Pen field — Interpretation of Mineral Analyses. 19 



Aet. III. — On the Interpretation of Mineral Analyses: a 

 Criticism of recent Articles on the Constitution of Toxirma- 

 line ; by S. L. Fen field. 



About eighteen months have elapsed since H. W. Foote 

 and the author published in this Journal* a joint article on the 

 chemical composition of tourmaline. Since that time two 

 articles have appeared, presenting views differing from one 

 another and from those of Foote and the author ; one on the 

 Constitution of Tourmaline, by Prof. F. W. Clarkef of Wash- 

 ington, the other Tiber das Mischungsgesetz der Turmaline, 

 by Prof. G. Tschermak+ of Vienna. 



In order to discuss the views presented in these articles it is 

 desirable to carefully consider some facts concerning chemical 

 analyses. In the first place, a perfect chemical analysis cannot 

 be made. There are, to be sure, a very few analytical processes 

 for the determination of single constituents, which, when care- 

 fully executed, can be relied upon to give results varying less 

 than O'Ol percent from the theory ; but when it comes to num- 

 eral analysis, necessitating the splitting up of a complex body 

 and the determination of a number of constituents, such accu- 

 racy cannot be attained. In making a mineral analysis, one 

 seldom feels confident that all determinations are correct, even 

 within 0*25 per cent of the truth, although if duplicate analyses 

 are made, it is expected that, for the majority of the con- 

 stituents at least, the two determinations will agree within 

 - 10 or 0*20 per cent of one another. At times, of course, 

 depending upon the difficulty of the analysis or the scarcity of 

 available material, variations of 0*50 per cent, or even more, in 

 duplicate determinations are not to be wondered at. 



Secondly, analytical work may be of a high order, the results 

 very accurate, and yet an analysis may not serve for the deter- 

 mination of a chemical formula because made on material 

 more or less impure. The chemists of to-day have a decided 

 advantage over those of a former generation, for the micro- 

 scope enables them to study their material, select that which is 

 best, and, if impurities cannot be avoided, to take their effect 

 into consideration in discussing the analytical results. Then 

 again the heavy solutions are invaluable for separating out 

 material for analysis, and, what is considered of very great 

 importance, for furnishing a guarantee of the purity of any 

 given material ; for if it can be stated that all of the mineral 

 floats on a solution of a certain specific gravity and sinks when 



* Vol. vii. p. 97, 1899. 



f This Journal, IY, viii, p. Ill, 1899. 



± Mineralog. und Petrogi'. Mitth., xix, p. 155, 1899. 



