=Pathologically= we find the urine strongly =alkaline= from the evolution of ammonia from urea, in fermentations occurring with prolonged retention in the bladder or in cystitis. The urine is =acid= even in herbivora in all fevers in which appetite is lost or seriously impaired, and in which the metabolism is excessive. =Chemical Changes in the Urine in Disease.= =Sodium Chloride=, is present in large amount in health (horse 25 to 35 grammes, dog 0.25 to 5 grammes daily) is =diminished= in fever, anæmia, visceral and exudative inflammations. It is =increased= during the absorption of false membranes and exudates. It is thrown down by adding solution of nitrate of silver, the curdy white precipitate being insoluble in nitric acid. =Phosphates of lime, soda, potash and, scantily, of magnesia= are normally present (horse 0.08 to 0.60 gramme phosphoric acid daily) and are present in excess in digestive disorders and in malnutrition of bones (rachitis, osteoporosis and rheumatoid arthritis). The =alkaline phosphates= are very soluble and never precipitated. =Earthy phosphates= dissolve in acid urine, but are precipitated from alkaline. To a little of the urine add a few drops of acetic acid, followed by a few drops of uranium acetate. A yellow precipitate of uranium and ammonium double phosphate is thrown down. =Indican= (C_{8}H_{7}NSO_{4}) is formed from indol which passes successively through the forms of indoxyle and indoxylid potassio-sulphate. This is normally present in the urine, the horse excreting 1 to 2 grammes daily, the dog 0.15 gramme. It is present in excess in intestinal indigestions, constituting indicanuria. It is tested by adding a drop of muriatic acid and one of a solution of chloride of lime to the urine, when it will show a blue ring, the depth of which indicates the relative amount. =Urea= (CON_{2}H_{4}) the principal waste product of nitrogenous matter, is always present in considerable amount. The sound horse may eliminate 100 to 200 grammes daily, the dog 5 to 180 grammes. It is present in excess in all fevers and inflammations unless urination is suspended or impaired, in cryptogamic diuresis, in mellituria, uræmia, nephritis and cystitis. Test: The addition to a filtered solution of urine, freed from phosphates, of solution of acid nitrate of mercury, precipitates it as nitrate of urea. A simpler test is to add to a drop or two of urine on a glass slide a drop of nitric acid and heat gently. The nitrate of urea is precipitated in the characteristic rhombic or hexagonal crystals as seen under the microscope. Heat urea crystals in a test tube: biuret is formed and ammonia escapes. Add a trace of a copper sulphate solution and a few drops of a 20 per cent. solution of caustic potash: a rose-red color is produced—the biuret reaction.