EXHALATION OF CARBONIC ACID. 427 



The absolute quantity of carbonic acid exhaled in a given 

 time is a more important subject of inquiry than the propor- 

 tion contained in the expired air ; for the latter is constant- 

 ly varying with every modification in the number and ex- 

 tent of the respiratory acts, and the volume of breathing 

 air is subject to great fluctuations, and is very difficult of 

 determination. The direct method, in which the actual 

 products of respiration are collected and estimated, has led 

 to very important results, which have been confirmed to a 

 certain extent by Boussingault, Barral, and others who have 

 employed the indirect method. It is by the direct method, 

 in the hands of Regnault and Reiset, Andral and Gavarret, 

 and more recently Dr. Edward Smith, that we have learned 

 so much regarding the physiological variations in the prod- 

 ucts of respiration ; one of the most important considerations 

 connected with the subject. 



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, 1 and Dr. Edward Smith. 2 The obser- 

 vations of Lavoisier and Seguin, 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 27 cubic feet, to which air was con- 

 stantly 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. 3 The mental condition of the 



1 Recherches sur la Quantite d'Acide Carbonique exhale par Ics Poumons dans 

 VEspece Humaine. Annettes de Chimie et de Physique, 3me serie, tome viii., p. 129. 



2 EDWARD SMITH, Experimental Inquiries into the Chemical and other Phenom- 

 ena of Respiration, and their Modifications by various Physical Agencies (Phi- 

 losophical Transactions, 1859, p. 681) ; and On the Action of foods upon the 

 Respiration during the Primary Processes of Digestion (Ibid., p. 715). 



3 Annales de Chim. et de Phys., tome viii., p. 488. Scharling recognized the 



