MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 287 



To the drop to be tested add a fragment of sodium phosphate 

 and a very little magnesium chloride, stir thoroughly. Beside the 

 drop place a drop of dilute solution of sodium hydroxide and 

 cause this drop to flow into the other. 



Ammonium magnesium phosphate separates in crystals having 

 the formula NHLiMgPC^ 6 H 2 0, belonging to the orthorhombic 

 system and exhibiting an exceptionally strong tendency to assume 

 hemihedral, hemimorphic and skeletal forms. This compound 

 usually separates first as an almost amorphous precipitate which 

 soon changes into star-like and X-shaped crystallites. Soon the 

 X's fill out and envelope-like crystals result and at the same time 

 rectangular prisms resembling roofs of houses appear. 



In preparations containing but little of the ammonium mag- 

 nesium phosphate the stars and X's are usually absent. 



Precautions. 



Since the amount of ammonia obtained upon distillation is 

 usually small it is quite necessary to avoid an excess of the mag- 

 nesium salt and also the phosphate, for the reason that magne- 

 sium phosphate is almost sure to be precipitated. This latter 

 salt appears as an amorphous deposit and if conditions are favor- 

 able it may eventually crystallize in star-like crystal aggregates, 

 distinct, it is true, from the ammonium magnesium phosphate, 

 yet very apt to confuse the beginner. 



If the phosphate test be applied directly to a solution of the 

 unknown salt it must be remembered that both phosphates and 

 hydroxides of a number of elements will probably be precip- 

 itated. 



EXPERIMENTS. 



Test as above for the presence of NEU in several different salts containing this 

 radical. 



CALCIUM. 



Crystal Forms and Optical Properties of Common Salts 

 of Calcium. 



A. ISOTROPIC. 



