Sir William Herschel discovered that invisible rays of high heating power exist beyond the red end of the solar spectrum, and Mr. Tyndall has shown that the reason of a substance being impervious to the light of the most brilliant flame and at the same time pervious to these extra red rays is, that the intercepted rays of light are those whose periods of recurrence coincide with the periods of oscillation possible to the atoms of the substance in question. The elastic forces which separate these atoms are such as to compel them to vibrate in definite periods, and when their periods synchronize with those of the ethereal waves, the latter are absorbed. Thus transparency in liquids as well as in gases is synonymous with _discord_, while opacity is synonymous with _accord_ between the periods of the waves of ether and those of the body on which they impinge. All ordinary transparent and colourless substances owe their transparency to the discord which exists between the oscillating periods of their molecules and those of the waves of the whole visible spectrum. The general discord of the vibrating periods of the molecules of _compound bodies_ with the light-giving waves of the spectrum may be inferred from the prevalence of the property of transparency in compounds, while their greater harmony with the extra red periods is to be inferred from their opacity to the extra red rays. Water illustrates this transparency and opacity in the most striking manner. It is highly transparent to the luminous rays, which demonstrates the incapacity of its molecules to oscillate in periods which excite vision. It is as highly opaque to the extra red oscillations, which proves the synchronism of its periods with more of the longer waves. If, then, to the radiation from any source water shows itself to be eminently or perfectly opaque, it is a proof that the molecules whence the radiation emanates must oscillate in extra red periods. It has been already mentioned that many substances which transmit radiant heat freely radiate badly, and vice versâ. Rock-salt is extremely permeable to radiant heat but radiates feebly; the reason according to Mr. Tyndall is, that the motion of the molecules of the salt, instead of being expended on the ether between them and then communicated to the ether external to the mass, is transmitted freely from molecule to molecule. Alum is exactly the reverse. Mr. Balfour Stewart proved that alum is an excellent radiator, and Mr. Tyndall proved it to be a very bad conductor, imparting freely and with ease the motion of its molecules to the external ether, and ‘for that very reason it finds difficulty in transferring the motion from molecule to molecule. The molecules are so constituted that when one of them approaches its neighbour, a swell is produced in the intervening ether; this motion is immediately communicated to the ether outside, and is thus lost for the purposes of conduction.’[6]