638 APPARATUS FOR MACROSCOPIC CATAPHORESIS 



Substituting for the viscosity (0.009) and dielectric constant of 

 water at 20° (80), and converting to volts it is found that p.d. in 

 millivolts = 12.6 X n per sec. X volts per cm. = 4.5 X mm. per 

 hour X volts per cm. 



In the experiments to be described no correction was made for a 

 change in the dielectric constant nor viscosity. 



The distance is taken as the average of the observed movement in 

 the two arms. This corrects for any gravity effect. As a rule, how- 

 ever, the two readings are nearly identical and any marked dis- 

 crepancy is apt to be the result of a leak in the apparatus or some 

 other accidental error. 



The apparatus has been found to work very satisfactorily for 

 gelati' 1, edestin, the bacillus of rabbit septicemia, and other fine sus- 

 pensions. With suspensions containing larger particles such as casein, 

 typhoid bacilli, etc., the line of demarcation becomes blurred and the 

 results are not satisfactory. For any particles that can be seen 

 without an oil immersion lens the microscopic method is better. 



Accuracy oj the Measurements. — The measurements can usually be 

 repeated as closely as can be read; i.e., ±0.1 mm. The error is 

 greater in very dilute and very concentrated salt solutions. 



