THE PAST HISTORY OF OUR MOON. 93 



sulphur in the form of acid gases ; which, with nitrogen, 

 watery vapour, and an excess of oxygen, would form an 

 exceedingly dense atmosphere, The resulting fused mass 

 would contain all the bases as silicates, and would probably 

 nearly resemble in composition certain furnace-slags or 

 basic volcanic glasses. Such we may conceive to have been 

 the nature of the primitive igneous rock, and such the com- 

 position of the primeval atmosphere, which must have been 

 one of very great density. 1 All this, with the single exception 

 of the italicised remark, may be applied to the case of the 

 moon. The lunar atmosphere would not probably be dense 

 at that primeval time, even though constituted like the ter- 

 restrial atmosphere just described. It would perhaps have 

 been as dense, or nearly so, as our present atmosphere. 

 Accordingly condensation would take place at a tempera- 

 ture not far from the present boiling-point, and the lower 

 levels of the half-cooled crust would be drenched with a 

 heated solution of hydrochloric acid, whose decomposing 

 action would be rapid, though not aided as in the case of 

 our primeval earth by an excessively high temperature. 

 'The formation of the chlorides of the various bases and 

 the separation of silica would go on until the affinities of 

 the acid were satisfied/ 'At a later period the gradual 

 combination of oxygen with sulphurous acid would eliminate 

 this from the atmosphere in the form of sulphuric acid.' 

 ' Carbonic acid would still be a large constituent of the 

 atmosphere, but thenceforward (that is, after the separation 

 of the compounds of sulphur and chlorine from the air) 

 there would follow the conversion of the complex alu- 

 minous silicates, under the influence of carbonic acid and 

 moisture, into a hydrated silicate of alumina or clay, while 

 the separated lime, magnesia, and alkalies would be changed 

 into bicarbonates, and conveyed to the sea in a state of 

 solution.' 



It seems to me that it is necessary to adopt some such 

 theory as to the former existence of lunar oceans in order to 

 explain some of the appearances presented by the so-called 



