RELATIONS BETWEEN OXYGEN AND CARBONIC ACID. 443 



processes cannot be immediately brought down to the sum- 

 mer 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. 



Some physiologists have attempted to fix certain hours of 

 the day when the exhalation of carbonic acid is at its maxi- 

 mum, or at its minimum ; but the respiratory activity is in- 

 fluenced by such a variety of conditions, that it is impossible 

 to do this with any degree of accuracy. 



Relations between the Quantity of Oxygen consumed and the 

 Quantity of Carbonic Acid exhaled. 



Oxygen unites with carbon in certain proportions, to form 

 carbonic acid gas, the volume of which is precisely equal to 

 the volume of the oxygen which enters into its composition. 

 In studying the relations of the volumes of these gases in 

 respiration, we have a guide in the comparison of the volumes 

 of the inspired and expired air. It is now generally recog- 

 nized that the volume of air expired is less, at an equal tem- 

 perature, than the volume of air inspired. Assuming, then, 

 that the changes in the expired air, as regards nitrogen, and 

 all gases except oxygen or carbonic acid, are insignificant, it 

 must be admitted that a certain quantity of the oxygen con- 

 sumed by the economy is unaccounted for by the oxygen 

 which enters into the composition of the carbonic acid ex- 

 haled. We have already noted that from -fa to -gL, or about 

 i'4 to 2 per cent, of the inspired air is lost in the lungs ; ' or 

 it may be stated, in general terms, that the oxygen absorbed 

 is equal to about five per cent, of the volume of air inspired, 

 and the carbonic acid exhaled only about four per cent. A 

 certain amount of the deficiency in volume of the expired air 

 is then to be accounted for by a deficiency in the exhalation 

 of carbonic acid. 



1 Vide page 405. 



