422 ILLUSTRATIVE EXPERIMENTS 



indicate no E.P.D. The cell x is of interest. It consists of two half cells 

 or electrodes. One of these is the ordinary standard calomel electrode, 

 i.e. an electrode of mercury covered with HgCl in the presence of a definite 

 concentration of KC1. The other half cell is a hydrogen electrode, i.e. 

 platinum black laden with hydrogen and immersed in a solution containing 

 H-ions. The difference of E.M.F. between electrode and solution depends 

 on the concentration of H-ions in the latter. 



The difference of potential between the calomel and normal hydrogen 

 electrode can be ascertained. This value is subtracted from the total 

 E.M.F. of the cell to give a value from which the ^>H may be calculated. 



46. Determination of the Relative Viscosity of Blood (see expt. 24). 

 Blood may be taken from the lobe of the ear or from the finger. The skin 

 is thoroughly cleansed with ether and pricked with a fine pointed lancet. 



Fid. 82. Denning-Watson Clinical Viscosimeter (Hawksley)." 



The viscosimeter (at body temperature) is held vertically under the bleeding 

 spot and the receiver filled (Fig. 82). Measure as in expt. 24. 



Immediately after an observation, the blood should be driven out by a 

 blast from an inflator attached by rubber tubing to the short arm of the 

 capillary. 



It is advisable to make a blood count at the same time (p. 242). 



47. Haematocrite. This apparatus consists of two similar graduated 

 capillary tubes, which, after clipping in a holder, may be spun hori- 

 zontally by a centrifuge. A small measured quantity of Miiller's fluid 

 (Na 2 S0 4 -l gm. ; K 2 Cr 2 7 - 2 gm. ; distilled water, 100 gm.) is placed 

 in a test glass. The same quantity of blood is added. Mix thoroughly 

 with a glass rod. Fix a short length of rubber tubing furnished 

 with a mouthpiece to each of the graduated tubes and suck up sufficient 

 of the mixture to fill the tubes. Place them in a holder and spin 

 for 5-7 minutes with a velocity of 8000 revs, per minute. Read off the 

 relative length of the column of corpuscles. As the glass walls of the 



