glass transmits luminous rays, but is athermanous to obscure rays; hence, a glass frame, in gardening, has been called 'a trap to catch sunbeams.' The light of the sun passes through the glass, and is absorbed by the earth as heat; but when this heat seeks to escape by radiation, the glass refuses to retransmit it.

LATENT HEAT.

96. Heat of Liquefaction and Evaporation.-When a solid body, such as ice, is watched whilst melting, a large quantity of heat is observed to enter it without raising its temperature in the slightest degree. This heat which enters the body serves only to melt or liquefy it, without rendering the liquid the least hotter than the solid was which yielded it. The water which flows from the melting ice is no warmer than the ice. The heat which thus renders a body liquid without warming it, is called latent or insensible heat, because it

does not affect our sensations, and does not raise the thermometer. The fact of heat becoming latent, is most decisively demonstrated by mixing a certain quantity— say an ounce by weight-of ice, or, still better, from its state of division, of snow at 32°, with an ounce of water at 172°. The result will be found to be, that the snow is all melted, and two ounces of water are procured at the temperature of 32°. The hot water, in cooling from 172° to 32°, has lost 140° of heat, which changes the snow into water, but does not raise its temperature above that originally possessed by the snow.

What we have illustrated here with ice, holds good for all solids. Each one of them renders latent a certain quantity of heat in becoming liquid, and retains that heat so long as it remains liquid; when the liquid solidifies, it is again given up. Thus, when water freezes, the 140° of latent heat all abandon it, and manifest themselves as sensible heat (see par. 239).

In evaporating a liquid, a similar disappearance of heat takes place. In boiling off a pound of water, or converting it into vapour, it can be shown by experiment that as much heat is absorbed as would have raised its temperature about 1000°, if it had not gone off in steam. Yet the water rises no higher than 212°, however hot the fire is, and the steam is of the very same temperature as the water it rises from. Thus 1000° have disappeared or become latent in the steam; and before the steam can be condensed into water again, all this heat must be given out.

The same is true of the vapour that rises slowly and silently from water at temperatures below boiling. This absorption of latent heat is the cause of the cold which always accompanies evaporation (see par. 227).

97. Latent Heat not Lost.-Heat which thus be- .