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124 the other common water, and, by the arrangement of the tubes, when I inspire I draw air through the water, and when I expire I blow air through the lime-water. You see the water remains clear but the lime-water becomes turbid in a few minutes. It becomes turbid by the carbonic acid in the breath combining with the lime in the lime-water so as to form carbonate of lime, which is not readily dissolved, and consequently gives the white appearance to the lime-water. Carbonic acid gas, then, comes from our lungs.

The air in the lungs, as I have said, receives the carbonic acid from the blood. Those who are unfamiliar with physiology can hardly conceive of the blood as containing a large amount of gas. Take a hundred cubic centimetres of blood: this quantity may contain about sixty cubic centimetres of gas, and perhaps two-thirds of this consists of carbonic acid, the other one-third being oxygen. Although it contains this large amount of gas, blood does not effervesce in the air, because the pressure of the air on its surface prevents it from escaping at ordinary temperatures, but if we allow blood to run into a vacuum, as I do when I allow it to