they actually appear. The clearness, also, of such an atmosphere would only serve to render the phenomena attending an occultation more distinctly visible; and we have seen that such phenomena would be sufficiently marked. Let us imagine, however, the effects of a further diminution of temperature. It is well known that when subjected to a loss of heat sufficiently great, many gases, elementary and compound, are reduced successively to the liquid and solid forms. Hitherto no process has enabled the chemist to convert oxygen, nitrogen, or hydrogen, into either the liquid or solid forms; * but there is no reason for supposing that they form exceptions to the general rule, that, under suitable variations of temperature, all substances in nature may assume any one of the three forms-gaseous, liquid, and solid. Now, it is probable that the variations of temperature with which we are familiar include but a small part of the range of possible variations. Thus it is conceivable that a planet parting with its heat by slow radiation might after the lapse of many ages have lost so much heat that all the gases upon its surface would be condensed to the liquid or solid forms. The length of time required to effect such changes would depend on the mass of the planet's globe-a large planet would obviously require a longer time to part with its internal warmth than a small planet. What relation such time would bear to the mass of a planet could not easily be determined, but it is certain that some such relation exists. Now, on Laplace's hypothesis of the development of the solar system, the moon was formed before the earth; and the mass of the moon is little more thanth part of the earth's Thus it is conceivable that the moon's mass may have become so intensely cold that the atmospheric envelope once clothing it has been condensed into the liquid, and thence into the solid form. It need not necessarily be assumed, however, that all the gases on the moon have been thus solidified. Small seas of liquefied gases may exist upon the moon's surface; and, again, some of the phenomena that have been supposed to indicate the presence of an atmosphere may be due to gaseous envelopes of small extent still uncondensed. We may imagine, for instance, that hydrogen would resist an intensity of cold that would liquefy or solidify all other gases. On these points we can only form vague conjectures, since as yet the more important gases have defied all attempts at liquefaction or solidification. mass. * A gas may be converted into the liquid and solid forms either by loss of temperature or by pressure. A combination of both processes is generally adopted to condense a gas into the liquid form; then part of this liquid being allowed rapidly to resume the gaseous form, the remainder is solidified, owing to the loss of latent heat. Greatest distance from the sun (earth's mean distance as 1) 10.072533 9.538850 9.005167 0.0559484 Least distance from the sun (same unit) Synodical revolution in days (at epoch) Longitude of the perihelion Annual variation of same (increase) Same variation referred to ecliptic (increase) Annual variation of same (decrease) 0.000003125 10759-2197106 377-767 378-092 90° 23′ 36"-4 19"-31 69"-41 112° 29′ 18′′-20 19"-54 30"-56 2° 29' 26"-15 Apparent equatorial diameter, at mean distance from earth Same, Saturn in opposition, in perihelio Same, Saturn in opposition, at mean distance from sun Same, Saturn in opposition, in aphelio 0"-15 2' 15"-3 2' 0"-6 1' 48"-2 63° 10' 32"-13 0"-321 61° 49′ 38"-05 0"-350 10h 29m 17s 17"-05 20"-31 19"-04 17"-92 The symbols thus bracketed refer to the corresponding symbols in Tables VII. and VIII. |