424 COMPOUND SALTS 



12. Ammonio-phosphate of soda. This salt, 

 If soda! e which has been so long known under the name 

 of microcosmic salt, and is so useful to mineralo- 

 gists as a flux in experiments made by the blow- 

 pipe, is easily obtained by mixing solutions of 

 phosphate of soda and phosphate of ammonia in 

 the atomic proportions, and concentrating the 

 mixture. The compound salt crystallizes in fine 

 four-sided rectangular prisms, terminated by 

 rectangles ; the solution yields these crystals to 

 the very last drop, showing that the salt contains 

 just an integrant particle of each constituent. It 

 has a saline and cooling taste, and dissolves with 

 facility in water ; when heated it melts in its 

 water of crystallization ; if the heat be continued 

 the water is dissipated ; in a red heat the ammo- 

 nia likewise escapes, leaving biphosphate of soda, 

 which melts into a transparent glass, dissolves in 

 water, and reddens vegetable blues. 20 grains 

 of the crystals of this salt, when thus treated, 

 lose 13 grains of their weight. Now, 11 is the 

 atomic weight of anhydrous biphosphate of soda ; 

 and 7 : ] 1 : : 13 : 20-43 = water and ammonia 

 combined with 11 of anhydrous bisulphate of 

 soda to constitute microcosmic salt. The am- 

 monia amounts to 2-125, which, added to 11, 

 make 13-125; and 20-43 2-125 = 18-305. 

 Thus it appears, that ammonio-phosphate of soda 

 is composed of 



