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 CHAPTER VII

there are any active volcanoes upon the Moon, it is evident that they must expel something. In other words, there must be some gaseous pressure to make them active. As we have already seen in Chapter III., the gases which they are most likely to expel, judging from the case of the Earth, are water- vapour and carbonic acid. Water in the liquid state cannot exist upon the Moon. Above the freezing point it would be wholly gaseous; below it, it would be partly gaseous and partly solid, the formation in the latter case being analogous to snow, or perhaps, more strictly speaking, to hoarfrost. Very many of the craterlets upon the Moon are lined with a white substance which becomes very brilliant when illuminated by the sun. The same white substance lines portions of some of the larger lunar craters and is found also on a few of the higher lunar mountain peaks. It may be noted in this connection that, owing to the bright yellow colour of a large part of the Moon's surface, the white regions present a greater contrast on photographs, and are thus more clearly defined than they are when observed visually through the telescope.

Besides these very bright patches and spots, there are other regions less brilliant, but exhibiting a curious characteristic. They are invisible for the first twenty-four hours after sunrise, but gradually appear as the sun rises higher and higher, becoming fairly conspicuous at the end of a couple of terrestrial days. Later they begin to fade, and finally disappear shortly before sunset. These "partly bright" regions, as they may be called to distinguish them from the wholly bright spots first noted, comprise considerable areas in the interior of some of the larger craters. They cover the upper slopes of many of the mountains, the rims and sometimes the central peak of numerous moderate-sized craters, and from a bright halo, so to speak, extending for miles around many of the smaller craters and craterlets. The most striking appearance, however, consists of long bright lines radiating in all directions—in some cases for hundreds of miles—from some prominent central crater.

It seems likely that these partly bright regions represent areas that are only partly covered with the white material of the more brilliant patches, which, perhaps in some 45