MICROCHEMIS TRY. 5 $ 



of which are illustrated after Haushofer (I, 92) in Fig. 2CV 

 belong to the rhombic system. A similar salt is also formed 

 by arsenic acid. 



A suitable reagent may be obtained by mixing 25 vol- 

 umes of a concentrated aqueous 

 solution of magnesium sulphate, 2 

 volumes of concentrated aqueous 

 solution of ammonium chloride, and 

 15 volumes of water. If there be 

 placed in this solution sections from 

 alcoholic material of the stem of 

 Stapelia picta, which have previously FIG. 20 . - crystals of ammonio 



magnesium phosphate. After 



been soaked for a time in water to Haushofer. 

 prevent the formation of a precipitate by the alcohol, there 

 appear after a time in the immediate vicinity of the sphae- 

 rites of calcium phosphate, in consequence of their gradual 

 solution, well formed crystals of ammonio-magnesium phos- 

 phate, among which the X-shaped skeleton-crystals appear 

 to be especially characteristic. This reaction may be 

 hastened by warming, but the crystals are then less regu- 

 larly formed. 



For the recognition of phosphoric acid within the tissues,, 

 this reaction is, according to Schimper (II, 216), preferable- 

 to the previously described one, since it is not interfered 

 with by the presence of organic compounds and is very 

 delicate. 



8. Silicic Acid, SiO 2 , and the Silicates. 



78. Silicic acid occurs in the vegetable kingdom partly irr 

 incrustations of cell-membranes and partly in the form of 

 variously-shaped silica masses in the interiors of cells (cf.. 

 Kohl's compilation, II, 197). 



For the microchemical recognition of silicic acid, one may- 

 utilize its peculiarity of not being changed by heating. Its. 

 insolubility in all acids except hydrofluoric acid serves to- 

 distinguish it from other inorganic substances. In case of 

 some strongly silicified organs it is possible by the combined 



