Fresh Notes on the Grater Linne. 221 



their condensation, as they become cooler, gives rise to the great 

 masses of cloud and smoke which form so striking a feature in 

 most eruptions. Water is acknowledged by all authorities 

 to play a very important part in all, or nearly all, varieties of 

 terrestrial volcanic action, and if the volcanoes of the moon 

 perform their functions without its assistance, they must differ 

 widely from those of the earth. 



The supposition of a mud volcano in the moon includes the 

 belief that water exists in that body in cavities below the 

 surface, if not upon it. But if water is poured forth in com- 

 pany with solid matters, might we not expect to see clouds and 

 vapours ? If the moon has no atmosphere, as is commonly 

 asserted, evaporation from mud or water emitted by a volcano 

 would be very rapid, and if the total quantity was very small, 

 condensation would be inconspicuous even when the lunar 

 temperature declined during the evening and night of our 

 satellite. 



The appearances of Linne are such, that if we are satisfied 

 it was an empty hollow, or crater, a few years ago, we must 

 regard it as having been filled to overflowing with some 

 erupted material. If mud, then much vapour might be ex- 

 pected. If liquid lava, vapour and cloud would, we should 

 suppose, have been formed at the time of the eruption, unless 

 lunar volcanoes differ in their action from those of our earth, 

 or the absence of an atmosphere causes instant evaporation, 

 and precludes noticeable condensation. Actions of this kind 

 might impair the visibility of particular objects while they were 

 in process. 



There are on the moon an immense number of spots, much 

 resembling the present appearance of Linne ; and if they had 

 a similar origin, what are we to suppose has become of all the 

 water vapour that must have been poured forth upon the mud 

 hypothesis ? Under the receiver of an air-pump, Dr. Miller 

 states, that water may be made to boil at a temperature of 

 70°,* and Sir John Herschel thinks it possible that, during 

 the lunar day, the moon's surface u may possibly be heated to 

 a degree much exceeding that of boiling water." 



Sir J. Herschel also says that we are entitled to conclude 

 the non-existence of an atmosphere one 1980th part as dense 

 as that of our earth, as the presence of such an atmosphere 

 would give rise to a refraction that would be observed during 

 the occultation and reappearance of stars. Under such cir- 

 cumstances, if any great quantity of water has at various 

 times been poured forth by volcanoes from internal cavities, it 

 must have been rapidly absorbed, and held fast by chemical 

 action, or an atmosphere of vapour would be detected. 

 * Elements of Chemistry. 



