CHEMICAL PHENOMENA OF RESPIRATION. 401 



the expired air appears from these estimates to be about 4 per cent, on the 

 whole ; but single experiments give a much higher estimate. Thus, in one 

 of those made by Allen and Pepys, in which fresh air was taken in at every 

 inspiration, the proportion was 8 parts in every 100. They found, however, 

 that if the air be already charged in some degree with carbonic acid, the quan- 

 tity excreted is much less ; for when 300 cubic inches were respired for three 

 minutes, only 28 cubic inches of carbonic acid were found in it, although the 

 rate of its production in a parallel experiment was 32 cubic inches in a minute. 

 Knowing, then, the necessity of a free excretion of carbonic acid, we are led 

 by this fact to perceive the high importance of ventilation ; for it is not suf- 

 ficient for health, that a room should contain the quantity of air requisite for 

 the support of its inhabitants during a given time ; since after they have 

 remained in it but a part of that time, the quantity of carbonic acid which its 

 atmosphere will contain, will be large enough to interfere greatly with the due 

 aeration of their blood, and thus to cause oppression of the brain and the other 

 morbid affections that result from the accumulation of carbonic acid in the cir- 

 culating fluid. On the other hand, it has been ascertained by the recent expe- 

 riments of Dr. Boswell Reid that, if the carbonic acid be removed as fast as it 

 is formed, an animal may remain in a limited quantity of air, without much 

 inconvenience, until nearly the whole of its oxygen is exhausted ; thus 

 showing that the respirability of air does not depend so much upon the pro- 

 portion of oxygen it contains, as upon its freedom from contamination with 

 carbonic acid or other poisonous gases. 



535. Although the statements just given may be regarded as representing 

 the average amount of carbonic acid evolved during the twenty-four hours, 



tion, &c. ; and were repeated several times on each individual. 1. At all ages beyond 

 eight years, the exhalation is greater in males than in females. 2. In mules, it regularly 

 increases in quantity from eight to thirty years of age ; from thirty to forty it is stationary, 

 or diminishes a little; from forty to fifty the diminution is greater; and from fifty to 

 extreme age, it goes on diminishing, till it scarcely exceeds the quantity at ten years. 



3. The quantity of carbonic acid exhaled in one hour by males of different ages, is as 

 follows; at eight years, 77-5 grains; at fifteen, 135 grains; at twenty, 176-7 grains; 

 between thirty and forty, 189 grains; between forty and sixty, 156 grains; between sixty 

 and eighty, 142-5 grains; and in a man of a hundred and two, it was only 91 5 grains. 



4. In females, nearly the same proportionate increase goes on to the time of puberty ; 

 when the quantity abruptly ceases to increase, and remains stationary so long as they 

 continue to menstruate. When, however, menstruation has ceased, the exhalation of car- 

 bonic acid begins again to augment, and then, again, in advancing years, decreases as it 

 does in men. Thus before puberty the quantity of carbon exhaled by girls in an hour is 

 99 grains; and so it remains during the continuance of menstruation; afterwards, from 

 thirty-eight to forty-nine years of age, it increases to 130 grains: from fifty to sixty it 

 again falls to 1 13 grains ; from sixty to eighty it is reduced to 105 grains ; and in a woman 

 of eighty-two, it was only 93 grains. 5. Should menstruation temporarily cease at any 

 time, the exhalation of carbonic acid immediately undergoes an increase, precisely as 

 at the final cessation of the function. 6. In pregnancy, the exhalation is equal to that 

 which is natural soon after cessation of the menstruation. 7. The more robust the indi- 

 vidual, cseteris paribus, the more carbonic acid is exhaled; but the differences are not con- 

 stant. 8. The influences of the weights of individuals, of the capacities of their chests, 

 and of the extent of the respiratory movements, are not great. 9. The maximum observed 

 was in a strong man of twenty-six, who exhaled at the rate of 21ci-5 grains of carbon per 

 hour. Another robust man of sixty years of age exhaled at the rate of 209 grains per 

 hour; another, of similar constitution, and sixty-three years of age, exhaled at the rate of 

 190 grains per hour; and an old man of ninety-two, who preserved an uncommon degree 

 of energy, and who, in his younger days, had boasted of uncommon muscular powers, 

 still consumed at the rate of 151 grains per hour. On the other hand, a slender man of 

 forty-five, in the enjoyment of good health, only consumed 139-5 grains per hour. These 

 facts demonstrate the influence of individual constitution on the amount of carbonic acid 

 generated. Interesting as these results undoubtedly are, they require to be confirmed by 

 a more extensive series of experiments, before they can be received as physiological truths. 



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