Urea is an amide—carbonic diamide. It very readily takes water into its molecule, changing into carbonate of ammonia. N₂H₄CO + 2H₂O = (NH₄)₂CO₃. This change is rapidly brought about by the influence of bacteria in urine exposed to the air. In thinking of the transformations which proteid substances undergo in the system, it is legitimate to regard their nitrogen as from the first united with hydrogen in the form of ammonia. Not that the grouping is so simple as this. An albumin is not an amide. But in the dance of atoms of its great molecule as it progresses through the system—forming part of the blood, taken up by the cells as floating protein, incorporated in the protoplasm of the cells, shaken into smaller aggregates in the muscles—nitrogen and hydrogen are partners. They leave the body hand in hand. Gusts of oxygen atoms enter through the lungs; use blood-corpuscles as carriages; dismounting, they traverse lymph, forcing their way into the interior of the cells; they join in the dance. With their strong arms they detach carbon atoms and hydrogen atoms from the huge albumin chain. As carbonic acid and water they bear them to the lungs. But nitrogen clings to hydrogen. Oxygen cannot detach its grasp. Out of the molecule of albumin this firmly united couple slips, without contributing anything to the energy which moves the body and keeps it warm. Nitrogen is not a source of energy. It even saves a portion of the hydrogen of albumin from combustion. Urea burnt in a calorimeter has a balance of energy to give up. Many attempts have been made to ascertain the stages through which proteins pass on their road to urea. The search for intermediate compounds is probably futile, since there is no sufficient reason for supposing that proteins disintegrate in stages, each a step less complex than the food and a step nearer to urea. Every nitrogenous extractive found in the tissues is, of course, on its road to urea. It will be removed as urea, unless indeed, like uric acid or creatinin, it has to be excreted without further change. But it appears to be impossible to discover in the tissues any nitrogenous compounds which occur in sufficient quantity to justify us in regarding them as inevitable halting-places on the downward road (_cf._ p. 146). The metabolism of albuminous substances, like other oxidations, takes place chiefly in muscles. Very little is known regarding the nature of the products. Urea is not amongst them. Whatever they may be (_cf._ p. 267), they are carried to the liver, in which they are turned into urea.