412 OF SECRETION. 



by old persons, a fact which corresponds with other indications of the 

 far greater rapidity of interstitial change in the earlier periods of life, 

 than in adult or advanced age. 



732. There is an organic compound, nearly allied to Urea in compo- 

 sition, but differing from it in its distinctly acid properties, and also in 

 its comparative insolubility. This substance, termed Uric or Litliic 

 Acid, forms but a small proportion of the solid matter of Human Urine 

 in the state of health ; but it is the chief element in the Urine of the 

 lower Vertebrata ; and its presence in too large a proportion is a fre- 

 quent source of disease in Man. Its ultimate composition is 10 Car- 

 bon, 4 Hydrogen, 4 Nitrogen, 6 Oxygen ; it crystallizes in fine scales 

 of a brilliant white colour and silky lustre ; and it is so sparingly solu- 

 ble in water, that at least 10,000 times its own weight of fluid is re- 

 quired to dissolve it. In healthy Human urine, it is in a state of perfect 

 solution, but it is precipitated by the addition of a small quantity of any 

 acid, even the Carbonic : it is evident, therefore, that it is held in solu- 

 tion by union with some base, and it seems probable that this base is 

 ammonia. According to Dr. Bence Jones, the first precipitate thrown 

 down by the addition of hydrochloric acid to ordinary urine, is urate of 

 ammonia, which is less soluble in acid than in neutral or alkaline urine ; 

 and it is only after prolonged contact with the hydrochloric acid, that 

 this salt is decomposed, and uric acid left behind. The solubility of 

 urate of ammonia is much greater in warm than in cold urine ; and hence 

 it frequently happens, that urine which is clear when voided, gives a pre- 

 cipitate of urate of ammonia on cooling. 



733. The proportion of Uric acid in healthy urine seldom rises above 

 1 part in 1000, and the quantity excreted daily is usually from 6 to 10 

 grains. The circumstances under which it varies, however, have not 

 been clearly determined. The absolute quantity in the urine bears no 

 proportion to its acidity, nor is it indicated by the amount of deposit ; 

 for the acidity of the urine depends upon the presence of other acids ; 

 and a deposit of urate of ammonia may be due to an excess of acid, 

 diminishing its solubility, rather than to an excess of the substance 

 itself. Thus it may happen that a precipitate of urate of ammonia 

 may be formed, when it is not present in any undue proportion, in con- 

 sequence of the acid state of the urine ; whilst, on the other hand, there 

 may be a large excess of urate of ammonia in the urine without any 

 precipitate, if the urine should be alkaline. In disordered states of the 

 system, there is often a great increase in the amount of uric acid in the 

 urine ; and there can be no doubt that this increase is partly controlla- 

 ble by the reduction of the proportion of azotized matter in the food. 

 In some of these cases, free uric acid is deposited, in consequence of 

 the decomposition of the urate of ammonia by a large excess of acid in 

 the urine. In attacks of gout, urate of soda is separated from the cir- 

 culating blood, and is deposited in the tissues around the affected joints, 

 forming the concretions termed " chalk-stones ;" and in this state of the 

 system, uric acid may be detected in the blood. 



734. There seems reason to believe that we are to regard Hippurio 

 acid as a normal element of the Urine of Man ; although it has been 

 usually supposed to be restricted to the Herbivorous quadrupeds, where 



