wonderful accuracy much about the actual components, and even actual physical condition of the most distant heavenly bodies. It is interesting to find that, as far as we can trace them in our telescopes, these are constituted of precisely similar matter to what we are familiar with, governed by precisely the same laws. Diverse and inconceivably far apart far enough for even Light, with its enormous velocity, to occupy hundreds of years in traversing the distance-all are yet one vast unity. We can trace their materials, and sort them out into groups according to their stages of development; we can tell if they are solid, or gaseous, and whether they have a surrounding atmosphere or not. The Light they send us is a true Revealer of all, and brings evidence of all these things in its beams.

CHAPTER VIII.

PHOSPHORESCENCE.FLUORESCENCE.-CALORESCENCE.

Effects of Absorbed Vibrations-The Invisible Rays of the Spectrum - Three Independent Spectra non-existent - Phosphorescence Fluorescence - Calorescence Relation of Fluorescence to Phosphorescence.

86. Effects of Absorbed Vibrations.-In previous' chapters, we have been led to adopt as our hypothesis of absorption, and of the cause of colour in coloured substances, that molecules of matter having certain periods of vibration, took up from ether-vibrations of all periods, such vibrations as synchronized with them. We are bound to ask, what becomes of these absorbed vibrations? Energy cannot be annihilated; and the motion apparently destroyed must produce certain effects. If molecules of matter thus take up vibrations from the ether, so quenching or weakening them, it may be urged that these molecules ought, in their turn, being set vibrating, to give out new light, or at least vibrations, of their own; and further it is conceivable, not to say probable, that such matter-vibrations should be excited in some measure, though not so strongly, by ether-vibrations not synchronous. Sound is our closest analogy; and we know that non-synchronal waves will set vibrating various sounding bodies. That this is so as regards heat and light, is beautifully shown by the allied phenomena of Fluorescence and Phosphorescence.

It readily appears, on reflection, that when small masses act by their motions upon large masses, the more common effect must be the conversion of quicker motions into slower ones. To use and expand a dynamical analogy which has been employed by Professor Stokes, let us consider short and choppy waves acting upon a large vessel anchored at sea. The quicker motions impart a slower pitching and rolling of the vessel; and these, again, cause new and slower waves in the water. But these are less perceptible than the primary waves, and may even be unnoticed, unless the water should become suddenly calm; when they would at once be conspicuous as long as the rolling continued, a period which would depend on the stability of the vessel. In the same way, long slow waves may more rarely be converted into quicker motion, and thence into quicker secondary waves. Regard the waves as motions of ether atoms, and the vessel as a molecule of matter, and the analogy is fairly complete.

87. The Invisible Spectrum.-But before we can fairly investigate these matters, we must take into our view more than the spectrum we "see" upon the screen. That spectrum has no sharply-cut ends; and we know well enough that it has other effects than visual ones. We can readily trace heat in it; and experiment in even a very rough way with a good thermometer soon shows us that the heat is much the greatest at the red end. If, on the other hand, we expose a photographic plate in the spectrum, we find very energetic chemical effects; and as regards salts of silver and many other compounds, we find that these powers of producing chemical changes are much more energetic at the violet end.

If we push our experiments further, with more delicate instruments, we find that some of the most energetic heat rays are quite outside of the visible red end, in a dark space,

representing still slower vibrations than the slowest red rays. And we also find that some of the most energetic chemical effects are produced in an invisible region outside of the visible violet end. Moreover there are broad absorption bands and Fraunhofer lines in these invisible regions; and there are bodies, alike in being perfectly clear and transparent to "visible" light, which differ widely in transparency as to these invisible rays. Clear rock-salt is the only body transparent to all the heat rays and quartz is one of the most transparent to the chemical rays, which are largely absorbed by glass; as the heat rays are almost totally stopped or absorbed by a solution of alum in water.

88. Three Independent Spectra non-existent. -Hence diagrams have been constructed showing the comparative intensity or working power of what is called the Light spectrum, the Heat spectrum, and the Chemical or Actinic spectrum; the energy of each in every region of the spectrum being shown by a curve, whose highest point is at the place in the spectrum where the intensity is greatest. Thus, the highest luminous intensity would be over the yellow. And it has been considered that there were in a beam-say of sun-light-rays of three distinct kinds, called heat rays, light rays, and chemical rays.

But this is now known to be a mistake. All the rays are subject to the same laws, being reflected, refracted, diffracted, &c., exactly as the luminous rays whose phenomena we have investigated. They differ solely in their periods of vibration; and their different effects are due simply to certain periods and lengths being best adapted to produce those effects. Just as with sounds, some persons can hear much graver sounds and others more acute sounds than others can, and probably insects can hear sounds inaudible to us; so while average visible light waves range only between those whose lengths are from gebooth to a booth of an inch

in air, some persons can see rays at one end or the other, invisible to the majority. Again, it has been said that "chemical rays" or chemical effects are almost nil in the yellow of the spectrum; and it is so as regards the salts of silver. But the action of light upon plants is also a distinctly chemical effect; and this is perhaps, if anything, the most powerful in that same yellow region. Science knows no real distinction but periods and lengths, between any of the rays in the spectrum; each period being more or less adapted, as a rate of Motion, to produce certain effects upon the molecules of bodies, or upon the nerves.

Now as respect these effects, we have a proof that the quickest motions act most powerfully in some respects upon the molecules of matter, in the effect of vibration upon wrought iron. Slow motions do not affect it; but quicker vibrations rapidly produce a crystalline structure, showing that the molecules are shaken, or at least forced in some way into new positions. We see the same thing exactly in the chemical power of the quicker waves of light. It is almost certain that the atoms upon which they act are literally shaken into new combinations, very much as in the crystalline iron; and thus we can see why it is the quickest vibrations which are often most powerful in their effects. Actinism is, in fact, itself one of the strongest proofs of the vibratory theory of light. And as the transference of motion from the ether we here suppose, is again to the atoms of bodies, we should expect to find it in some respects most evident from the quickest waves.

We thus see, in a general way, what becomes of light when it meets bodies opaque to any given periods of vibration. It can always be traced somewhere; but is largely converted into heat in the body, while many vibrations which would be true visible waves as regards period, are too weak to be discerned. We should expect that the quicker motions would