Now in Light, if we assume 424 billions for the vibrations per second of the fundamental wave, we have these relations.

=

9: 8=477 billions = red ray exact; 5: 4 530 billions (535 Hersch.) = yellow ray; 3: 2 = 636 billions (644 = b intermediate to blue and indigo); 4: 3565 billions (b in the green); 5: 3 = 707 billions (H. centre of violet).

Thus the three primary ratios, answering to the major tone, major third, and fifth in music, correspond with red, yellow, and blue, which by general consent are the three primary and simplest colours. The ratios next in simplicity, where three is the divisor, or the musical fourth and sixth, correspond with green and violet, which are the colours next in order of simplicity, so that Dr Young selected red, green, and violet for the three simplest. The next ratio is 6: 5 = 509 billions, and corresponds exactly with the orange ray, and in music with the minor third.

The agreement, then, between the scales of light and sound is complete, when once we assume a ray beyond the red for a natural unit of comparison, or a fundamental pitch in the series of luminous waves.

Again, the waves beyond either limit will be octaves of those near the opposite limit. They may be rendered luminous by union with another ray, because the new mean, from the half sum of the lines of their vibrations will be within the limit of visibility. In this case they must seem nearly identical in their properties with the last visible rays at the other limit, just as a note and its octave sound together almost like a perfect unison.

53. It will result, from this view, that the rays of the spectrum are not, strictly speaking, compound; but that specific ratios in their vibrations, referred to a fixed limit

of visual power, give rise to the sensation of each primary colour; and the combination of two of these, because the period of the half sum prevails over that of the half difference in the compound period, will resemble the tint half way between them, but with some defect in purity and brightness of tone.

Again, the most usual variety of the imperfect vision, called colour blindness, is the incapacity to discern more than two fundamental colours, yellow and blue. This is easily accounted for on the present hypothesis. For the next ratio to the major tone is 17: 16, and is insensible as a concord, or as a fundamental shade of colour. Now if this limitation in the power of the ear or eye were carried one step further, then the eye will appreciate the ratios 32 and 54, but not 9: 8. Consequently only yellow and blue, to the exclusion of red, will be left for the fundamental colours, of which all the rest will appear to be varied combinations.

54.

Luminous and Non-luminous Matter.

Another subject, on which theory needs to throw fresh light, is the source and nature of luminosity. It is often assumed that the sun and stars are composed of matter different in kind from the substances with which we are familiar. The present hypothesis would exclude all possibility of such an absolute contrast. We are thus led to the inference that the sun's luminosity is due, in some way or other, to its immense relative mass, and its central position.

The chief known sources of light are combustion or incandescence, phosphorescence, that is, a weaker and insensible kind of combustion, and solar radiation.

In combustion it is plain that the chemical atoms enter

into new and more intimate combinations. The effect must be that vis viva is largely increased; which may take the form of rapid oscillation, that is, high temperature of solid products; rotation of the new atoms, or the specific heat of fluidity; or rapid translation, where the products are gaseous. But in all cases alike there must be those ethereal vibrations, from which pulses both of light and of radiant heat usually arise.

Again, the whole pressure of an atmosphere on its lowest stratum equals the weight of the column, and hence the total pressure throughout the column will be one half the square of the mass into the mean force of gravity. But the pressure on each part must equal the repulsive force developed. This, again, in a gaseous medium, must depend on the centrifugal atomic motion, which varies as the square of the velocity into the mass of each particle. The same product must represent the vis viva of gaseous motion, on which the power to excite vibrations must be conceived to depend. Hence it seems probable that the collective power to disturb the surrounding ether will be measured, at least in some rude approximation, by the total pressure, or the square of the mass in each column, multiplied by one half the force of gravity at the mean height.

The mass of the solar atmosphere is unknown. Let us assume that it bears the same ratio to that of the earth as the two spheres bear to each other, reaching from their centres to the distance of equal attraction. Then will be the relative mass of the sun's atmosphere, μ3

its

με square, the relative amount for the unit of surface, 2.2

-6

the relative luminosity, or collective pressure for the

unit of surface, and ur the relative amount of light. On this hypothesis, the luminosity of the sun would be 12127 millions, compared with that of the earth, and compared with Jupiter 1 millions. By this formula, the earth would have a native light, forty thousand times less than that of the sun which falls upon it, and Jupiter, on the contrary, a native light, equal to or greater than what it receives.

Now if Venus and Jupiter shone only by reflected light, the amount they transmit from a unit of surface should be as 50 to 1; and when she is in her maximum brilliance, and Jupiter in opposition, the disc of Jupiter is very little larger in visible size than hers. Assuming the crescent to be one fourth of her disc, her light on this view should be twelve times greater. The disparity, however, it seems certain, is considerably less. And hence there seems to be some direct presumption that Jupiter, the "lustrous star" of astrology, has really a native light, distinct from that which he receives from the sun. The auroras, again, are a native earth-light, and offer a further illustration of the same view. Luminosity, it is highly probable, is some function of the mass of each sun or planet, and of its velocity of motion, or the height of its atmosphere, though the exact function by which the two are related may not be easy to ascertain.

CHAPTER VI.

THE CHEMICAL ELEMENTS IN GENERAL.

Occurrence.

55. THE Elements of the ancients were fire, air, earth, and water, answering to four general forms or conditions. of matter, which is either solid, fluid, gaseous, or igneous. Modern science, however, applies the name to those kinds of homogeneous substance, which have not at present been resolved into each other, or into any simpler form. The total number of these is rather more than sixty, but one or two are doubtful, and several others of rare and limited Hydrogen, oxygen, nitrogen, chlorine and fluorine, are gases; bromine and mercury are fluid at common temperatures; and the rest are solid, but the heat required for their fusion differs widely, and carbon, one of the most important, is infusible. The greater part, about fifty in number, have sufficient general resemblance to receive the name of metals. In their density and other properties, however, they differ widely. Potassium and sodium are lighter than water, while platinum and iridium are more than twenty-one times heavier. When binary compounds are analyzed by a galvanic current, the same