Action of the Atmosphere upon neichj -deepened Soil. 433 



constant although slow evolution of carbonic acid, which is ab- 

 sorbed by the moisture contained in the soil, and exerts a solvent 

 action on its constituents. In fact, although a very feeble acid, 

 carbonic acid, by its continuous action, is constantly effecting the 

 solution of new quantities of the constituents of the soil." (^Enc. 

 Brit., 8 ed. ii. 395.) 



On the backs of veins containing sulphuret of lead, exposed to 

 the air, tlie carbonates of that metal are often found. A pot of 

 Roman copper coins was got, a few years since, in Cardiganshire, 

 not far from the surface, and the coins themselves had been 

 exposed to the action of atmospheric influences ; the waters, con- 

 taining common air and carbonic acid, finding their way to them, 

 had produced the red oxide of copper on the surface of some of 

 the coins, beautifully crystallised, while in others the further 

 change into the carbonate had been so effected as to present the 

 usual mamillated character of malachite. Illustrative specimens 

 of these coins are now in the Museum of Practical Geology. 

 Many ancient bronzes shew similar changes. In the refuse of 

 the old Derbyshire mines the small fragments of sulphuret of 

 lead are found wholly changed into the carbonate, and a similar 

 action is observed upon the sulphuret of zinc. {De la Beche.) 



Aqua regia is known as about the most powerful solvent we 

 have, dissolving even gold and platinum ; but the long- continued 

 action of water containing carbonic acid will dissolve what even 

 aqua regia fails to do in a short time, as the following experi- 

 ment by i-'olstorff and Wiegmann clearly shows. Tliese chemists 

 boiled some white sand with a mixture of nitric and muriatic 

 acids, and, after completely removing the acid by washing the 

 sand with water, they exposed it, thus purified, to the action of 

 water saturated with carbonic acid gas. After the lapse of 30 

 days this water was subjected to analysis, and was found to 

 contain in solution silica, carbonate of potash, and also lime and 

 magnesia ; thus proving that the silicates contained in the sand 

 were unalde to witlistand the continued action of water con- 

 taining cart)onic acid, although the same silicates had resisted 

 the short action of the aqua regia. And it is also known that 

 felspar is unable to withstand the solvent action of water 

 saturated with carbonic acid, although it is scarcelv affected by 

 being left in contact with cold muriatic acid for 24 hours. 

 (Lichif/, Cliem. of A(/riculliirc, ord ed. p. 92.) 



The air contained in the pores of the soil is often propor- 

 tionally much richer in this gas than tlie atmosphere, espe- 

 cially when it contains much decaying organic matter, the 

 earthy substance having the property of condensing the carl)onic 

 acid within its interstices. I^oussingault, investigating this 

 subject in lbo2, found tliat in soil rceentlv niaiuued the air frc- 



