ELEMENTS OF COMPOSITION. 59 



phoric acid, like calcium, mentioned in the preceding section. Its amount, 

 however, is everywhere smaller than that of calcium. 



PHOSPHATE OF MAGNESIUM, Mg 3 P 2 8 + 5H 2 0, or MgHP0 4 + 7H 2 . 



We are not yet able to state which of these two salts it is which occurs 

 in the body. Like phosphate of calcium, the corresponding combination of 

 magnesium is met with in all the fluids and solid portions of the body. It 

 is one of the hardening constituents of bones and the teeth, but only in 

 a minor degree. The preponderance of phosphate of magnesium over the 

 corresponding salt of calcium in muscle and the thymus gland (Liehig) is of 

 interest. It is taken up as such from without, and is offered to the body 

 in superabundance by a vegetable diet, so that the greater part of all that 

 is received into the body passes through the intestinal canal unabsorbed. 



PHOSPHATE OF MAGNESIUM and AMMONIUM, MgNH 4 P0 4 + 6H 2 0. 



During putrefactive decomposition, or indeed with every generation 

 of ammonia in the system, the latter combines with phosphate of mag- 

 nesium to form a crystalline salt known as phosphate of magnesium and 

 ammonium. This salt (fig. 38) is found in crystals of rhomboid funda- 

 mental form, and appears most generally 

 in three-sided prisms bevelled at both 

 ends on one of their edges ; this form is 

 known as the "coffin-lid crystal." Further 

 varieties are produced by the bevelling of 

 two polar opposed angles, or finally of the 

 two remaining ones. 



Crystals of phosphate of magnesium and Fig. 38. Crystals of phosphate of mag- 



, v / j n i ,, nesium and ammonium. 



ammonium are to be found in isecal matter, 

 alkaline urine, and all putrefying animal substances. 



CARBONATE OF MAGNESIUM 



Is of very minor importance in animal life. It is met with in the urine 



2CO ) 

 of the vegetable feeders, probably as a bicarbonate M TT > 4 also, per- 



M 8 11 a ) 



haps, in bones. It is very difficult, namely, to determine whether it is 

 the carbonate or phosphate of magnesium that exists in the latter, however. 



CHLORIDE OF MAGNESIUM, MgCl 2 . 

 This salt is said to be present in the gastric juice. 



43. 

 Sodium Compounds. 



While the lime compounds appear, as a class, to possess in part the 

 characters of hardening materials for the animal body, those of soda seem 

 entirely devoid of these qualities as far as we know. On the other hand, 

 however, they appear to play an important part in the chemical occur- 

 rences of the economy, although as yet we have not arrived at satisfactory 

 conclusions in regard to all their purposes. It has been mentioned before 

 (pp. 15-17) that soda is combined with the protein substances of the system, 

 and retains the latter in solution ; also that, combined with the two biliary 

 acids, it forms the most important constituents of the bile (pp. 40 and 41). 



