By reaction is meant the condition of urine as to acidity or alkalinity. This is determined by test paper for the purpose. Alkaline urine turns red paper blue. Acid urine changes blue paper to red. In health, the urine is naturally slightly acid, especially in persons who employ a flesh diet. It is noticeable that animals whose diet is made up wholly of vegetables have alkaline urine, while in carnivorous animals, whose diet is made up almost wholly of flesh, the urine is very strongly acid.

Persons who eat much meat, have a very acid urine, while in those who subsist upon vegetables, the urine is only very faintly acid, is neutral, or distinctly alkaline. This fact should be remembered by persons who are subject to gout, rheumatism, neuralgia, and various nervous diseases, which are known to arise from a superabundance of acid in the blood, or, at least, are associated with a very acid state of the urine. Very acid urine is also likely to produce gravel or stone in the bladder, catarrh of the bladder, irritation of the urethra, and various other diseases of the urinary organs. Great acidity of the urine generally gives rise to a brick dust deposit when the urine is allowed to stand a short time. The fine, reddish particles composing this deposit are crystals of uric acid.

Great alkalinity of the urine sometimes occurs in consequence of decomposition taking place in the bladder. This occurs most frequently in chronic catarrh of the bladder, and in cases in which the bladder is not completely emptied as frequently as it should be. When this condition exists, the urine will have a very unpleasant and distinct ammoniacal odor, as though it had stood for a few hours in a warm room.

Acidity of the urine is relieved by the treatment to be presently recommended for uric acid. An excessive degree of alkalinity will be relieved by the proper treatment of the disease to which it is due.