PHOTOGRAPHS OF THE LUNAR SURFACE. 119 



space of time from the gaseous to the solid state. It may be assumed 

 from this standpoint that the moon is enveloped, even at present, by an 

 uninterrupted coating - of ice. This view has been adopted by various 

 skillful observers, especially by Raynard. It is my opinion that it can 

 no longer be defended to-day, and that the modern photographs do not 

 permit us to assume the presence of any considerable quantity of ice 

 upon the surface of the moon. 



The hypothesis of an ice coating appears seducing on account of the 

 fact that a general cooling appears to progress more rapidly than the 

 absorption of the water, at least on the earth's surface. Accordingly 

 it may be predicted that at some future time the greater part of the 

 ocean will have been transformed into the solid condition. The moon, 

 therefore, would offer us an image of the future state of our globe. 



It is also true that the measurements of radiant heat made by various 

 physicists, especially by Langley, have led to the view that, on the 

 whole, the lunar disk is at a very low temperature even when in 

 opposition. 



On the other hand, if the lunar disk were covered with ice one ought 

 to see a luminous spot periodically appear following the motion of the 

 sun. Ice, indeed, possesses a marked power of reflecting light, although 

 to a smaller degree than water. Neither photography nor visual obser- 

 vation indicate anything of this kind, besides the measurements of M. 

 Landerer give a value for the angle of polarization of the surface of 

 the moon differing from that of ice and agreeing much better with that 

 of volcanic rocks. 



It seems also very improbable that a bed of ice spread over the 

 equatorial zones could undergo for two consecutive weeks the direct 

 action of the solar rays, to be for several times twenty-four hours 

 beneath the sun near its zenith, without being heated to the melting 

 point and consequently giving rise to the production of bodies of water 

 and of clouds. Although the high summits of terrestrial mountains 

 remain cold in the summer and although the snows which cover them 

 melt but slightly, this may be attributed to the violent winds which 

 there abound and which prevent the temperature of the surface from 

 exceeding that of the air. But the melting of the snow does take 

 place under the sun's exposure in localities sheltered from the wind. 

 On the moon, however, the air is too rare to remove by convection any 

 considerable quantity of heat from bodies, and hence the action of the 

 sun is exercised in an extremely energetic and prolonged manner. The 

 supposition that the glacial coat of the moon is covered by another, 

 formed of scoriae and cinders, is in better agreement with its superficial 

 aspect, but the resistance past and present offered by the ice to fusion 

 must still be regarded as most mysterious. The scoriae indeed should 

 have an elevated temperature, for they heat up under the action of the 

 sun even more readily than ice. Water surfaces should therefore 

 appear periodically at least in the bottoms of great depressions. 



