The percentage of the solid and liquid materials in urine varies as the secretion alters in strength, but on an average it may be said to contain about 4 per cent, of solids and 96 per cent, water.

The following are the more important solid matters: - Urea is the most important, and at the same time most abundant solid constituent, commonly forming about 2 per cent, of the urine. It is regarded as the chief end-product of the oxidation of the nitrogenous matter in the body, so that the amount excreted per diem gives us the best estimate of the amount of chemical change taking place in the nutrition of the tissues. It is readily soluble in alcohol and water, but insoluble in ether. It forms acicular crystals with a silky lustre. From a chemical point of view it may be regarded as the monamide of carbamic acid, with the formula:

CO

NH2 NH2

It is isomeric with ammonium cyanate

CN NH1

O, from which it was first prepared artificially.

On exposure to the air bacteria develop in the urine, and, acting as a ferment, change the urea into ammonium carbonate, two molecules of water being at the same time taken up thus:-

CO(NH2 )2+ 2H20 = (NH1)2C03.

This gives rise to a change in the reaction of the urine, which, after a time, becomes increasingly alkaline, and the change is commonly spoken of as the alkaline fermentation of the urine. This change is extremely slow in solutions of pure urea, which do not support bacterial life.

With nitric and oxalic acids, urea forms sparingly soluble salts - a fact made use of in its preparation from urine.

The amount of urea eliminated in the 24 hours is about 500 grains (35 grammes). The amount varies (1) in some degree with the amount of urine secreted; an increase in the amount of water being accompanied by a slight increase in the urea eliminated. Some materials, such as common salt, increase the water, and thereby also increase the urea. (2) The character and quantity of the diet influences most remarkably the quantity of urea given off, the amount increasing in direct proportion to the quantity of proteid consumed. Fasting causes a rapid fall in the amount of urea; even in the later days of starvation it continues to fall, but very slowly. (3) The amount differs with age, being relatively greater in childhood than in the adult (about half as much again in proportion to the body weight). (4) Many diseases have a marked influence on the amount of urea. In most febrile affections it increases with the intensity of the fever, while in disease of the liver it often notably decreases. In diabetes, if the consumption of food be very great, the daily 34 excretion of urea may reach nearly 4 oz. (100 grammes) or three times as much as normal.

Preparation

To obtain urea from human urine it is evaporated to one-sixth of its bulk, an excess of nitric acid is added, and it is left to stand in a cool place. Impure nitrate of urea separates from the fluid as a yellow crystallized precipitate. This sparingly soluble salt is caught on a filter, dried, dissolved in boiling water, mixed with animal charcoal to remove the coloring matter, and filtered while hot; when the filtrate cools, colorless crystals of nitrate of urea are deposited. The precipitate is dissolved in boiling water, and barium carbonate added as long as effervescence takes place, barium nitrate and urea being produced. This is evaporated to dryness, and the urea extracted with absolute alcohol, which, on evaporation, leaves crystals of pure urea.