1 8 8 2 . ] Microscopy. 6 1 5 



preserved. If we mix a drop of the four per cent, solution of the 

 silica solution with a drop of the one per cent, sodium bicarbon- 

 ate solution, we fail to obtain any plant forms, but find polarized 

 spheres, which, when the Nicol prism is at 90 , exhibit a dark 

 cross, just such as are obtained with calcspar ; on further turning 

 of the prism it seemed to revolve visibly, and at o° almost entire- 

 ly disappears or passes over into a green cross. The most min- 

 ute 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 man- 

 ner previously indicated. 



It is just as easy to microscopically determine aluminum 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 drop of a 

 four 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 o°, the rays of the little 

 spheres appear as if composed of a number of small black grains; 

 piacing it at at 6o°, they appear as two blue rays opposite to each 

 other, which at 90 assume a corresponding position, and on fur- 

 ther turning of the prism disappear entirely. If we mix a satu- 

 rated aluminum oxide solution in potassium hydrate with suffi- 

 cient water to produce a two per cent, solution, and place a drop 

 or two of it on the slide, then mix the sample with a drop of a 

 one per cent, solution of sodium bicarbonate, after evaporation, 

 there will remain a dull white spot, which when still moist shows 

 Peculiar spheres; by means of these alumina can easily and 

 positively be distinguished from silica; for they appear when the 

 Pnsm is at 90 as a white cross whose diagonal axis ends in two 

 round or rhombic scales. If we mix the alkali solution of silica 

 and aluminum oxide with a drop of sodium bicarbonate solution, 

 the silica will appear as silvery, partly closed dendrites, while the 

 alumina assumes lengthy forms which, when covered with a mica 

 Plate, seems blue, while the dendrites of silica are seldom colored. 



Crlucina may be very easily distinguished microscopically from 

 both of the preceding earths. A drop of a four per cent, solution 

 °» glucium sulphate when evaporated on the slide leaves large 

 stars, which may be detected by the naked eye ; whose fern-like 

 'eaves spread themselves over the entire surface of the drop. The 

 st ar in the center, when the prism is at 90°, exhibits prismatic 

 colors, the leaves appear of a dull silver white or brownish color, 

 and they are often perforated. 



■ooric acids is likewise very easy to detect, for from its two per 

 cent, aqueous solution there is obtained; after evaporation, a series 

 very small plates hardly 2 mm. in diameter, which, when they 

 are ma gnified 80 times, do not show any cross. If the residue of 



