CHANGES IN AIR AND BLOOD IN RESPIRATION. 675 



apparatus is arranged so that the blood to be examined is brought 

 into a vacuum while kept at the temperature of the body. Under 

 these conditions all of the oxygen and nitrogen and part of the car- 

 bon dioxid are given off and may be collected by suitable means. 

 A portion of the carbon dioxid present in the blood is in such stable 

 combination that to remove it it may be necessary to add some 

 dilute acid, such as phosphoric acid. This portion of the carbon 

 dioxid is designated in this connection as the fixed carbon dioxid. 



The principle of the gas pump may be explained most easily by describing 

 the simple 91 devised by Grehant. The essential parts of this pump are 

 represented in Figure 272. The mercury pump consists of two bulbs, one 

 movable (M), the other fixed (F). M may be raised and lowered by the wind- 

 lass (P) . Above F, there is a three-way stopcock (m) by means of which the 

 chamber F may be put into communication with the outside air by way of C, 

 or with the bulb B, which is to contain the blood, or may be shut off com- 

 pletely. If M is raised so as to fill F entirely, and the stopcock ra is shut off, 

 then on lowering M the mercury will flow into it, leaving a perfect vacuum 

 in F, since the distance between F and M is greater than the barometric 

 height. If the stopcock m is turned so as to throw F into communication 

 with B, the chamber of this latter is brought under the influence of the vac- 

 uum and any gases that it may contain will be distributed between B and 

 F. If stopcock m is again turned off and M is raised, the gases in F will be 

 condensed at its upper end, and by turning the stopcock m properly these 

 gases may be forced to the outside by way of C or may be collected, if de- 

 sired, in a burette filled with mercury and inverted over the opening from 

 F contained in the bottom of C. In performing an experiment the flask 

 B, which is to contain the blood, is connected with F, as shown in the figure, 

 all joints being protected from leakage by a seal of water outside, as shown 

 at h, which represents a piece of wide rubber tubing filled with water so as 

 to protect a joint between two pieces of glass tubing. B is next exhausted 

 completely by raising and lowering M a number of times in the way described 

 above until on throwing B into communication with a vacuum in F no further 

 gas is given off. The last particles of air may be driven out from B by boil- 

 ing a little water in it. After a complete vacuum has been established in B 

 a measured amount of blood is introduced from a graduated syringe, S, as 

 represented in the figure. This blood must be taken directly from the vessels 

 of the animal and be introduced into B at once. B is kept immersed in water 

 at the temperature of the body, and the bulb M is now raised and lowered a 

 number of times so that the gases given off from the blood are drawn over 

 into F and then by proper manipulation of the stop-cock are driven into 

 a burette fastened over the opening of the tube in C. To drive off all of the 

 carbon dioxid a little dilute phosphoric acid must be added to the blood in 

 B by means of the syringe, S. The gases thus collected into the burette are 

 first measured and are then analyzed for the three important constituents 

 by some of the accepted gasometric methods. The principle involved is to 

 absorb first from the mixture all of the CO 2 by introducing a solution of sodium 

 or potassium hydrate. The reading of the volume left after this absorption 

 is completed compared with the first reading gives the volume of CO^ Next, 

 a freshly made alkaline solution of pyrogallic acid is introduced into the tube. 

 This solution absorbs all of the oxygen, whose volume is thus easily determined. 

 The gas that is left unabsorbed after the action of these two solutions is nitro- 

 gen. The volumes of gases are reduced, as is the custom, to unit pressure 

 and temperature, that is, to zero degree centigrade and 760 mms. barometric 

 pressure. A correction must also be made for the tension or pressure exerted 

 by the aqueous vapor in the gases. These corrections are made by means 

 of the following formula: 



