458 SUMMARY OF CURRENT RESEARCHES RELATING TO 



are either old ones of small angular aperture or supplied with a movable 

 stop to reduce their larger apertures when required. 



Utility of the Microscope in Chemistry.* — In a paper by H. 

 Eeinsch on the detection and separation of certain minerals under 

 the Microscojoe, it is claimed that the use of the Microscope in 

 chemical analysis is not only rapidly increasing, but that it is ap- 

 proaching the spectroscope, and, in some respects, suri)assing it in 

 usefulness. It is admitted, however, that great skill is required in 

 manipulation, and in preparing test objects to verify results, as 

 appearances vary according to the degree of concentration of the 

 solutions used, and different reactions will sometimes be obtained 

 from the same salt. The following are some of the more interesting 

 exjjeriments : — 



Silica, of all substances, yields the most varied and beautiful 

 forms, resembling plants and ferns, often presenting, in the most 

 glowing colours, five-leaved flower-forms in infinite varieties. To 

 obtain these forms, we place a drop of a 4 per cent, solution of 

 potassium silicate on an object slide, and then add a drop of a 2 

 per cent, solution of sodium bicarbonate, and then allow the liquid to 

 evaporate at the ordinary temperature ; after a few hours have elapsed 

 the most beautiful flower-forms will be found spread over the slide, 

 and will be readily recognized by a pocket lens, but when examined 

 by the Microscope with the Nicol at 90°, will exhibit the crystals 

 gleaming with a most magnificent play of colours. By moistening 

 the object with a drop of copal varnish, and covering it with a thin 

 glass, these forms may be i)ermanently preserved. If we mix a drop 

 of the 4 per cent, solution of the silica solution with a drop of the 

 1 per cent, sodium bicarbonate solution, we fail to obtain any plant- 

 forms, but find polarized spheres, which, when the Nicol prism is at 

 90°, exhibit a dark cross, such as are obtained with calcspar ; on 

 further turning of the prism it seems to revolve visibly, and at 0° 

 almost entirely disappears or passes over into a green cross. The 

 most minute traces of silica can, by this means, be readily detected 

 in a mineral, by melting a small sample of the substance with a little 

 potassium hydrate, and dissolving it in a little water, and then placing 

 a clear drop of the solution on an object slide in the manner previously 

 indicated. 



It is just as easy to microscopically determine aluminium oxide 

 as it was to detect the silica. It may be recognized as well from its 

 sulphates as from its alkali solutions. If we place a droji of a 4 per 

 cent, solution on an object slide, and allow it to evaporate, spherical 

 crystals will be obtained, which, turning at 90*^, show a white cross 

 formed of pencils of rays ; if we cover the object with a mica plate, 

 and place the Nicol at 0'^, the rays of the little sjiheres appear as if 

 composed of a number of small black grains ; placing it at GO^, they 

 appear as two blue rays opposite to each other, which at 90° assume 

 a corresponding position, and on further turning of the prism dis- 



* Cf. Journ. Chem. Soc, xlii., Abstracts (1882) p. 245, from Ber. Deutseh. 

 Chem. Gesel., xiv. (1881) pp. 232.5-31. Amcr. Natural., xvi. (1882) pp. 614-8, 

 from 'Scientific American,' Supplemeut 1. 



