. The Origin of Rock-Salt. 29 



question whether any of the saline constituents of the 

 oceans really do make their way into the lithosphere by 

 osmosis, as I hold that they do, we may go on to notice 

 the changes which take place through the medium of the 

 atmosphere. 



Probably no one is prepared to question the statement 

 that, during storms, large quantities of sea-spray are driven 

 far inland by the winds, for the salt crystals outside our 

 window panes after storms give sufficient ocular demonstra- 

 tion of the fact. Nor, it is to be presumed, will anyone be 

 disposed to call in question the statement that salt originat- 

 ing in this manner is driven far inland by the same cause. 

 The use of the spectroscope (to say nothing of evidence 

 afforded by many other methods of observation) proves in- 

 contestably that chloride of sodium does exist, in a diffused 

 state, as salt-dust, and in other forms, throughout a large part 

 of the inner portion of the Earth's atmospheric envelope. 



These facts being taken for granted, we may go on to con- 

 sider what becomes of this atmospheric salt-dust thus derived 

 from the spray of the sea. In this connection we may fit- 

 tingly make reference to the numerous and important experi- 

 ments carried out by Mr John Aitken of Darroch, Falkirk. 

 As is now well known, he has demonstrated that condensation 

 of aqueous vapour cannot take place, except upon solid par- 

 ticles ; and he has further shown that particles, or dust, of 

 particular kinds are much more potent agents than others in 

 bringing about condensation. Amongst those which play the 

 most important part in this respect, dust particles, consisting 

 of chloride of sodium, rank foremost. Mr Aitken points out, 

 therefore, that, in consequence of the fact, there is a constant 

 circulation in progress. Salt is carried inland along with the 

 sea-spray. Then it forms the nuclei of rain-drops, of flakes 

 of snow, or of other condensed forms of HgO, and thus it 

 descends to the land, and is afterwards transported seaward, 

 and so it completes the round. 



That such is really what happens can be easily proved by 

 analyses. Those of rain-water, fresh from the clouds, show, 

 in the most unmistakable manner, that salt-dust is abun- 

 dantly present in the atmosphere, especially in the parts next 



