40 EXTRACTION OF THE BLOOD GASES. 



each flask a perfectly uniform mixture. The phenomenon is called the diffusion of gases. If 

 * porous ,. mbrane be previously inserted between the gases, tho exchange of gases still goes on 

 through the membrane. But (as with endosraosis in fluids) the gases pass with unequal rapidity 

 through the pores, so that at the beginning of the experiment a larger amount of gas is found 

 on one side of the membrane than on the other. According to Graham, the rapidity of the 

 dilfusion of the gases through the pores is inversely proportional to the square root of their 

 specific gravities. (According to Bunsen, however, this is not quite correct. ) 



Different Gases in a Gaseous Mixture do not Exert Pressure upon one another. Gases, 

 therefore, pass into a space filled with another gas, as they would pass into a vacuum. If the 

 surface of a fluid containing absorbed gases be placed in contact with a very large quantity of 

 another gas, the absorbed gases diffuse into the latter. Hence, absorbed gases can be removed 

 by (3) passing a stream of another gas through the fluid, or by merely shaking up the fluid with 

 another gas. 



Partial Pressure. If two or more gases arc mixed in a closed space over a fluid, as the 

 different gases existing in a gaseous mixture exert no pressure upon each other, the several gases 

 are absorbed. The weight of each absorbed is proportional to the pressure under which each 

 gas would be, were it the only gas in the space. This pressure is called the partial pressure of 

 a gas (Bunsen). The absorption of gases from their mixtures, therefore, is proportional to the 

 partial pressure. The partial pressure of a gas in a space is at the same time the expression for 

 the tension of the gas absorbed by a fluid. 



The air contains 0*2096 volume of 0, and 0*7904 volume N. If 1 volume of the air be placed 

 under a pressure. P, over water, the partial pressure under which O is absorbed = '2096 P; 

 that for X = 0*7904 P. At 3 C, and 760 mm. pressure, 1 volume of water absorbs 0*02477 

 volume of air, consisting of 0*00862 volume O, and 0*01615 volume N. It contains, therefore, 

 34 per cent. O and 66 per cent. N. Therefore, water absorbs from the air a mixture of gases 

 containing a largt r p> rcentage of than the air itself. 



34. EXTRACTION OF THE BLOOD GASES. [The blood to be analysed must be collected 

 over mercury so as to avoid contact with air. This is done by means of a special apparatus, 

 consisting of a graduated tube filled with mercury and communicating with a glass globe also 

 filled with mercury, which can be lowered as the blood flows into the graduated tube.] The 

 extraction of the gases from the blood, and their collection for chemical analysis, are carried out 

 by means of the mercurial pump (C. Ludwig). Fig. 22 shows in a diagrammatic form the 

 arrangement of Pfliiger's gas-pump. 



It consists of a receptacle for the blood, or " blood -bulb " (A), a glass globe capable of con- 

 fining 2f)0 to 300 c.c, connected above and below with tubes, each of which is provided with 

 a stop-cook, a and b ; b is an ordinary stop-cock, while a has through its long axis a perforation 

 which opens at .', and is so arranged that, according to the position of tho handle, it leads up 

 into the blood-bulb (position x, a), or downwards through the lower tube (position xf, a'). This 

 blood-bulb is first completely emptied of air (by means of a mercurial air-pump), and then care- 

 fully weighed. One end {x) of it is tied into an artery or a vein of an animal, and when the 

 lower stop cock is placed in the position x, a blood flows into the receptacle. When the 

 necessary amount of blood is collected, the lower stop-cock is put into the position x', a', and the 

 blood-bulb, after being cleaned most carefully, is weighed to ascertain the weight of the amount 

 of blood collected. The second part of the apparatus consists of the froth-chamber, B, leading 

 upwards and downwards into tubes, each of which is provided with an ordinary stop-cock, c and 

 d. The froth-chamber, as its name denotes, is to catch the froth which is formed during the 

 energetic evolution of the gases from the blood. The lower aperture of the froth-chamber is 

 connected by means of a well-ground tube with the blood-bulb, while above it communicates 

 with the third part of the apparatus, the drying-chamber, G. This consists of a U-shaped tube, 



f>rovided below with a small glass bulb, which is half filled with sulphuric acid, while in its 

 irabs are placed pieces of pumice-stone also moistened with sulphuric acid. As the blood 

 gases pass through this apparatus (which may be shut off by the stop-cocks c and f), they are 

 freed from their watery vapour by the sulphuric acid, so that they pass quite dry through the 

 stop-cock, /. The short well-ground tube, D, is fixed to /, and to the former is attached the 

 small barometric tube or manometer, y, which indicates the extent of the vacuum. From D we 

 pass to the pump proper. This consists of two large glass bulbs, which are continued above 

 and below into open tubes ; the lower tubes, Z and w, being united by a caoutchouc tube, G. 

 Both tho bulbs and the caoutchouc tube contaiu mercury the bulbs being about half full, and 

 F being larger than E. The bulb, E, is fixed ; but F can be raised or lowered by means of a 

 pulley with a rack and pinion motion. If F be raised, E is filled ; if F be lowered, E is emptied. 

 The upper end of E divides into two tubes, g and h, of which g is united to D. The ascending 

 tube, h (gas-delivery tube), is very narrow, and is bent so that its free end dips into a vessel 

 containing mercury, v (a pneumatic trough), and the opening is placed exactly under the tube 

 for collecting the gases, the eudiometer, J, which is also filled with mercury. Where g and H 

 unite, there is a two-way stop-cock, which in one position, H, places E in communication with 

 A, B, G, D, the chambers to be exhausted, and in the position K shuts off A, B, G, D, and 



