SODIUM AND POTASSIUM CARBONATES. 51 



in the herbivora the carbonates are the more abundant of the two. In 

 animals fed at the same time upon both animal and vegetable food the 

 two kinds of salts are found to be present in nearly equal proportion, 

 and in the same animal either the phosphates or the carbonates may 

 be made to predominate by increasing the relative quantity of animal 

 or vegetable food respectively. This is readily understood when we 

 remember that muscular flesh and the animal tissues generally are com- 

 paratively abundant in phosphates, while vegetable matters, as we shall 

 hereafter see, abound also in salts of the organic acids, which give 

 rise by their decomposition in the system to carbonates of the same 

 bases. 



The alkaline phosphates are taken in with the food, as they exist 

 widely in both animal and vegetable matters. They circulate with the 

 animal fluids, and are finally excreted with the perspiration, the mucus, 

 and the urine. In the urine a portion of the alkaline sodium phosphate 

 is replaced by the acid sodium biphosphate, which gives to this fluid its 

 property of reddening blue litmus paper, although it contains no free 

 acid. The manner in which this change is supposed to take place is the 

 following. A nitrogenous organic acid of new formation, namely, uric 

 acid, makes its appearance in the system, and is excreted by the urine. 

 It exists, however, in this fluid only in the form of combination, as 

 sodium urate. It is, therefore, believed to combine, at the time of its 

 formation, with a portion of the sodium which forms the base of the 

 sodium phosphate; and the remainder of this salt, converted into a 

 biphosphate, is then eliminated by the urine, which thus acquires an 

 acid reaction. 



There is evidence that phosphoric acid is also generated in the inte- 

 rior of the body by oxidation. A substance to be described hereafter, 

 containing phosphorus in the form of organic combination, exists in 

 various parts of the system, especially in the blood and in the tissue of 

 the brain and nerves, and is taken with certain kinds of food ; but no 

 such substance is to be met with in the excretions. In the fluids dis- 

 charged from the body phosphorus exists only in the form of the phos- 

 phatic salts. It is, therefore, without doubt oxidized in the internal 

 transformation of the organic substances, thus becoming phosphoric 

 acid, which in turn unites with the alkaline bases to form neutral or acid 

 phosphates. In this way a certain portion of the superabundant acid 

 is produced, which gives rise to the acid reaction of the excreted fluids. 



The sodium and potassium phosphates, including the acid biphosphate, 

 are discharged with the urine to the amount of about 4.5 grammes per 

 day. 



8. Sodium and Potassium Carbonates, Na 2 C0 3 and E^CO.,. 

 The alkaline carbonates, as mentioned above, are associated with the 

 phosphates in all the more important fluids of the body. They are 

 readily soluble, and assist in producing the necessary alkalescence of 

 the blood and secretions. 



