We have seen that during natural sleep the carbon dioxide expired (during wakefulness between three and four volumes per hundred of air from the lungs) diminishes as the circulation diminishes. Many claim that a succession of rapid but longdrawn respirations will quickly bring on drowsiness, and often sleep even ensues. This can be attributed only to a small overcharging of the circulation with carbonic acid. It must be recalled also that carbonic-acid waters and effervescent drinks generally produce with many a decided though transient sedative effect. But in this way doubtless none of the gas reaches the blood, except the little thrown off from the stomach by eructation, this being incidentally and partially inhaled. The gases contained in arterial and venous blood appear to vary somewhat. Several authorities agree, however, that the amount of free carbonic acid in arterial is much smaller than in venous blood, while the oxygen in the former (combined mainly with the hæmoglobin) is much larger. Pflüger made the free carbon dioxide, mostly in solution, in red arterial blood of a dog thirtyfour volumes and a half per hundred, while in the blue venous blood it rose as high as fifty, or half the volume of the blood.
The generalization referred to is simply that normal sleep and sleepiness, or drowsiness, are due to a small increase over the average of the carbonic acid in solution in the blood, arising through its overproduction from the greater amount of muscular and other tissue that undergoes oxidation during the waking hours. During the sleeping hours this overload of the anæsthetic gas is gradually discharged until wakefulness results.
In this brief discussion no room has been occupied with what are called hypnotism, clairvoyance, trance, mind-reading, etc. These are outside of our scope—being, if authentic, not natural but supernatural phenomena, pertaining to the realms not of law but of miracle.
Haldane discussed the subject of the fatality to miners of "after-damp," hitherto believed to owe its effects to carbon dioxide. He shows that absence of oxygen is the real cause of death in these cases. He introduces the novel and highly rational suggestion that all parts of mines should be provided with reservoirs of highly compressed oxygen. A pint of this, he shows, would keep a man alive for an hour. Doubtless, with our present means and knowledge, oxygen could be stored in mines and elsewhere, in its liquefied form, for this and other valuable uses.—H. W.
