194 DIRECTIONS FOR CONDUCTING 



standing a few hours, put this also upon . filter^ and 

 wash the acid liquid through. 



a. Wash the residue upon the filter witn successive 

 portions of clear water, until it no longer tastes acid ; 

 it may then be burned until all of the organic, part is 

 consumed, and weighed when it is cool. This weight 

 gives the percentage of insoluble siliceous matter in 

 the soil. 



b. To the filtered acid solution, is first added am- 

 monia (common aqua ammonia), till it is no longei 

 acid but alkaline; a flocculent precipitate then imme- 

 diately falls, being iron and alumina. If it is of t 

 deep red color, then iron predominates, and the con- 

 trary if it is nearly white. If the precipitate has a 

 whitish green color, and reddens when exposed to the 

 air, then the soil contains the protoxide of iron, in 

 place of the peroxide. The first, it will be remem- 

 bered, was spoken of on p. 62, as injurious to plants. 

 It is for this reason important to know which oxide is 

 present. 



If it is shown by the above test to be the protoxide, 

 the solution must be boiled again with an addition of 

 a little nitric acid : this will convert all of the iron 

 into peroxide, and it will thus remain upon the filter; 

 the protoxide would have been partially washed 

 through. Another filtering is now necessary. Thli 

 should be done as soon as the precipitate has settled, 

 and while the liquid is warm, so that it may filter 

 more rapidly. The whole operation should be done 

 in the shortest practicable time, and the liquid covered 

 as far as possible from access of air. 



From the apparent quantity of the iron and alu- 

 mina, as weighed after burning, may be judgnd with 

 tolerable accuracy the proportion present in tb.2 soil. 



c. If the soil conta.Ded much lime, an efTe "vescence 

 would have been seen at first, when the acid was 

 added : this is supposing the lime to be contained as 



