Respiration. 85 



not ascertained ; probably it was a mixture of these gases, 

 and I have spoken of them as ' undetermined '; they did 

 not, however, amount to 183 cubic inches in the 24 hours. 

 Nitrogen in small quantities may probably be given off 

 from the lungs of horses during rest, and Regnault and 

 Reiset found this to be the case in dogs, though it is denied 

 by Pfliiger. 



We have previously learned the changes occurring in the 

 blood during its passage through the lungs, we have now 

 to study the way in which the interchange of gases 

 between this fluid and the air are brought about. 



The law regulating the absorption of gases by fluids is 

 very clear; every fluid in which a gas is soluble absorbs the 

 same volume of gas, no matter what the barometric pres- 

 sure may be ; but as the number of molecules in a gas 

 depends upon the pressure, it is evident that the weight 

 of the absorbed gas rises and falls in proportion to the 

 pressure. This is known as the law of Dalton and Henry. 



The volume of gas absorbed by a fluid depends 

 upon the gas ; for instance, 1 volume of water will absorb 

 1180 volumes of ammonia gas, whilst the same volume 

 of water will only absorb -00193 volumes of hydrogen. 

 The temperature of the water is also an important factor, 

 for the higher the temperature the less the gas absorbed. 



If now, instead of taking a single gas to be absorbed 

 by a fluid, Ave take a mixture of gases, it is found 

 that the volume of each gas forming the mixture is ab- 

 sorbed as perfectly as if it were the only gas present ; 

 no more and no less is absorbed whether the gas be 

 by itself or whether it forms only a proportion of the mixed 

 gases present ; this is explained by Dalton as resulting 

 from the tact that one gas does not exercise any pressure 

 upon the other gases with which it forms a mixture, and 

 the weight of the gas absorbed depends upon the pressure. 

 The term used by Bunsen to define the pressure exerted by 

 one gas in a mixture of gases is termed the ' partial pressure." 

 For example, 100 volumes of air contain at freezing-point 

 and standard barometric pressure (30 inches) 21 volumes of 



