340 ABOUT COMMON WATER. 



contact with lime, carbonate of lime will be formed. 

 Knowing this, you can make the following experi- 

 ment : — Drawing your breath inward so as to fill your 

 liiDgs, you breathe, by means of a glass tube, through 

 the lime-water. Before you have emptied your lungs the 

 clear lime-water will have become quite milky, the milki- 

 ness being due to fine particles of carbonate of lime — 

 otherwise chalk — formed by the union of the carbonic 

 acid of your breath with the lime of the water. 



Take a well-corked champagne-bottle, from which 

 the wine has been half removed, but which still re- 

 tains, above the remaining wine, a quantity of carbonic- 

 acid gas. It is easy to devise a means of causing this 

 gas to bubble through lime-water. A heavy white pre- 

 cipitate of chalk is immediately formed. 



We now come to a point of great practical impor- 

 tance. The carbonate of lime exists in two forms: 

 the simple carbonate, of which chalk is an example, 

 which embraces a certain amount of carbonic acid ; and 

 the bicarbonate, which contains twice as much. But 

 the bicarbonate is far more soluble in water than the 

 simple carbonate. Pure water dissolves only an ex- 

 tremely small quantity of the simple carbonate of lime. 

 But carbonic acid is sparingly diffused everywhere 

 throughout our atmosphere, and rain-water always 

 carries with it, from the air, an amount of carbonic 

 acid, which converts the simple carbonate of the chalk 

 into the bicarbonate, of which it can dissolve a con- 

 siderable quantity. Every gallon of water, for example, 

 taken from the chalk contains more than twenty grains 

 of the dissolved mineral. 



By boiling, or by evaporation, this bicarbonate is 

 re-converted into the insoluble carbonate, which renders 

 our flasks of boiled chalk-water turbid, forms incrusta- 

 tions in our kettles, and deposits itself as stalactites and 



