106 Evolution of the Earth. 



phurous and hydrochloric acid gases ; the former soon takes into 

 composition the steam already formed, and sulphuric acid in great 

 quantities is produced. Acid vapors therefore constitute a large 

 and important part of the hot atmosphere. Beneath these the 

 oxygen unites also with calcium, magnesium, silicon, aluminium, 

 potassium, sodium, etc., producing a dry solid crust as a boundary 

 between the hot liquid or viscous matter within and the hot mix- 

 ture of gases without. It is well known that silica at very high 

 temperature behaves like an acid; it would therefore form sil- 

 icates with the alkalies and alkaline earths just named. The free 

 hydrogen is in time all used up in combination, and the cooling 

 water-gas at the outside is reduced to its condensing point. It 

 forms an envelope of clouds such as now float in the atmosphere 

 of Jupiter and Saturn. With still further cooling these become 

 precipitated as mist, which descends into the hotter regions below 

 and becomes charged with acid. Reaching the solid crust some of 

 it is re-vaporized with more or less explosive violence, and carries 

 back to the outside the heat which has been imparted to it from 

 the crust. At the same time the acids are largely taken up by the 

 earthy and alkaline silicates, producing sulphates and chlorides 

 of these, while silica is precipitated. The silicates of iron and 

 aluminium are not easily decomposed; hence the clays remain af- 

 ter the rest have been broken up into sulphates, chlorides and pre- 

 cipitated silica. 



The law of diffusion of gases, though it never ceases to be op- 

 erative, is generally masked by other laws. The process just de- 

 scribed implies convection on an immense scale, so that diffusion 

 may be largely thrown out of account after the first stages of con- 

 densation are passed. Acid rain-storms continually recur, assum- 

 ing a degree of constancy and violence far in excess of anything 

 conceivable at the earth's surface to-day. Meanwhile the cooling 

 earth has been contracting unequally over different areas, accord- 

 ing to the variation in conductivity of its materials for heat. 

 Some large areas of the crust thus become regions of depression 

 in comparison with those of less conductivity. When the sur- 

 face temperature of the solid falls below the condensing point 

 of the acid vapors, the rain falling on the hot ground dissolves all 

 soluble compounds, and gathers into the great depressions, form- 

 ing oceans of water that are from the outset strongly charged with 

 salts. The leaching process continues even to the present day; 

 but it is highly improbable that the saltness of the ocean, or even 

 of inland seas, has been appreciably modified by the addition of 



