H. B. Biggs — Composition of Tourmaline. 37 



difficulty. This fact together with the fact that the iron in 

 tourmaline, possibly for some inherent organic reason, as the 

 high and unstable degree of oxidation of the boric acid, oxidizes 

 with unusual ease, has rendered the determination of the condi- 

 tion of this constituent most troublesome On account of its 

 refractory nature, usually not more than 0*2 grams of the finely 

 ground mineral was taken for this determination. Several 

 methods of decomposition were tried without any satisfactory 

 results other than to lead to the conviction that the presence of 

 ferrous iron in large amounts was doubtful, until the digestion 

 of the mineral with hydrofluoric acid, in a sealed platinum cru- 

 cible over the direct flame, was resorted to with fairly satisfactory 

 and conclusive results. 



1st. The mineral was digested with hydrofluoric and sul- 

 phuric acids in a sealed platinum crucible in boiling water from 

 one to seven days, a treatment which decomposes most minerals 

 in the course of a few hours. In a few cases complete decom- 

 position seemed to have been effected, in the greater number 

 however it was at the best very incomplete. In no case was 

 more than one per cent of ferrous oxide found, though between 

 thirty and forty experiments were made, and some of the tour- 

 maline analyzed contain as high as eleven per cent of the metal. 

 These attempted determinations were made in a deep heavy 

 platinum crucible of about 60 c. c. capacity. This crucible is made 

 with a flaring mouth into which fits a platinum head secured in 

 place by a gallows screw clamp. Though the platinum joints 

 are ground, a rubber washer is inserted between the cap and the 

 lip of the crucible to ensure tightness. Before sealing the cru- 

 cible a little carbonate of ammonia is thrown in to expel the 

 air. During the period of digestion the crucible is kept com- 

 pletely immersed in water, so that it is hard to comprehend how 

 the outside air can play any part in the oxidation of the iron. 

 I am accordingly tempted to think that we have here to deal 

 with a very slow reduction of a highly oxidized condition of 

 the boric acid at the expense of which there is a corresponding 

 oxidation of the iron. Either the change goes on very slowly 

 or the explanation proves too much. 



2d. From 0*2 to - 5 grams of the mineral were. treated with 

 sulphuric acid — 4- parts acid to 1 of water — in sealed glass 

 tubes, from one to four days, at temperatures varying from 

 150° C. to 250° C. Although the tubes were taken out of the 

 bath frequently and shaken, decomposition, in none of the 

 fifteen attempts, was more than partial. As even the best 

 acid contains organic matter, sometimes in considerable quan- 

 tities, a higher heat than 200° C. is likely to be attended by a 

 reduction of the acid, thus vitiating the results. This was the 

 method used by Mitscherlich (Journ. prakt. Chemie, lxxxvi, 1) 



