LXVI.] VITAL CAPACITY, ETC. 313 



(". ) Suppose the gases of the blood to be extracted ; they are collected in a 

 eudiometer over mercury (fig. 237). Or, for practice, and merely to grasp the 

 principle how the relative proportion of the gases in a mixture is ascertained, 

 the student may use air containing a small quantity of carbon dioxide. 



(b.) Fuse a ball of potash on the end of platinum wire (best done in a bullet- 

 mould). Introduce this under the mercury into the gases in the eudiometer. 

 The caustic potash absorbs all the C0 2 (twenty-four hours), and the diminution 

 in volume represents the proportion of C0 2 in the mixture. 



(c.) With a curved pipette introduce a solution of pyrogallic acid into the 

 eudiometer containing the remainder of the gases ; this unites with the potash 

 to form pyrogallate of potash, which rapidly absorbs the oxygen. The decrease 

 in volume represents the amount of 0. The remainder of the gas present 

 represents N. 



FiQ. 237. Gases collected 

 over mercury. A ball of 

 FIG. 236. Heywood's Experiment. caustic potash absorb- 



ing the C0 2 . 



There are other methods of estimating the proportion of the gases, but this 

 simple experiment is sufficient to explain the general principle on which such 

 estimations are made. Of course there are corrections for temperature and 

 pressure, and other precautions which require to be taken, but we do riot enter 

 into these here. (See Appendix.) 



A simple form of gas-pump has been devised by L. Hill (Journ. of I'hys., xvii. 

 P- 353)' by means of which results of sufficient accuracy are obtained from 10 

 cc. of blood. 



6. Analysis of Expired Air by Hempel's Method. 1 



A burette, A (fig. 238), containing 100 cc., and graduated into tenths of 

 a cc., is used to measure the expired air. It communicates below by means 

 of an india-rubber tube with the movable tube or reservoir for water, B. 

 Above, A is connected to an absorption pipette bv means of a short india- 

 rubber tube of 1-2 mm. diameter with thick walls, and provided with a 

 Mohr's clip. The tube, A, is placed in connection successively with the 

 pipettes, px, which contain a solution of caustic potash to absorb the C0 2 

 and fig. 239, which contains sticks of red phosphorus in water to absorb the 0. 



1 Methods qf Gas Analysis, by W. Hempel. London, 1892, 



