1082 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



characteristic of that element. The test afforded by studying the 

 colour of the flame when a small fragment is acted upon by the blow- 

 pipe is often valuable but this, of course, hardly forms part of 

 microscopy. 1 



The discovery of the presence of foreign inclusions in all minerals 

 has led to a remarkable revolution in mineral-chemistry. In earlier 

 days it was customary to analyse a mineral without questioning its 

 purity. Hence the early analyses and the formulae developed there- 

 from express the actual constitution plus the inclusions. Methods 

 have now been invented by which the foreign matter can be removed. 

 Advantage is taken of the difference that is usual between the 

 specific gravity of the mineral and that of its inclusions, the so-called 

 * heavy solutions ' being employed for the separation. 2 Most satis- 

 factory results have been obtained by such means. In cases where 

 the greatest accuracy is necessary, the apparatus designed by Dr. 

 P. Mann had better be employed. 3 It is well microscopically to 

 examine the isolated substance before executing the analysis, for 

 the optical test with polarised light is so sensitive as to detect the 

 smallest impurities. Also, in the case of ordinary bulk analyses of 

 rocks, it is advisable to follow the same course, as by doing so one 

 is often enabled to make a qualitative analysis with the microscope 

 alone. 



A valuable adjunct to petrology is to be found in micro- 

 chemistry. 4 Instances sometimes occur where a mineral cannot 

 be satisfactorily determined by its optical characters, and in such 

 cases micro- chemical methods are resorted to. Let us suppose it is 

 desirable to see whether any of the rock -components are silicates 

 containing soda and soluble in acids. The cover-glass is accordingly 

 removed and the balsam dissolved in alcohol. A weak solution of 

 hydrochloric acid is then poured over the surface, when, if soluble 

 silicates are present, gelatinisation will take place. Upon allowing 

 the gelatinous mass to evaporate little squares of salt will form if 

 such a silicate is present. Sometimes colouring substances may be 

 used for the same purpose. By the treatment of a slide with nitric 

 acid a silicate like nepheline becomes porous and permeable to 

 anilin blue, fuchsin, &c. In the case of nepheline the colouring 

 matter cannot be washed out, and hence * staining ' proves a delicate 

 test. 



Where such a course is possible, minute pieces of the question- 

 able minerals should be isolated and treated singly. There are two 



1 It was suggested by Professor Szabo and is well described in G. A. J. Cole, Aids 

 in Practical Geology, Part ii. ch. viii. 



2 For their mode of preparation see Rosenbusch, Mikroskopische Physiographic, 

 p. 206 et seq. (English edition by Iddings.) 



5 Neues Jahrbuchfiir Mineralogie, &c. Bd. ii. 1884, p. 172. 



4 The following works can be consulted on this subject: E. Boricky, Elemente 

 einerneuenchemisch-mikroskopischen Mineral- und Gesteinsanalyse ,Prague, 1877 ; 

 T. H. Behrens, Mikrochemische Methoden zur Miner alanalyse, Amsterdam, 1881 ; 

 Haushofer, Mikroskopische Beactionen, Braunschweig, 1885 ; Klement et Renard, 

 Reactions microchimigues a cristaux, &c., Bruxelles, 1886 ; Rosenbusch, Mikro- 

 skopische Phyeiographie\ vol. i. 1885, pp. 195-238 (English edition by Iddings) ; 

 F. Rutley, Rock-forming Minerals, London, 1888. A useful summary of a number 

 of microchemical investigations is given by C. A. McMahon, Mineralog. Magazine, 

 vol. x. p. 79. 



