Page:All the Year Round - Series 2 - Volume 1.djvu/261

Charles Dickens] appreciate the peculiar characteristics of each individual light, from whatever radiant source it reaches us.

If we put a glass prism in the course of a ray of light, that light, by traversing the prism, is decomposed into its primitive elements. It is an experiment which may be tried any sunshiny day; and sometimes an icicle, drawing-room ornament, or a gem, will try it for us. At first sight, the eye perceives a series of lovely hues ranged one above the other, calling to mind the colours of the rainbow, with which, in fact, they are identical. On inspecting the party-coloured ribbon so obtained more closely, we discover, when the light comes from the sun, hundreds of black stripes of extremest fineness. When the light proceeds from an incandescent solid or liquid body, the stripes disappear, and the coloured ribbon or spectrum, is continuous. If, on the contrary, the light is given out by burning gas, bright and brilliant stripes appear in the spectrum. If, lastly, the source of light is an incandescent nucleus enclosed in a gaseous envelope, the image, as is the case with the sun, is traversed by a series of black lines.

Both the black and the brilliant lines were long a puzzle to natural philosophers. In 1822 Herschell remarked that when salts of lime, copper, and strontian were introduced into a flame, luminous lines were produced in the spectrum of that flame. Not long afterwards, Brewster and Talbot ascertained that the brilliant stripes varied with the nature of the body put into the flame. Common salt, for instance, gives a bright yellow stripe. Potash causes the simultaneous appearance of a red stripe and a violet stripe. It was clear, therefore, that the bright lines of the spectrum resulted from the presence of determinate compounds in the flame. But what of the black stripes?

The labours of several other philosophers helped Messieurs Kirchoff and Bunsen to demonstrate undeniably, in 1860, that every bright light in the spectrum is transformed into a dark one, when a source of intense light exists behind the flame. Example: soda gives a bright yellow stripe. Put an electric light behind the flame producing the spectrum, and instantly the bright stripe disappears, to give place to a corresponding black one. The fact is easily accounted for, when we remember that the property of emitting light, like that of radiating heat, is combined with the property of absorbing it in inverse proportion. The more light a luminous object gives out, the less it will take in. The more capable a flame is of emitting light, the more does it, from that very circumstance, extinguish a light placed behind it. It is therefore clear that every line which is more luminous than the neighbouring portions of the spectrum of a flame, will necessarily become darker as soon as a source of light is placed behind it. Such is the answer to the black line enigma.

But the spectrum of the solar light is cut up and riddled with black lines through and through. The conclusion is that incandescent flames or vapours, containing a great number of volatilised substances, surround the sun, and that behind those flames there exists a source of light still more powerful and intense than they are. MM. Kirchoff and Bunsen carefully examined the position of the lines produced in the solar spectrum by the principal substances found on the earth, and they then turned them black by the application of a more intense source of light. Now, they found that there was an absolute identity in the situation and distance of the black stripes in the solar spectrum, and of the stripes thus artificially produced. This precise coincidence allows us to conclude the existence, both in the sun and the earth, of certain constituent elements. The light emitted by the sun indicates the presence in it of iron, magnesium, sodium, potassium, barium, copper, manganese, zinc, &c. Hitherto they have been unable to ascertain the existence of gold, silver, lead, tin, antimony, cadmium, arsenic, mercury, &c.

We have thus a telegraph established between the stars and ourselves. The telegrams reach us in letters of fire. The lines of the spectrum replace the letters of the alphabet. Every element has its characteristic signs; but the reading of this alphabet is very complicated, and we have scarcely as yet begun to spell it. Evidently discoveries will be greatly multiplied when we have learnt to read it fluently. Nevertheless the principal stars, comets (one of which has been found to contain carbon), and nebulæ have already been explored with considerable success.

It was an inevitable consequence of the preceding facts that the habitable condition of the sun is a fallacy, and that we do not see the sun's soil at the bottom of his spots. Our central life-giving luminary must consist of a gaseous incandescent atmosphere containing metallic vapours, inclosing a solid or liquid burning nucleus. The spots in this case would be veritable clouds, produced by the partial and local condensation of solar vapours. There is a discrepancy between Kirchoff's observations and Arago's experiments on polarised light; but the apparent contradiction has been reconciled by an able French astronomer, M. Faye. Kirchoff's spectral observations are quite consistent with those afforded by a perfectly gaseous sphere holding solid particles in suspension.

The sun, therefore, must be set down as neither solid nor liquid, but gaseous, as might be inferred from its slight mean density. This theory has the further philosophical advantage of being applicable to the other heavenly bodies, each one of which would pass through successive phases corresponding to the divers epochs of their evolution and progress. Each heavenly body must successively experience the gaseous, liquid, and solid states. The sun, the earth, and the moon, for instance, offer us the three distinct ages in the life of worlds. The earth once must have been for the moon what the sun is now for us. The moon's smaller mass would sooner grow cold. Then the earth, after having been what the sun is,