-4 Ail Account of the Soil [ft. i 



dioxide, a gas breathed out bv ourselves, bv animals 

 and by plants. As aU soil water contains some of this 

 gas the chalk readily dissolves, so much so that in many 

 districts,, especially in chalk districts, the spring and 

 weU waters become very rich in this constituent. It 

 deposits on boiling and forms a fur in boilers, kettles, 

 etc. ; sometimes it even deposits on standing, forming a 

 sediment in the vessel or a crust on any object Ivinor 

 in the water. Chalk is decomposed by strong heat givincr 

 off carbon dioxide gas and leaving hme behind: this is 

 the change that goes on in a hme kiln. Careful studies 

 of the decomposition have proved that 100 parts by 

 weight of pure chalk, after the removal of all impurities, 

 invariably give rise to 56 parts by weight of hme and 

 4A parts by weight of carbon dioxide. This relationship 

 .is very important for it shows how chalk is built up: it 

 may be expressed thus : 



Chalk or calcium carbonate = lime or calcium oxide -i- carbon dioxide 



IW 56 44 parts by weight. 



The effect of carbon dioxide in promoting the solution 

 of chalk is shown by the following experiment: Take 

 some fresh rain-water and divide into two lots of 100 c.c. 

 each : to each add 1 gram of finely powdered quarrv or 

 precipitated chalk. Shake one lot occasionally: blow 

 your breath (which contains carbon dioxide) through 

 the other at intervals during five minutes. Then leave 

 for a Httle time to settle. Pour each hquid through a 

 separate filter, measure 50 c.c. of each into beakers and 

 evaporate on the water-bath. Although the filtered 

 Hquid was perfectly clear it is seen to leave a distinct 

 residue after the experiment, showing that some of the 

 chalk has been dissolved: a larger quantity of residue is 



