ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 737 



solution of ammonium molybdate, of the minerals of the hauyn group 

 by means of sulphur vapour, and of opal by means of a magenta 

 solution. 



In the meanwhile the methods of optical examination were being 

 greatly improved ; the use of gypsum and quartz plates for increasing 

 the double refraction, determiniug the depolarizing directions, and 

 distinguishing between positive and negative double refraction, were 

 borrowed in a complete form from the accessories of zoologists and 

 botanists ; through Tschermak the test of dichroism was applied 

 (1869), and through Descloizeaux the stauroscope of Von Kobell was 

 added (1875). The now tolerably complete instrumental methods 

 were united by Eosenbusch in a convenient form (1876), and rapidly 

 made known by his treatise on the whole subject. New cutting and 

 polishing machines, Microscopes, and accessories were afterwards 

 introduced, and principally from the workshops of Fuess, in Berlin, 

 and Seibert, in Wetzlar. The advantages of the purely optical 

 method of examination are that with a compact apparatus and without 

 any damage to the preparation, it can be examined and determined 

 quickly, and in comparison, simply, in a way impossible with hand 

 specimens. The physical properties at first relied on for the dis- 

 crimination of minerals, and then replaced by Werner and Mohs for 

 the chemical properties, have again, although under altered conditions, 

 become of primary importance in modern micro-mineralogy and 

 micro-petrography. Unevenness of the faces and partial opacity 

 ("milkiness, muddiness "), which interfere so greatly with the use of 

 the goniometer, polariscope, and stauroscope, are removed by the use 

 of thin sections, or nearly so ; cleavage directions, which otherwise 

 have to be sought for with a hammer and chisel, are at once detected ; 

 crystal enclosures can only be completely studied under the Micro- 

 scope, and their constant occurrence in certain mineral species affords 

 a new means of detecting such species, i. e. hauyn, noseau, leucite, 

 quartz, garnet, &c. The success obtained by clever observers by 

 these methods during the last fifteen years has been such as to place 

 on a new basis the study of rocks, but even in these methods much 

 practice is necessary, while in not a few cases, especially where decom- 

 position has set in, in spite of all endeavours deductions can only be 

 regarded with uncertainty. 



The method of acting on a rock powder with hydrochloric acid, 

 and examining before and after treatment mounted in balsam, is not 

 a very successful one. Tho solution of the soluble part can indeed 

 be filtered off and chemically examined, but the uncertainties still 

 remain considerable. E. Boficky was the first to make known a 

 connected system of micro-chemical reactions, he excludes filtration, 

 and his method is simple. It depends on the action of hydrofluo- 

 silicic, or of hydrofluoric acid on small fragments of the mineral or 

 on the rock section itself, the separation of crystalline silicofluorides 

 by evaporation, and the recognition of the several compounds by their 

 form under a magnifying power of 200-400 diams. 



But though the method is capable of rendering service in some 

 cases, yet in others it is very insufficient, and there are considerable 



