EXHALATION OF CARBONIC ACID. 151 



In walking at the rate of two miles an hour, the exhalation of carbonic acid during 

 one hour was equal to the quantity produced during If hour of repose with food, and 2 

 hours of repose without food. 



Walking at the rate of three miles per hour, one hour was equal to 2- hours with, 

 and 3 hours without food. 



One hour's labor at the tread-wheel, while actually working the wheel, was equal to 

 4 hours of rest with food, and 6 hours without food. 



The various observers we have cited have remarked that, when muscular exertion is 

 carried so far as to produce great fatigue and exhaustion, the exhalation of carbonic acid 

 is notably diminished. 



Influence of Moisture and Temperature. Lehmann has shown that the exhalation of 

 carbonic acid is much greater in a moist than in a dry atmosphere. This conclusion 

 was the result of a number of experiments on birds and animals confined in air at differ- 

 ent temperatures and different degrees of moisture. He found that 35-J- oz. av. weight of 

 rabbits, at a temperature of about 100 Fahr., exhaled during an hour before noon, in a 

 dry air, about 15 cubic inches of carbonic acid ; while, in a moist air at the same tempera- 

 ture, the exhalation was about 22 cubic inches. 



Disregarding observations on the influence of temperature in cold-blooded animals as 

 inapplicable to the human subject, it has been ascertained that the exhalation of carbonic 

 acid is much greater at low than at high temperatures, within the limits of heat and cold 

 that are easily borne by the human subject ; thus following the rule which governs the 

 consumption of oxygen. 



The experiments of Yierordt on the human subject show that there is an increase in 

 the exhalation of carbonic acid of about one-sixth, under the influence of a moderate 

 diminution in temperature. In these observations, the low temperatures ranged between 

 37-5 and 59, and the high temperatures between 60-5 and 75'5 Fahr. He found the 

 quantity of air taken into the lungs slightly increased at low temperatures. The abso- 

 lute quantity of carbonic acid exhaled per minute was 18-27 cubic inches for the low 

 temperatures, and 15 '73 cubic inches for the high temperatures. 



Influence of the Season of the Year. It has been pretty well established by the re- 

 searches of Dr. Smith, that spring is the season of the greatest, and fall the season of 

 the least activity of the respiratory function. 



The months of maximum are : January, February, March, and April. 



The months of minimum are : July, August, and a part of September. 



The months of decrease are : June and July. 



The months of increase are : October, November, and December. 



W. F. Edwards, in 1819, showed in a marked manner the influence of the seasons 

 upon the respiratory phenomena in birds. In a series of very curious observations, which 

 he repeatedly verified, it was demonstrated that the increase in the activity of respiration 

 during the winter was to a certain extent independent of the immediate influence of the 

 surrounding temperature. In the month of January, he confined six yellow-hammers in 

 a receiver containing 71 '4 cubic inches of air, carrying the temperature from 69 to 70 

 Fahr. The mean duration of their life was 62 minutes 25 seconds. In the months of 

 August and September, he repeated the experiment on thirteen birds of the same species, 

 at the same temperature. The mean duration of life was 82 minutes. These experiments 

 have an important bearing on our views concerning the essential nature of the respira- 

 tory function. They seem to indicate that the respiratory processes are intimately con- 

 nected with nutrition. Like the other nutritive phenomena, they undoubtedly vary at 

 different seasons of the year and are to a certain extent independent of sudden and 

 transitory conditions. During the winter, more air is habitually used than in summer, 

 and the respiratory processes cannot be immediately brought down to the summer 

 standard by a mere elevation of temperature. 



Observations on the influence of barometric pressure are not sufficiently definite in 

 their results to warrant any exact conclusions. 



