Action of the Atmosphere iqion newly -deepened Soil. 459 



ciated frequently with carbonate of lime ; and that the soluble 

 silica in these rocks is dissolved out with perfect ease by boiling 

 them in solution of potash or soda ; and they point out tliat soils 

 derived from these strata are celebrated for their fine crops of 

 wheat and the strength of the straw grown upon them. 



Professor Fuchs has shown that silica exists in minerals in 

 two conditions, a crystallized and an amorphous one, and that in 

 the latter it is much more readily acted upon by solvents than in 

 the former. 



A communication was made to the Royal Society by Julius 

 Jeffreys, to the effect that siliceous materials exposed to the 

 action of steam at high temperature were partially consumed, and 

 a siliceous crust deposited on several vessels placed in the upper 

 part of the furnace, which crust was re-dissolved when again 

 subjected to a hotter steam. {Proceedings of Royal Soc, iv. 233.) 

 Dr. WoUaston also had observed that steam under high pressure 

 becomes a rapid solvent of alkaline silicates ; and Dr. Turner 

 found that glass exposed to the vapour issuing from a high-pres- 

 sure engine was rapidly corroded, rock crystal remaining un- 

 changed, and that the silica taken up was again deposited in a 

 beautiful stalactitical form. {Proceedings of Geolog. Soc, ii. 95.) 

 Turner also showed that rain-water must have the power of dis- 

 solving silica by contrasting the chemical composition of felspar 

 with that of the porcelain clay, or kaolin, which results from its 

 decomposition, pointing out that the water had carried off in 

 some way all the potass, and 8^ out of 12 proportions of silica, 

 leaving all the alumina and the remainder of the silica untouched. 

 {Phil. Mag. 1833, vol. iii. p. 20.) Dr. MacCulloch also found 

 an interesting case of the artificial sublimation of silica by a 

 strong heat, in whicli it was deposited in the state of fine crystals, 

 but he was unable to reproduce the results. {Trans, of Geological 

 Soc, 1st series, ii. 275.) 



When glass contains 70 per cent, of silica and 30 per cent, of 

 potash or soda, it becomes soluble in boiling water, and may be 

 spread over any surface like varnish, the solubility of it varying 

 according to the proportion of the alkali it is combined with. 

 When a silicate is dissolved in water it is easily decomposed by 

 the addition of an acid, even the carbonic, which displaces it from 

 the base with which it is united, leaving it free. The resulting 

 precipitate of silica has a gelatinous appearance, and is to a cer- 

 tain extent soluble in even pure water ; if this gelatinous sub- 

 stance, or hychate of silica, is dried, it presents the aj)poarance 

 of a white powder, and loses altogether its solubility on Ix'ing 

 strongly heated. The decomposition of the silicates of the soil 

 is therefore probably effected by the action of the carbonic acid, 

 which has been shown to be so plentifully generated therein, and 



