ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 461 



Uranium sulphate assumes the most beautiful forms of all the 

 metals ; a 4 per cent, solution is used, and at least twelve hours 

 are necessary to produce the desired formation. It can readily be 

 recognized with a pocket lens, and resembles beautifully coloured 

 asters or corn-flowers. Less frequently it occurs in the form of 

 envelopes with velvet-blue, narrow, and purple-coloured broad 

 triangles, which may also be recognized without the Nicol, and 

 therefore are not produced by polarized light, but result from the 

 mechanical arrangement of the crystals. 



The mercuric sulphate is soluble with difficulty, but it can easily 

 be brought into solution by the addition of a few drops of nitric acid. 

 It forms figures similar in shape to a Maltese cross, of superimposed 

 scales, which are very unstable. 



Silver may easily be determined, and in such a way that it is not 

 easily mistaken for any other metal. A drop of a 2 per cent, 

 solution of silver sulphate deposits bright points which may be 

 detected with the naked eye ; at 0° these appear as complete rhombic 

 octahedrons, with the edges cut off, at 90° they glisten with the most 

 beautiful play of colours, like the diamond ; at times groups are 

 formed which seem exactly like a set of diamond jewellery. 



Preparing Thin Slices of Rocks and Minerals. — Dr. A. Geikie, 

 the Director-General of the Geological Survey of Great Britain and 

 Ireland, in his ' Outlines of Field-Geology,' * deals with the advan- 

 tages of microscopical investigation in the study of minerals and rocks 

 by which means we are enabled to trace the minuter structures of the 

 earth's crust, and to follow many of the stages in the formation of its 

 rocks. We can tell which mineral of a rock crystallized first, and, 

 indeed, can follow eveiy phase of crystallization in such a way as to 

 explain many otherwise unknown parts of the history of the rocks. 

 Moreover, by this method we can trace the subsequent changes which 

 rocks have suffered in the chemical alteration of their minerals by 

 percolating water, with the resulting secondary products. While a 

 chemical analysis informs us of the ultimate chemical constitution of 

 a rock, a microscopic analysis brings before us its mineralogical com- 

 position, showing in what forms the chemical elements have been 

 combined, and how diverse two rocks may be in structure and textui'e, 

 though in chemical composition nearly alike. 



A cutting machine will greatly facilitate the process of preparing 

 rock slices. The thickness of each slice must be mainly regulated by 

 the nature of the rock, the rule being to make it as thin as can be 

 conveniently cut, so as to save labour in grinding down afterwards. 

 Perhaps the thickness of a shilling may be taken as a fair average. 

 This thickness may be still further reduced by cutting and polishing 

 a face of the specimen, cementing that on glass, and then cutting as 

 close as possible to the cemented surface. The thin slice thus left on 

 the glass can then be ground down with comparative ease. 



Excellent rock sections, however, may be prepared without any 



* 3rd ed. 1882 (Macmillan & Co.), pp. 30, 201-15. See also ' Test-Book of 

 Geology' by the same author (Macmilh.n & Co.), 1882, pp. 94-108, 182-91 (fio-s.). 



