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 flow 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, and 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. * 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 Eeaction. 
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 liases, 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. 
