54 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



part of the new moon taken in the earth-light clearly show these differences of 

 hue ; they are, indeed, plain enough to enable one to map the more brilliant rays 

 of the Tycho system. We may therefore dismiss the idea that these features 

 are evolved in the progress of one lunar day to be reconstructed in the next, and 

 regard them as permanencies made visible when they may reflect to us the light 

 which comes to them at a high angle. 



As to the conditions which bring about the large amount of reflection under 

 a high sun from those parts of the moon which appear very white when it is full, 

 the experience of geologists suggests the following working hypothesis : First, 

 that the bright area may be covered by an incrustation of a smooth nature such as 

 ice or other material which forms a sheet. It cannot be frozen water, but various 

 volcanic emanations may be conceived as forming like surfaces of glassy smooth- 

 ness. Or we may suppose that some part of the material which came forth during 

 eruptions was distributed as vapor to become crystallized on the surface. Such 

 solfataric action is common enough in terrestrial volcanic districts ; it would often 

 be sufficient to cover extensive fields were it not for the erosive agents which 

 scour the surface. It appears to me, however, that the suggestion of a smooth 

 surface, such as an incrustation, is insufficient to meet the facts, for the reason 

 that such coating could not be formed save of frozen water or of materials laid 

 down by fluid water. With the low temperature of the moon's crust and the lack 

 of an atmosphere, the idea of a quick crystallization of mineral substances from a 

 vaporous state seems more consonant with the known facts. 



It is possible that the sudden-coming brilliancy of the bright patches and 

 streaks is due to the fact that these shining areas are covered with crystals which 

 have their planes so arranged that they are prevailingly parallel with the surface 

 on which they lie, so that they reflect their light toward the earth only when the 

 sun is high. This hypothesis has some support in the appearance of certain steep 

 slopes, as those of the cones in the greater vulcanoids, where the face of the cliffs 

 may be observed to shine brightly, when the sun's rays strike them, some time 

 before the adjacent nearly horizontal surfaces gain the intensity of light which 

 they afterwards acquire. A close study of this matter may afford data for a 

 determination as to the nature of the action. So far I have been able to do no 

 more than prove that the brilliancy is due mainly to the angle of illumination, by 

 noting that it appears in earth-shine as well as sunshine, though the brilliance 

 of the glow on the margins of the moon suggests that there is also an element of 

 fluorescence or other action in the phenomenon. 



Although light rays distinctly appear to be connected by series with the light 

 patches, there are certain peculiarities about the former which demand explana- 

 tion. Their exceeding length and their generally slight width make them very 

 puzzling features. It has been frequently suggested that they are due to certain 

 dust-like emanations from the craters which have been blown by the wind which 

 bore them and lodged in crevices or in the lee of projecting points. The current 

 of air which bore them is conceived as produced by the gaseous emanations from 

 the crater. This view appears to me to be ill-founded, because the volume of 



