ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 459 



appear entirely. If we mix a saturated aluminium oxide solution in 

 potassium hydrate with sufficient water to produce a 2 per cent, 

 solution, and place a drop or two of it on the slide, then mix the 

 sample with a drop of a 1 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 prism 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 aluminium oxide with a drop of bicarbonate solution, the 

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

 alumina assumes lengthy forms which, when covered with a mica 

 plate, seem blue, while the dendrites of silica are seldom coloured. 



Glucina may be very easily distinguished microscopically from 

 both of the preceding earths. A drop of a 4 per cent, solution of 

 giucium sulphate, when evaporated on the slide, leaves large stars, 

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

 spread themselves over the entire surface of the drop. The star in 

 the centre, when the prism is at 90°, exhibits prismatic colours, the 

 leaves appear of a dull silver white or brownish colour, and they are 

 often perforated. 



Boric acid is likewise very easy to detect, for from its 2 per 

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

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

 magnified eighty times, do not show any cross. If the residue of the 

 boric acid be moistened with a drop of the 2 per cent, solution of 

 sodium bicarbonate, the dried drop will be found to consist of beautiful 

 polarizing spheres, which in their centre enclose a small white cross ; 

 this, on turning the Nicol prism, also revolves. Occasionally dendritic 

 stars instead of the spheres are formed. 



The alkalies possess such optic properties that they can be definitely 

 and certainly distinguished by the Microscope. In making these 

 tests it is best to employ the sulphates for the examination, as they 

 are the most constant in their composition, and in the drying the 

 samples will not absorb moisture from the air, and so produce forms 

 which may readily be recognized. Four per cent, solutions were 

 made of the alkalies soluble in water. 



The test with potassium sulphate gives, at 0° of the Nicol, a series 

 of rhombic plates, which are not very well defined ; at 90° blue rims 

 with yellow or red spots are developed; these cannot be taken for 

 any other alkali. 



Sodium sulphate will be recognized as soon as it becomes dry by 

 its precipitation. In the darker field of the Microscope it appears 

 dull, and silvery- white in hopper- shaped quadratic crystals. 



The ammonium sulphate assumes such peculiar shapes that it 

 cannot be mistaken for any other salt. At 0° the crystals are hardly 

 recognizable ; at 90° they appear like partly decomposed walls built 

 of grey blocks, with blue and brown rims. 



Lithium sulphate forms clusters of prismatic needles which at 0" 

 show beautiful colours and a blue cross, which at 90° becomes black. 



