OXYGEN. 413 



sidered. The oxygen may be simply absorbed by the blood, or it may be 

 that there is some compound in the blood which unites with this oxygen. 

 In the former case the absorption of oxygen must follow the gas laws, 1 of 

 which we shall briefly sketch the most important particulars. 



The absorption of a gas by a liquid, when there is no chemical reaction 

 between the two, is dependent upon the nature, the temperature, and the 

 pressure of the gas; and, in fact, the weight of gas which is absorbed by a 

 definite liquid is proportional to the pressure under which the gas is placed. 

 Now according to Boyle's law, 2 the weight of a definite volume of a gas is 

 directly proportional to the pressure, so that evidently the volume of gas 

 absorbed is independent of the pressure. Again, if instead of a single gas 

 a mixture of gases stands above a liquid, then each individual gas will be 

 absorbed independently of the others, and the amount absorbed is gov- 

 erned entirely by the pressure which this gas exerts (Dalton's law). 

 This pressure is called the partial pressure. The partial pressure can be 

 computed as soon as one knows the total pressure exerted by all of the 

 gases present in the mixture, and the percentage composition of the mix- 

 ture. The partial pressure is the same percentage of the total pressure 

 that the gas in question is present in per cent by volume in the mixture. 3 



If a liquid is allowed to remain in contact with a definite gas mixture 

 for some time, the liquid will become saturated with gas. When this 

 has taken place, then the pressure exerted by each gas in the liquid is 

 equal to the partial pressure of the same gas in the mixture above the 

 liquid. There is a state of equilibrium between the gas in the atmosphere 

 and that in the liquid. If this equilibrium is disturbed, for example, by 

 diminishing the amount of gas in question in the gaseous mixture, then 

 the liquid will give up this gas until once more the pressure in the mixture 

 is in equilibrium with the pressure exerted by the gas in the liquid. This 

 fact may be taken advantage of, if we wish to determine in a simple 

 manner what pressure is exerted by a gas which is dissolved in a liquid. 

 The liquid is placed in contact with a gas mixture of a definitely known 

 composition and pressure (whereby, as stated above, the partial pressure 



1 Cf . Text-books on Physics. For comparative purposes the volumes of gases are 

 reduced to C. and 760 millimeters, barometric pressure. As regards the methods 

 used for examining the gases in blood, see E. Pfliiger's Untersuchungen aus dem physi- 

 ologischen Laboratorium zu Bonn, Berlin, 1865. Alexander Schmidt : Verhandl. Sachsi- 

 schen Gesellsch. Wissensch. 19, 30 (1867). A. Kossel and A. Raps: Z. Physiol. Chem. 

 17, 644 (1893). Neesen: ibid. 22, 478 (1897). Miiller: Pfliiger's Arch. 103, 541 (1904). 

 J. Geppert: Die Gas-analyse und ihre physiologische Anwendung nach verbesserten 

 Methoden, Berlin, 1886. 



2 The name Mariotte's law is often given to this principle (earlier discovered by 

 Boyle), that at any given temperature the volume of a given weight of gas varies 

 inversely as the pressure which it bears. 



3 The partial pressure of a gas in a mixture is the same pressure that the gas exerts 

 when present by itself in the volume occupied by the mixture. 



