ANALYSIS OP ARABLE SOILS. 47 



and the water charged with all the soluble portions flows 

 into the vessel destined to receive it. 



We find, by this operation, three distinct products ; 

 first, the precipitate deposited at the bottom of the vase 

 in which the ebullition was performed, consisting of the 

 finest sand ; secondly, that remaining upon the filter, 

 and which consists of a mixture of earths and insoluble 

 salts ; and, thirdly, that which contains in solution all the 

 salts and animal and vegetable substances capable of 

 being dissolved in boiling water. The two first, after 

 they have been dried with c^re, and their weight ascertain- 

 ed, must be examined in order to know the nature and 

 proportions of the substances which compose them. 



I have previously observed, that the deposit constitut- 

 ing the first product, is generally composed only of sil- 

 ica ; if otherwise, it could be tested by acids, which will 

 dissolve all the calcareous portions of it, while those parts 

 which are insoluble may be treated by the means, which 

 I have already pointed out, for separating alumina from 

 silica. 



For the second part, which is the one remaining on 

 the filter, it is sufficient to make an analysis of it, by 

 pouring upon it muriatic acid diluted with four parts of 

 water, till it will effervesce no longer ; this will dissolve 

 the carbonates of lime, and of magnesia, should there 

 be any present, as well as any oxide of iron ; the solu- 

 tion being filtered, any substance not dissolved will re- 

 main on the filter, and must be washed with water, till 

 the water runs off tasteless ; the residuum must be dried 

 and weighed ; it generally consists of alumina, and some 

 animal and vegetable matter. 



In order to ascertain if the muriatic acid has dissolved 

 any oxide of iron, stir it with a bit of oak bark ; if the 

 liquor renders it brown or black, it contains iron ; in or- 

 der to ascertain the quantity, throw into the liquor prus- 

 siate of potash till it will no longer form a blue precipi- 

 tate ; let it settle ; collect the deposit, and heat it to red- 

 ness ; that which remains after this operation is the oxide 

 of iron, and must be carefully weighed. 



When the solution has been freed from the oxide of 

 iron, there remains in it only lime, and perhaps a little 

 magnesia; these can be precipitated by means of a so- 

 lution of carbonate of soda, which must be poured into 

 the muriatic acid till a precipitate is no longer thrown 



