116 PHOTOGRAPHY OF THE LUNAR SURFACE. 



prehendert except on the assumption that they were emitted in a pow- 

 dered state and held in suspension in an atmosphere sufficiently dense 

 to temporarily support them. 



IX.— CAUSES WHICH MAY HAVE LED TO THE DISAPPEARANCE OF 

 THE WATER AND THE ATMOSPHERE. 



The preceding conclusions, taken together, lead to the result that 

 the moon is incapable of preserving in a gaseous state the substances 

 distributed over its surface. Its feeble gravitational attraction would 

 permit the lighter gases, such as hydrogen, etc., to escape if they be 

 raised to a high temperature or be endowed with sufficient kinetic 

 energy. The denser gases, having entered into stable combinations, 

 would have become incorporated with the core. The liquid portions 

 would have disappeared by means of mechanical absorption. The 

 continuity and the efficiency of this tendency are manifested by the 

 phenomena which are daily taking place about us. 



Thus the continually increasing predominance of solid compounds in 

 a gradually cooling body would result in accordance with the most 

 general laws of chemistry. Most saline compounds contain water of 

 combination which they set free on being heated and which they take 

 up again on cooling in a moist atmosphere. This |>henomenon would 

 be limited, both by the complete saturation of the salts and by the total 

 disappearance of free water vapor. These two conditions can be 

 experimentally realized in laboratories. The first is that generally 

 presented on the surface of the earth, but the second would be found 

 on a planet less well provided with water. 



It is even probable that the evolution of the earth on which we dwell 

 will not cease to continue in the same direction. The incessant cycle 

 of changes which water undergoes under our very eyes, and which 

 seems a necessary condition for vegetation and life, is not fixed and will 

 not endure forever. The formation of deposits of rock salt, gypsum, 

 and nitrates effectively abstracts a very considerable quantity of water 

 from circulation. The water uniting with the solid salts can then no 

 longer be separated from them by the ordinary operation of natural 

 forces. What we have said above in regard to the water applies 

 equally well to the other components of the atmosphere. From it have 

 been abstracted the carbon contained in limestone formations and in 

 coal beds, the nitrogen in vegetable soils and in the Peruvian nitrate 

 beds, and finally the oxygen of siliceous rocks. Nothing indicates that 

 this transformation has already been completed, and that it will not be 

 continuously manifested by the lowering of the ocean level and of the 

 barometric column. 



There is, moreover, a special cause operating to abstract water from 

 circulation apart from chemical action. The major part of the terres- 

 trial core is formed, as is well known, of permeable rocks. The water 

 carried down by rain filters through them, saturating them more or 



