402 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



^^ 



separate values from the mean was less than 0.1 per cent except in the 

 case of the extreme dilutions, where it amounted in a few cases to 0.3 

 per cent. The probable error of the mean was always less than 0.1 per 

 cent except in the case of water. Both the slide-wire and the rheostat 

 used were carefully calibrated, and the corrections were found in no case 

 to exceed 0.1 per cent. The resistance vessel for 

 holding the solution whose conductance was to be 

 measured was a platinum cylinder 15 cm. high and 

 5 cm. in diameter. Into this was inserted, so as to 

 rest on the bottom of the cylinder, an inverted glass 

 bell 4.4 cm. in diameter attached to a glass tube 

 sealed near the bottom, as shown in Figure 1. 



Into the bell an unplatinized but meclianically 

 roughened platinum disk was sealed by means of a 

 stiff platinum wire. The electrode was not platinized, 

 so as to reduce as far as possible absorption effects. 

 It had an area of 15 sq. cm., and the distance be- 

 tween its lower surface and the bottom of the platinum 

 cylinder was 1.9 cm. There was enough space along 

 the side of this disk to allow the liquid to pass, and 

 near the top of the bell a large hole was blown so 

 that raising and lowering the glass plunger gave an 

 Figure 1. efficient means of stirring. It was proved that the 



" cell constant " of the apparatus varied less than 0.05 

 per cent with different settings of the plunger in the vessel. The use 

 of a platinum vessel made it possible to collect the water at a little be- 

 low the boiling temperature as it came off fresh from the still, whereby 

 better water could be obtained, as there was less chance for the absorp- 

 tion of carbon dioxide. The plunger passed through a vulcanite cap whicli 

 fitted the top of the cylinder, and thus kept tlie solution from much con- 

 tact witli the air. The cylinder was immersed almost to the top in a 

 thermostat kept always within 0.02° of 18°. The temperature was 

 measured by a thermometer that had been standarized and corrected to 

 the scale of an air thermometer. The room was also kept nearly at the 

 same temperature to prevent condensation at the top of the cell. Con- 

 tacts were made by mercury. 



The solutions were made by adding successively a number of portions 

 of standard acid (approximately 0.01 normal) from a weighed Beckman 

 pipette to a weighed quantity (about 180 grams) of water in the vessel. 

 The weight of the added portion could easily be determined to less 



