THE ASH CONSTITUENTS OF PLANTS. 217 



made the determination directly in the crucible, but employed the wash 

 bottle {(j) of his apparatus (p. 212) in collecting the carbonic acid. 



ESTIMATION OF SILICIC ACID. 



After driving off the carbonic acid the contents of the generating 

 tiasK are transferred to a porcelain dish, evaporated to dryness on a 

 water bath with aqua regia, dried at 105 to 110^, as in' analysis of sil- 

 icates, moistened with concentrated hydrochloric acid, and after a half 

 hour heated with warm water. The silicic acid and any unconsumed 

 carbon are filtered out with a Gooch crucible,^ and the filtrate made 

 up to a definite volume (250 to 500 cc). The contents of the crucible 

 are dried at 120^ and weighed for silicic acid and carbon. After incin- 

 eration the usually very small amount of carbon is determined hj 

 difference. 



When silicates not decomposed b}^ hydrochloric acid are formed, the 

 precipitate must be dried at 120° and collected on a filter previously 

 dried at that temperature, and weighed for silicic acid and carbon, 

 after which it should be incinerated, weighed, and the residue sub- 

 tracted from the previous weight to determine the carbon. The resi- 

 due from the above, containing silicates, is evaporated with sulphuric 

 acid and h3^drofluoric acid or ammonium fluoridto free the silicic acid, 

 and then weighed as sulphate. The sulphuric acid must be determined 

 in the latter and subtracted, in order to determine the bases of the 

 silicates to be deducted from the weight of crude silicic acid, giving 

 the pure silicic acid. 



All this is a complicated operation and requires much time and care. 

 It is important, therefore, to avoid the formation of difiicultl}' decom- 

 posable silicates. This is accomplished b}^ employing the lowest pos- 

 sible temperature for incineration, and by mixing sodium carbonate, 

 lime, or baryta with the substance. In the writer's laboratory the 

 formation of silicates is avoided by the use of the Shuttleworth or 

 Tucker apparatus and by the addition of calcium acetate. 



SEPARATION OF SAND AND SILICIC ACID. 



The silicic acid which is obtained by the above-described methods is that 

 which is present in the substance as amorphous silicic acid. Crystallized 

 silicic acid and silicates may also be present in the substance examined, 

 especially with roots, in the form of sand or earth adhering to theuL 



For the separation of this "sand" from the true silicic acid of the 

 plant, various means have been suggested. Prominent among these is 



' 111 the methods of analysis adopted by the Association of Official Agricultural 

 Chemists, November, 1S98, p. 77, there is descril)ed a hardened filter of Schleicher 

 andSchiill. The silicic acid, sand, and carbon are washed from this into a iilatinum 

 dish and before incinerating are boiled with a saturated solution of sodium carbonate, 

 sodium hydroxid added, and the Hiii<"ic a(-id and sand sejiarated. 



