378 PHYSIOLOGY CHAP. 



briefly as possible, to condense the more important conclusions 

 arrived at in the actual state of science, we must pass from the 

 historical exposition of the subject to a summary of the experimental 

 data. 



Laws of Absorption and Diffusion of Blood. In order to \ understand 

 what follows in regard to the mechanism of respiratory gas exchanges, it is 

 necessary to recapitulate certain physical laws which are closely bound up 

 with this process. 



Since gases have no definite shape like solids, nor definite volume like 

 liquids, and since the molecules which constitute them have the property of 

 mutual repulsion, so, when two gases that do not enter into chemical com- 

 bination, are brought into contact, they promptly expand one into the other, 

 until they form a uniform mixture independent of their different densities. 

 This phenomenon is called the diffusion of gases. 



The force with which the molecules of the gases tend to expand in a space, 

 and by means of which they exert uniform pressure in every direction, is 

 called the tension of gases. Obviously, the greater the number of gaseous 

 molecules brought together in a confined space, the greater will be the pressure. 

 It follows that the tension of a gas is inversely proportional to its volume 

 (Mariotte's law). 



Again, when two gases are separated by a porous septum, there is reciprocal 

 diffusion, but the velocity with which the molecules of each diffuse across the 

 septum varies according to their densities ; the lighter gases, such as H and 

 CH 4 , diffuse more rapidly than Cl and CO, which are heavier. It may be 

 said approximately, with Graham, that the rate at which gases traverse the 

 pores of the septum is inversely proportional to the square root of their 

 densities. 



There is a marked attraction between gases and particles of solid porous 

 bodies, whereby the former are attracted and condensed between the pores of 

 the latter. Thus, for example, 1 vol. of boxwood charcoal may condense at 

 12 C., and at ordinary barometric pressure, 35 vols. of CO 2 , 9-4 vols. of O 2 , 7'4 

 vols. N, 175 vols. H 2 . This process is termed the absorption of gases by solid 

 bodies, and is invariably accompanied by evolution of heat in ratio with the 

 energy with which the absorption proceeds. Non-porous bodies, too, are 

 capable of condensing, if not of absorbing on their surface, a layer of the gases 

 with which they may be brought into contact. 



More important for us is the absorption of gases by liquids. In this con- 

 nection it has been found that the volume of a gas absorbed by a liquid is 

 independent of its pressure. Since, however, the density of a gas is pro- 

 portional to the pressure under which it is placed, and since its weight is 



_., pressure is reduced to zero by 

 the Torricellian vacuum, the liquid can be deprived of all the gases which it 

 has absorbed.; 



The absorption coefficient of a liquid for a gas is the figure which indicates 

 that volume of gas which at C. and 760 mm. Hg pressure, is absorbed by 

 the unit volume of the liquid (Bunsen). 



Temperature has great influence on coefficients of absorption. A liquid 

 absorbs less gas, in proportion as its temperature is higher, and at boiling- 

 point there is no longer any absorption. It is therefore sufficient, in order to 

 extract the gases absorbed by any liquid, to heat it to boiling-point. 



The degree in which different liquids absorb the same gas, and in which 



