Penfield and Foote — Composition of Tourmaline. 125 



gases were first freed from all traces of oxygen by conducting 

 them over a roll of heated bright copper gauze at one end of 

 the tube. Decomposition is thus readily effected without any 

 appreciable oxidation. After fusion the material can be dis- 

 solved in dilute sulphuric acid and titrated with potassium per- 

 manganate. With small quantities of iron the method gives 

 very satisfactory results, but ferric iron suffers a slight reduc- 

 tion during fusion, hence the method can not be recommended 

 as a quantitative one when accuracy is required. The amount 

 of reduction is nearly proportional to the amount of ferric iron, 

 and when about 6 per cent of Fe 2 3 is present the error 

 amounted to about 0*3 per cent. It is certain, however, that very 

 satisfactory determinations of ferrous and ferric iron might be 

 obtained by means of this method, by applying a correction 

 which could be determined by experimenting with some arti- 

 ficial mixture of silicates containing approximately the same 

 amount of ferric oxide as the mineral. It is difficult to under- 

 stand the cause of this reduction, which was first noted by 

 Rammelsberg,* and afterwards carefully studied by Suida.f 



Fusion with borax furnishes, however, a most excellent 

 means for decomposing insoluble silicates, when qualitative 

 tests for ferrous and ferric iron are desired. The application 

 of the method is as follows : The mineral and some borax glass 

 are heated in a closed-tube over a Bunsen-burner flame until 

 decomposition is complete, or nearly so. Water is then 

 dropped upon the hot glass in order to crack it, and, when cold,- 

 the end of the tube is broken up and transferred to a test-tube. 

 About 3 CC of dilute hydrochloric acid are then added, and the 

 acid is boiled vigorously until the material is dissolved, when 

 about 5 CC of cold water are added. The liquid, after being 

 divided into two portions, may then be tested with potassium 

 ferricyanide for ferrous iron, and with either ammonium thio- 

 cyanate or potassium ferrocyanide for ferric iron. The results 

 are very satisfactory and decisive, and although there is a 

 tendency for ferric iron to undergo slight reduction during 

 fusion, and also a tendency for ferrous iron to undergo slight 

 oxidation during the manipulation, these defects are so trifling 

 that practically they may be disregarded. 



Mineralogical-Petrographical Laboratory, 

 Sheffield Scientific School, January, 1899. 



* Zeitschr. der deutsch. geolog. Gesell., xxiv, p. 69, 1872. 

 f Min. Mittheilung, Tschermak, 18*76, p. 175. 



