SOLUTION OF COPPER IN AMMONIA LIQUOR. 119 



remaining ammonia solution becomes blue, now that the air too 

 has got access to the copper. 



Copper is thus dissolved in large quantity in manufacture by 

 the simultaneous action of air and ammonia solution, the blue 

 solution of copper being employed for preparing certain kinds of 

 waterproof paper, canvas, and such like materials, by what is called 

 the " Willesden process." 



We have already had illustrations of the solution of solids 

 where the chemical action taking place resulted in the evolution 

 of gas: thus, in Expts. 99 and 104, the solution of chalk and 

 of zinc in hydrochloric acid produced effervescence, due to the 

 formation of carbon dioxide and of hydrogen gases, in each 

 case respectively, and in Expt. 13 sulphuretted hydrogen gas 

 was evolved by the action of hydrochloric acid on sulphide of 

 iron. The following experiments further illustrate this kind of 

 action. 



Expt. 115. To produce Carbon Dioxide Gas from Barium Car- 

 bonate. Chalk and limestone consist of carbonate of calcium (or 

 carbonate of lime, both names being familiarly employed, the 

 former being somewhat the more scientifically exact), which for 

 present purposes may be regarded as a compound of quicklime and 

 carbon dioxide ; so that when limestone is heated, it breaks up or 

 decomposes into carbon dioxide, which escapes as a gas, and quick- 

 lime which remains as a solid, this operation being the essential part 

 of the manufacture of quicklime from limestone, ordinarily termed 

 lime burning. Conversely, when lime is dissolved in water and 

 carbon dioxide gas brought in contact with the solution, the lime 

 and carbon dioxide reunite and produce carbonate of lime, chemi- 

 cally identical with chalk and limestone (Expt. 152). Barium 

 carbonate is a mineral somewhat resembling limestone; when acted 

 upon by hydrochloric acid, it is decomposed in a similar fashion ; 

 in the case of limestone, carbon dioxide gas is evolved, and the 

 lime becomes transformed into a soluble substance called chloride 

 of calcium, which remains dissolved in the fluid formed in the 

 carbon dioxide generator (Expt. 99) ; in the case of barium carbonate, 

 carbon dioxide is also evolved, whilst the fluid contains in solution 

 a substance analogous to chloride of calcium, called chloride of 

 barium or barium chloride. 



Treat barium carbonate with hydrochloric acid diluted with 

 water ; when the effervescence has ceased, filter the liquid (Expt. 

 56), and evaporate it to a small bulk ; on allowing it to cool and 

 stand crystals of barium chloride will be formed. 



In the same kind of way, by treating barium carbonate with 

 diluted nitric acid, carbon dioxide gas will escape, and a liquid 



