on the Moon's surface on account of its weight, and the latter because on account of the low pressure the rapid evaporation would cause it immediately to freeze. Its action, indeed, would be very like that of compressed carbonic acid when suddenly released at terrestrial atmospheric pressure. As is well known, the solidified gas which immediately forms appears like snow, and slowly disappears by evaporation without melting. Its temperature is -109° F.
The cold produced in the lunar snow at this low pressure would be not far from -80° F., and at this temperature the evaporation would be very slow, even when exposed to the Sun. During the night time the Moon's surface temperature must fall far below this figure, so there would be little evaporation then. The Moon's atmosphere probably consists chiefly of these two gases, while we should expect to find the water vapour also in the solid form, as hoar frost, in considerable quantities. Water in the liquid form cannot exist upon the Moon. This is because it requires a pressure of at least 4.6 millimetres to condense it from the gaseous condition, and such a pressure nowhere exists on the Moon's surface. While carbonic acid may also be found there in the solid form, yet such a very low temperature would be required to maintain it in this condition at this pressure that it is more likely to occur in the daytime at all events, chiefly as a gas.
Although the Moon's atmosphere is so rare, yet the quantity of carbonic acid (which is to plants what oxygen is to animals) contained in a cubic foot of the Moon's atmosphere may in certain places be quite as large, or even larger, than that contained in an equal bulk of our own. In the Earth's atmosphere we find about three parts of this gas in ten thousand of oxygen and nitrogen. The Moon's atmosphere a mile or two above the surface we have assumed does not exceed one ten-thousandth of our own in density, but close to the small volcanic vents and cracks where the gas would be given off there is every reason to suppose it would be much more dense.
It was first pointed out by Schlösing that the supply of carbonic acid in our own atmosphere is maintained chiefly by volcanic craters and springs, and that the quantity furnished by animal life is comparatively insignificant. As volcanic energy is undoubtedly diminishing upon the Earth, this question of supply may in future ages become one of serious interest.
The temperature of the night side of the Moon must be about that of interplanetary space. This, according to Professor Langley, is not far from absolute zero, or -273° C,
