MICROCHEMISTRY. 65 



slowly soluble in acetic acid, but readily so in nitric and 

 hydrochloric acids, of course without any evolution of gas. 



In sulphuric acid they are quickly dissolved with the for- 

 mation of gypsum needles. If sections are quickly heated 

 on the slide in a mixture of two parts concentrated sul- 

 phuric acid to one part water, the masses of gypsum needles 

 then formed show the same outline as the sphaerocrystals 

 previously present. They may be distinguished from the 

 latter by being wholly opaque and therefore black by trans- 

 mitted light, and white by reflected light. But if the sec- 

 tions are placed in dilute, e.g., i#, sulphuric acid, the gyp- 

 sum needles are gradually formed in the vicinity of the. 

 sphaerites. 



On burning, the calcium phosphate sphaerites at first 

 become black in consequence of the organic admixture to 

 be mentioned in the next section, but on further heating 

 they yield a pure white ash. 



With nitric acid and ammonium molybdate, as well as 

 with magnesium sulphate and ammonium chloride, they 

 give the reactions for phosphoric acid (cf. 77)- 



When examined with a polarizing microscope these 

 sphaerocrystals show the well-known dark cross, with crossed 

 nicols. By the interposition of a gypsum plate it can be 

 determined that the orientation of the optical axis is the 

 same in them as in starch-grains and in the sphaerocrystals 

 of inulin. 



In Canada balsam these sphaerites may be preserved for 

 as long as one wishes, and in glycerine gelatine at least for 

 a considerable time. 



97. The various sphaerocrystals do not represent an even 

 approximately chemically pure compound, but always con- 

 tain a considerable quantity of organic substance, which 

 often forms an amorphous nucleus at the centre of each, 

 but is also often contained in the separate layers. It is to 

 be ascribed to this circumstance that calcium phosphate 

 sphaerites take up pretty freely various coloring matters 

 like methylene blue and borax-carmine (cf. Leitgeb III). 

 The chemical composition of these organic substances is 



