12 CHEMICAL MANUEES. 



guano, under the name of stercorite. Its solution on standing gives 

 off ammonia. 



4. Monacid PJiosjjliate of Ammonmm is, likewise, found in 

 Peruvian guano. It is formed from the neutral salt through loss of 

 ammonia. It has an alkaline reaction, like all monacid soluble 

 phosphates. The acid phosphates of potassium, of sodium and of 

 ammonium are still more soluble in water than the preceding. Their 

 solutions turn red litmus paper blue. 



5. Magnesium PJiospJiates — (1) Basic PJios2}hate of Magnesium, 

 Mg3(PO^).2, is found in small quantities in seeds. It is ver}' soluble 

 in acids ; it is precipitated from its acid solution by ammonia as 

 phosphate of ammonia and magnesia. (2) Monacid PliosjjJiate of 

 Magnesium. — This, like the corresponding lime compound, is a very 

 soluble salt. (8) Double Pliosphate of Anwionium and Magnesiinn, 

 NH^MgPO^ + 6H.,0, is precipitated from solutions which contain 

 at one and the same time phosphoric acid, magnesia salts and 

 ammonia, as a white crystaUine powder slightly soluble in water 

 and almost insoluble in ammonia water. Ignited, it leaves a 

 residue of pyrophosphate. This interesting compound was found 

 in large crystals as clear as water in excavating the soil for the 

 foundations of the church of St. Nicolas in Hamburg. The name 

 of struvite was given to it. In this locality there was a cesspool, 

 from w^hich the matter spread into the peaty soil. As peat contains 

 a large amount of ammoniacal salts, the phosphate of magnesia 

 from the urine had formed the double phosphate of ammonia and 

 magnesia, which remained in solution for a somewhat lengthened 

 period in the ammoniacal salts in excess. By slow precipitation 

 crystals were formed, some as large as a hazel nut. This same 

 salt is met with in different varieties of guanos, but in much 

 smaller crvstals. Its formation is analogous to that which occurs 

 in putrefying urine : the phosphate of magnesia is brought by the 

 urine, the ammonia results from the putrefaction of the nitrogenous 

 constituents of the guano. 



6. Phosphates of Iron. — The basic phosphates of iron are ^\ddely 

 distributed in nature, but their composition is very variable. 

 They are formed wherever phosphates dissolved by the moisture 

 in the soil come in contact with oxide of iron. In arable land, 

 which as is well known always contains oxide of iron, a portion 

 of the phosphoric acid is in all probability combined with that 

 oxide. The phosphate of the protoxide of iron (ferrous phosphate) 

 is formed in an analogous manner ; it is only reduced to pro- 

 toxide by decomposing organic matter. This product is found in 

 large masses in certain peaty districts. A basic artificial phos- 

 phate is obtained by mixing perchloride of iron with excess of 

 phosphate o£ soda. It possesses a yellowish colour, w^hich turns 

 brown on heating. But the compound so obtained is never pure, 



