Opalescent glass, slightly milky liquids, pure water with particles of sulphur floating in it, are impervious to the chemical rays, whence Professor Roscoe infers that the atmosphere, more especially its lower regions, possesses that property in consequence of multitudes of solid particles floating in it. What they are is unknown, but infinitesimal particles of soda seem to be everywhere, and no doubt particles of other substances mixed with them may be often seen as motes dancing in the sunbeams. Besides, it is clearly proved that myriads of the eggs and germs of organized beings, though invisible to the naked eye, are continually floating in the air, and that they are more abundant in the lower than in the higher strata of the atmosphere. Since opalescent matter reflects the blue rays of light and transmits the red, Professor Roscoe ascribes the blue colour of the sky and the bright tints at sunrise and sunset to the opalescent property of the air. The atmosphere is permeable to every kind of chemical rays, which is far from being the case with bodies on earth, some of which though transparent to all the visible rays, vary greatly in their transparency to the chemical rays. The atoms and molecules of matter not only have the power of turning the rays of the solar beam out of their rectilinear path, but of changing their refrangibility. The myriads of ethereal waves or rays of light that constitute the seven colours of the solar spectrum, decrease in refrangibility and increase in rapidity of vibration and length of wave from the extreme violet to the end of the red; each ray having its own rate of vibration, its own length of wave, and its own colour. From the middle of the yellow, which is the luminous part of the spectrum, the chemical spectrum extends invisibly, but with increasing refrangibility and increasing velocity of vibration, to a point far beyond the violet. On the contrary, the heat spectrum, which may also be said to begin in the yellow light, extends invisibly but with decreasing refrangibility, and decreasing velocity of vibration to some distance beyond the visible red. The rays of heat are absorbed by the humours of the eye, but were they to reach the retina we should see that they differ from one another as much as those of the luminous spectrum; the chemical spectrum from its greater length is still more diversified.