Co.MI'orNDs OF CARBON WITH OXYGEN AND NITROGEN 375 



significance in nature, because by this means water acquires the 

 property of destroying and dissolving many substances which are not 

 acted on by pure water; for instance, calcium phosphates and carbonates 

 are soluble in water containing carbonic acid. If the water in the 

 interior of the earth is saturated with carbonic acid under pressure, 

 the quantity of calcium carbonate in solution may reach three grams 

 per litre, and on issuing at the surface, as the carbonic anhydride 

 escapes, the calcium carbonate will be deposited. 10 Water charged 

 with carbonic anhydride brings about the destruction of many rocky 

 formations by removing the lime, alkali, &c., from them. This process 

 has been going on and continues on an enormous scale. Rocky 

 formations contain silica and the oxides of various metals, amongst 

 others oxides of aluminium, calcium, and sodium. Water charged 

 with carbonic acid dissolves both the latter, transforming them into 

 carbonates. The waters of the ocean ought, as the evolution of the 

 carbonic anhydride proceeds, to precipitate salts of lime ; these are 

 actually found everywhere on the surface of the ground in those places 

 which previously formed the bed of the ocean. 



The presence of carbonic anhydride in solution in water is essential 

 to the nourishment and growth of water plants. Although carbonic 

 anhydride is soluble in water, yet no definite hydrate is formed ; 11 

 nevertheless an idea of the composition of this .hydrate may be 



10 If such water trickles through crevices and enters a cavern, the evaporation will 

 be slow, and therefore in those places from whence the water drips growths of calcium 

 carbonate will be formed, just like the icicles formed on the roof-gutters in winter-time. 

 Similar conical and cylindrical stony growths form the so-called stalactites or pendants 

 hanging from above and stalagmites formed on the bottom of caves. Sometimes these 

 two kinds meet together, forming entire columns filling the cave. Many of these caves 

 are remarkable for their picturesqueness ; for instance, the cave of Antiparos, in the 

 Grecian Archipelago. This same cause also forms spongy masses of calcium carbonate 

 in those places where the springs come to the surface of the earth. It is therefore very 

 evident that a calcareous solution is sometimes capable of penetrating plants and filling 

 the whole of their mass with calcium carbonate. This is one of the forms of petrified 

 plants. Calcium phosphate in solution in water containing carbonic acid plays an im- 

 portant part in nourishing plants, because all plants contain both lime and phosphoric 

 acid. 



11 The crystallohydrate COo,8H.,O of Wroblewski (Chapter I. Note 67), in the first 

 place, is only formed under special conditions ; in the second place, it still requires con- 

 firmation ; and in the third place, it does not correspond with that hydrate HoCO^ which 

 should occur, judging from the composition of the salts. 



It is easy to demonstrate the acid properties of carbonic anhydride by taking a long 

 tube, closed at one end, and filling it with this gas ; a test-tube is then filled with a 

 solution of an alkali (for instance, sodium hydroxide), which is then poured into the long 

 tube and the open end is corked. The solution is then well shaken in the tube, and the 

 corked end plunged into water. If the cork be now withdrawn under water the water 

 will fill the tube. The vacuum obtained by the absorption of the carbonic anhydride by 

 an alkali is so complete that even an electric discharge will not pass through it. This 

 method is often applied to produce a vacuum. 



