Bulk of the air expired. —The quantity of air thrown out of the lungs at each expiration has also an influence upon the percentage and absolute quantity of carbonic acid gas in the expired air. Vierordt, in six experiments, found that while the average of carbonic acid gas in the expired air in a normal expiration in a state of rest was 1.78 per cent., in the deepest expiration he could make, it was 4.05 per cent.
The stoppage of the respiratory movements for a time has also a marked effect upon the quantity of carbonic acid in the expired air. Vierordt has made four series of experiments upon himself to ascertain the extent of this influence upon the quantity of carbonic acid evolved from the lungs. In the first series he shut his mouth and held his nose from 20 to 60 seconds (the longest period he could con tinue the experiment), and then made the deepest possible expiration. In the second series he made the deepest inspiration pos sible, then suspended the respiratory move ment for a longer or shorter time, at the ter mination of which he made the deepest ex piration. This experiment he was able to prolong to 70, 90, and even 100 seconds. In the third series he made an ordinary inspira tion before suspending the respiratory move ments, and after this suspension had con tinued for different periods up to 30 seconds, he made an ordinary expiration. 'The fourth series of experiments was to ascertain the period of time after the stoppage of the respi ratory movements when the percentage of carbonic acid gas becomes uniform in the different parts of the lungs and air passages, and this he found took place after 40 seconds. He has arranged the results of the three first series of experiments in several tables, ex hibiting the difference between the percentage and absolute quantity of carbonic acid gas in the expired air at various periods, after the suspension of the respiratory movements under the circumstances mentioned, and when the respiratory movements proceed in the normal manner. In the first series of ex periments, the percentage of the carbonic acid in the expired air, after the respiratory movements had been suspended 20 seconds, was higher by 1.73 than when these move ments were normal, but the absolute quantity evolved from the lungs had diminished by 2.642 English cubic inches, and at the end of
55 seconds its percentage had increased 2.32, but its absolute quantity had diminished to the extent of 12.382 cubic inches. In the second series of experiments, where the deep est possible inspiration preceded, and the deepest possible expiration followed, the sus pension of the respiratory movements, the absolute quantity of carbonic acid gas evolved from the lungs, for the first 15 seconds, was somewhat more than what would have oc curred had these movements proceeded in the normal manner, but after this it began to diminish ; and when the respiratory move ments had been suspended for 95 seconds, it was diminished to the extent of 14.078 English cubic inches, though its percen tage had considerably increased. At the end of the 100 seconds, the percentage of' the expired air was 3.08 above the normal quan tity in ordinary respiration. In the• third series of experiments, the carbonic acid in the expired air, at the end of 30 seconds, was 1.55 pt.r cent. above the normal quantity. These experiments prove, therefore, that when the respiratory movements have been suspended for a time, the percentage of carbonic acid in the expired air will increase, but the absolute quantity evolved from the lungs will be diminished, so that the increase in the per centage of this gas does not by any means compensate for the diminished quantity of air passing through the lungs.
When the same air is breathed more than once, the quantity of carbonic acid in it is increased. Allen and Pepys state that air, passed 9 or 10 times through the lungs, con tained 9 5 per cent. of carbonic acid gas ; and the greatest quantity obtained, in air breathed as often as possible, was 10 per cent. Mr. Coathupe + found the average quantit) of carbonic acid gas, in air in which warm blooded animals had been confined until they were becoming comatose, to be 10-42 per cent.; while, if they were allowed to remain in it until they had become asphyxiated, it con tained 12-75 per cent. Vierordt, in three experiments, breathed, from to 3 minutes, a volume of air amounting to 427 English cubic inches, and found, on an average, the carbonic acid gas 1-5 per cent. above that contained in air breathed only once.