PHOSPHATES. 431 



PHOSPHATE OF SODA. 



Dr. Pearson, of London, introduced it into medicine, and his 

 process for forming it, is as follows 



1. PREPARATION. 



(a.) To a solution of 1400 grs. crystallized carbonate of soda, at 

 150, in 2.100 grs. of water; add by degrees, 500 grs. phosphoric 

 acid of the sp. gr. 1.85 ; boil, filter while hot, and crystals will con- 

 tinue to form for several days. From the above quantity of materi- 

 als, Dr. Pearson obtained from 1450 to 1550 grs. of crystals. 



(b.) The usual process is to add carbonate of soda in excess, to 

 the impure phosphoric acid,* procured from the decomposition of 

 bone ashes by sulphuric acid. (See phosphate of lime.) The solution 

 is filtered, and crystals are obtained by slow evaporation. f 



2. PROPERTIES. 



(a.) The phosphate of soda of the shops has an excess of alkali, 

 which is said to be essential to the formation of good crystals ; they 

 are rhomboidal prisms, with pyramidal terminations, and their solution 

 turns blue vegetable colors green. 



(b.) Its taste is much like that of common salt. 



(c.) From this circumstance, it is advantageously employed as a 

 purgative, as it may be taken in broth, &c. without disgust, and even 

 without the patient's knowledge. Dose, from six drachms to one 

 ounce. 



(d.) Soluble in about 4 parts of water, at 60, and in 2, at 212. 



(e.) Suffers the aqueous fusion, and loses .62 of water, dries, and 

 melts at a red heat ; by cooling after blowpipe fusion, assumes a poly- 

 hedral form. 



(f.) Effloresces rapidly on the surface. 



(g.) The strong acids decompose it partially, and the free phos- 

 phoric acid forms a very soluble bi-phosphate. 



(h.) With most of the earths, fuses into vitreous compounds, be- 

 ing an excellent flux. 



(i.) In the humid way, baryta, strontia, and lime, attract its acid j 

 it is doubtful whether potassa decomposes it. 



3. COMPOSITION of the dry salt. 



Phosphoric acid, 53.48 100 



Soda, - 46.52 



100.00J: 



* Holding phosphate of lime in solution, which is precipitated. 

 t For processes, vide Black's Lectures, Vol. II, 233 and 4. 

 t Bevzelius, Ann. de Chim. et de Phys. II, 164. 



