ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 730 



a high refractive index. Size 0-010-0 -030 mm. The separation of 

 the silico-fluoride is not so rapid, nor are the crystals nearly so easily 

 recognized. The phosphomolyhdate greatly resembles in colour and 

 form the platinum double salt, but separates very much more slowly. 

 Cerium sulphate quickly produces a precipitate of a double salt (see 

 under Sodium). 



Sodium. — The reagent used is a concentrated solution of cerium 

 sulphate. If much sodium is expected place near the test-drop one 

 of the reagent, and connect them by a small thread of glass, the latter 

 drop then becomes turbid and under a power of 600 diameters is seen 

 to contain whitish, translucent particles of scarcely 0*002 mm., and 

 if potassium were present also larger spheroids greatly resembling 

 potato starch, size 0*005-0 "008 mm. If less than 1 per cent, of 

 alkaline sulphate is supposed to be present, the two drops are at once 

 allowed to touch each other, and the potassium salt forms in lumps, or 

 occasionally in truncated rhombs, six or eight-sided, while the sodium 

 salt forms short pointed prisms, like the Navicellia, size 0*003- 

 • 005 mm. These are not to be confounded with crystals of the 

 cerium sulphate itself, which have the same form, but are five or six 

 times the size. Any great excess of sulphuric acid must be avoided. 

 The separation of the sodium silico-fluoride is not so delicate (see 

 under Fluorine). 



Lithium. — After precipitating any lime present as gypsum, the 

 lithium is thrown down by addition of an alkaline carbonate. The 

 monoclinic crystals resemble those of gypsum, but are yet quite 

 distinguishable, size 0" 050-0* 075 mm.; they are moreover distin- 

 guished by their solubility in dilute sulphuric acid. Crystalline 

 magnesium double carbonates can only be formed if a large excess 

 of alkaline carbonate is employed. Phosphoric acid may entirely 

 prevent this test for lithium. 



Barium and Strontium. — These exist as sulphates in the insoluble 

 residue left in the platinum spoon. The residue is heated wi'h con- 

 centrated sulphuric acid, and the solution brought by a capillary 

 pipette on to a slide. On cooling and absorbing water the crystalline 

 sulphates separate. Barium sulphate forms small crossed lens-shaped 

 crystals, size 0*005-0*012 mm. Strontium sulphate separates after 

 the barium salt, the crystals likewise form crosses, but are distinguished 

 by their greater complexity and size, viz. 0" 020-0 '045 mm. If 

 much calcium is present in the mineral, gypsum crystals will be 

 formed, they appear last of all, and in their usual forms. Lead would 

 also appear here, the crystals have the same size as those of barium 

 sulphate, but the form of strontium sulphate. 



Magnesium. — To the test-drop is added a little ammonium chloride 

 and ammonia until alkaline, and left a minute or two for any iron 

 and manganese present to oxidize. At one cm. from this drop is 

 placed a drop of water containing a fragment of microcosmic salt, the 

 two drops are connected by a thread or two of glass. The crystals 

 are very characteristic, being hemimorphous, if formed quickly 

 peculiar skeleton growths of 0*060 mm. result, but if formed slowly 

 only well-defined crystals of 0*010-0*020 mm. 



