CH. iij Carbonates in the ISoil 27 



properties are identical with those of the gas obtained 

 from chalk treated in the same manner, and we can 

 therefore conclude that the gas evolved is carbon dioxide 

 and further that a carbonate is present in the soil. We 

 cannot say what carbonate, because as a matter of fact 

 all carbonates would decompose under the same circum- 

 stances to give carbon dioxide. There is evidence to 

 show that calcium carbonate is the chief one in the soil 

 and it has become customary to speak as if this were 

 the only carbonate, although it is known that others 

 also occur. Thus in forming estimates of the amount of 

 carbonate in the soil it is usual to determine the amount 

 of carbon dioxide evolved and then express this in terms 

 of calcium carbonate. 



The experiment made with the calcium carbonate 

 (Fig. 9) should now be repeated with soil. The precise 

 amount to be used depends on the amount of effer- 

 vescence with the acid : if this is vigorous 10 to 20 grams 

 may be sufificient: if not 25 grams may be needed, while 

 in many cases the method may break down altogether 

 and another has to be adopted. If there is only shght 

 effervescence it is unlikely that the soil contains more 

 than 0-5 per cent, of carbonates, while many soils con- 

 tain less. 



Colhn's calcimeter^ is a more convenient form of appa- 

 ratus for measuring the amount of carbon dioxide given 

 off from soil, and therefore the amount of carbonate 

 present. 



The calcium carbonate in the soil arises from several 

 sources. The huge masses of chalk represent the remains 

 of minute sea animals, as may be seen by examining 



^ See Journ. Soc. CJmri. Ind. 1906, 25, 518. The apparatus is made 

 by Messrs Brady and Martin, Newcastle-on-Tyne. 



