574 THE CHEMISTRY OF THE URINE. 



Both factors, however, may vary through much wider limits than 

 those given, without any departure from conditions of health. The 

 chief causes which lead to increase of quantity and diminution of density 

 are increased consumption of liquid and diminished activity of the sweat- 

 glands. With abstention from liquids, or increased activity of the skin, 

 the amount necessarily falls, and the density is raised. 



Increase in the quantity may follow, not alone from a heightened 

 quantity of water in the blood, but from any influence, normal or 

 pathological, which increases the blood now through the kidneys. 



Pathologically the quantity is increased in diabetes mellitus and 

 insipidus, in certain stages of chronic nephritis, and in some neurotic 

 conditions ; it is decreased in the early stages of acute nephritis, in the 

 congestive condition of cardiac disease, find when large quantities of fluid 

 are lost by the bowel, as in cholera. The specific gravity is increased in 

 diabetes, and diminished in chronic nephritis. 



The specific gravity is roughly an indication of the amount of the 

 urinary solids. It cannot indicate the amount with exactness, as the 

 substances in solution are of various physical properties, and are not all 

 capable of increasing the density in like proportions. Thus, while a 10 per 

 cent, solution of common salt has (at 15) a specific gravity of about 1073, 

 a 10 per cent, solution of urea indicates only 1028. 1 An increase in the 

 urinary salines would therefore have a much greater effect in raising the 

 specific gravity than a like increase in the urea. A knowledge of the actual 

 weight of solids present seldom becomes of much importance. It may be 

 obtained with sufficient accuracy by multiplying the last two figures of the 

 sp. gr. by 2-2 ; the result indicating the total solid matter in grammes per 

 litre. Thus a specimen of sp. gr. 1020 contains about 44 grms. per litre of 

 substances in solution. 



CHEMICAL BEACTION. 



Acids and bases are so proportioned in human urine that the mixed 

 excretion of twenty-four hours generally reacts acid to litmus paper. It 

 may sometimes exhibit the so-called amphoteric reaction a phenomenon 

 to be later discussed but under strictly normal circumstances the 

 accumulated excretion of the day is never alkaline to litmus. On the 

 other hand, during limited periods of the daily cycle, it may sometimes, 

 though not commonly, become alkaline. 



Litmus is reddened both by acids and by acid salts ; but there are 

 other coloured indicators which behave differently in the presence of 

 free acids and acid salts respectively. When such are applied to urine, 

 they show unequivocally that the former are never present, and we are 

 thus forced to the conclusion that urine owes its acidity to acid salts. 

 It will be shown immediately that we may conclude with some certainty 

 that the reaction is due, as a matter of fact, to the presence of acid 

 phosphates. 



The nitrogen, carbon, phosphorus, and sulphur of food-stuffs are all 

 capable of oxidation to acid anhydrides, and the last three elements are 

 in fact oxidised to this acidic form in the body. The chief product of 

 the oxidation of carbon, carbon dioxide, may play a not unimportant 

 role in the equilibrium of urinary acids and bases, and the existence of 

 oxidised carbon in the molecules of certain organic compounds in the 

 1 A. H. Allen, "Chemistry of Urine," 1895, p. 12. 



