EXHALATION OF CARBONIC ACID. 145 



ing point has been studied by Yierordt, in a series of ninety-four experiments made upon 

 his own person, with the following results : 



"When the respirations are frequent, the quantity of carbonic acid expelled at each 

 expiration is much less than in a slow expiration ; but the quantity of carbonic acid pro- 

 duced during a given time by frequent respirations is greater than that which is thrown 

 off by slow expirations." 



The air which escapes during the first period of an expiration is naturally less rich in 

 carbonic acid than that which is last expelled and comes directly from the deeper por- 

 tions of the lungs. Dividing, as nearly as possible, the expiration into two equal parts, 

 Vierordt found, as the mean of twenty-one experiments, a percentage of 3'72 in the 

 first part of the expiration and 5'44 in the second part. 



Temporary arrest of the respiratory movements, as we should expect, has a marked 

 influence in increasing the proportion of carbonic acid in the expired air ; although the 

 absolute quantity exhaled in a given time is diminished. In a number of experiments 

 on his own person, Vierordt ascertained that the percentage of carbonic acid becomes 

 uniform in all parts of the respiratory organs, after holding the breath for forty seconds. 

 Holding the breath after an ordinary inspiration, for twenty seconds, the percentage 

 of carbonic acid in the expired air was increased 1*73 over the normal standard; but the 

 absolute quantity exhaled was diminished by 2*642 cubic inches. After taking the 

 deepest possible inspiration and holding the breath for one hundred seconds, the per- 

 centage was increased 3 '08 above the normal standard; but the absolute quantity was 

 diminished more than fourteen cubic inches. Allen and Pepys state that air which has 

 passed nine or ten times through the lungs contains 9'5 per cent, of carbonic acid. 



Vierordt gives the following formula as representing the influence of the frequency of 

 the respirations on the production of carbonic acid : Taking 2-5 parts per hundred as 

 representing the constant value of the gas exhaled by the blood, the increase over this 

 proportion in the expired air is in exact ratio to the duration of the contact of the air 

 and blood. 



The absolute quantity of carbonic acid exhaled in a given time is a more important 

 subject of inquiry than the proportion contained in the expired air ; for the latter is con- 

 stantly varying with every modification in the number and extent of the respiratory acts, 

 and the volume of breathing air is subject to great fluctuations and is very difficult of 

 determination. 



Among the most reliable observations on the quantity of carbonic acid exhaled by the 

 human subject in a definite time and the variations to which it is subject, are those of 

 Andral and Gavarret and of Dr. Edward Smith. The observations of Lavoisier and SSguin, 

 Prout, Davy, Dumas, Allen and Pepys, Scharling, and others, have none of them seemed 

 to fulfil the necessary experimental conditions so completely. Scharling's method was to 

 enclose his subject in a tight box, with a capacity of about twenty-seven cubic feet, to 

 which air was constantly supplied ; but the observations were comparatively few, being 

 made on only six persons. In his observations, the quantities of gas exhaled must have 

 been considerably modified by the elevation of temperature and exhalation of moisture in 

 so small a space. The mental condition of the subject of an experiment has an influence 

 upon the products of respiration, and the function is sometimes modified from the mere 

 fact that an experiment is being performed ; an influence which Scharling did not fail to 

 recognize, but which frequently cannot be guarded against. 



The observations of Andral and Gavarret were made on sixty-two persons of both 

 sexes and different ages and under absolutely identical conditions as regards digestion, 

 time of the day, barometric pressure, and temperature. The products of respiration were 

 collected in the following way : A thin mask of copper covering the face and large 

 enough to contain an entire expiration was fitted to the face by its edges, which were 

 provided with India-rubber so as to make it air-tight. At the upper part was a plate of 

 glass for the admission of light, and at the lower part, an opening, which allowed the 

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