It is quite otherwise with regard to the liberation of carbonic acid. The problems presented by the solution of this gas in blood and its elimination in the lungs are difficult to solve. Less than one-tenth of the volume of carbonic acid which can be extracted from blood by the air-pump is simply in solution. The remainder is in loose chemical combination, the chief agents in holding it being the alkaline carbonates which the plasma contains. With an excess of carbonic acid they form acid carbonates, which give up carbonic acid and again become normal carbonates in the lungs. About one-third of the carbonic acid is, however, held by the blood-corpuscles—partly in virtue of their alkaline carbonates and phosphates, partly in combination with their globulin. The affinity of these several vehicles for carbonic acid is sufficient to enable them to take it from the lymph, and to hold it while the blood is in the veins. When they reach the capillaries of the lungs, they part with their burden of carbonic acid to the air. It is in connection with this renunciation that certain difficulties remain to be explained. The carbonic acid is given up with greater readiness than our knowledge of the chemistry of the compounds into which it enters in the blood would lead us to expect. Why does oxygen enter blood as it circulates through the lungs, and carbonic acid leave it? We have referred to the immense surface which the lungs expose to air. If a soap-bubble be filled with a mixture of oxygen, nitrogen, and carbonic acid, and if the oxygen be in smaller proportion, and the carbonic acid be in greater proportion, than in the air of the room, oxygen will enter the bubble, and carbonic acid will leave it, by diffusion. If, instead of filling a bubble with gas, we fill a bladder with water charged with carbonic acid, but destitute of dissolved oxygen, a similar exchange with the gases of the air will take place. It is merely a question of “gaseous tension.” The tension of the gases in the lungs is measured by passing a small tube down the trachea, and along one of the two chief bronchi until it becomes blocked in a bronchus just large enough to admit it. Respiration is carried on under normal conditions in the remainder of the lung; but in the lobe which the catheter blocks diffusion from stationary air to tidal is no longer allowed. At the same time, since the circulation is not interfered with, the gases in the blood of the occluded lobe of the lung are not in markedly different proportions from those in the air-chambers of other parts. If at the end of a sufficient interval the air of the occluded lobe is drawn off and its gases measured, their tensions can be compared with the tensions of gases in specimens of arterial and of venous blood. If from 10 c.c. of fluid 1 c.c. of gas can be removed by the air-pump, the volume of gas dissolved is 10 per cent. of the volume of the fluid which dissolved it. Commonly