New York AGRICULTURAL EXPERIMENT STATION. 215 
METHOD OF MEASURING THE CONDUCTIVITY OF THE FILTRATE. 
The flask containing the filtrate is closed at once after filtration 
by a paraffined rubber stopper, prepared in the manner previously 
described; it is then placed in the thermostat at 25° C. and allowed 
to remain until the solution has reached this temperature, usually 
about one hour. ‘The electrical conductivity is then measured in an 
ordinary Ostwald or Arrhenius cell. 
After the filtrates in a number of flasks have been brought to 
the temperature of the thermostat, the conductivities can be taken 
in rapid succession. We found the following method of manipula- 
tion to combine satisfactorily accuracy and rapidity in running a 
series of measurements :— The cell and electrodes are dried with 
clean filter paper after examining each filtrate, rinsed with the next 
filtrate to be tested and then placed in the thermostat, after which 
some of the solution to be tested is placed in the cell. After the 
resistance is taken, the solution is stirred with the electrodes a 
short time to make sure that the solution has reached the tempera- 
ture of the bath. We rarely observed any change in the second 
reading. 
The reading is then taken with care, using in the resistance box 
a resistance which brought the knife contact near the center of the 
bridge, where the error is least. The solution in the cell is replaced. 
by a second portion from the same flask and the reading repeated. 
The two readings are usually identical. In case of disagreement, a 
third portion of solution is used, which practically always agrees 
with the second. In the case of the last portion used, three read- 
ings are taken on different parts of the bridge. The results usually 
agree within a limit of one part in a thousand. 
All measurements of conductivity were taken with a calibrated 
Wheatstone bridge by the usual Kohlrausch method at a tempera- 
ture or 25° C. + 0.03° C, maintained in a thermostat by an Ostwald 
regulator. 
The capacities of the Ostwald and Arrhenius cells were deter- 
mined and frequently checked with N-50 and N-500 KCI solutions, 
respectively. The solutions were made by successive dilutions from 
a standard normal solution having a specific gravity at 18° C. of 
