Section 8. Standardization of the Apparatus. 29 



Some 0.02 normal potassium chloride solution* is then boiled to free it 

 from air, and, right after cooling, enough of this to fill the bottom part of 

 the bomb to within 1 or 2 mm. of the flange is weighed in from a pipette. 

 The mouth of the pipette is kept under the surface to diminish the absorp- 

 tion of air. The cover is next put on and screwed down, care being taken 

 not to tip the bomb enough to get any of the solution into the mouth 

 of the capillary tube. By means of the water pump the air pressure in the 

 bomb is reduced to about 2 cm., and the valve is then closed. If the air is 

 not removed from the solution at the start, it comes out rapidly upon 

 reducing the pressure and spatters some of the solution up into the tube, 

 thus allowing it to be swept out by the air current. 



The lead wires are now bolted on and the bomb is placed in the liquid 

 xylene bath, serving ordinarily for the 26 measurements, and the temper- 

 ature of the latter is raised by means of the heating coil. The liquid level in 

 the bomb is at the start about 3 mm. below the point of the auxiliary elec- 

 trode, so that the resistance of the upper cell is shown by the conductivity 

 apparatus to be infinite ; but upon heating, the level rises and finally 

 touches the electrode, whereupon the resistance suddenly sinks to perhaps 

 1,000 ohms. The temperature of the bath (perhaps about 130) is now 

 held constant until the solution in the bomb has also attained it, as will be 

 indicated by the resistance of the lower and, far more sensitively, by that 

 of the upper cell becoming constant. Both these resistances are then 

 noted, and the temperature is measured. 



The temperature is now raised by steps of three or four degrees until 

 that ratio of the conductances is reached which corresponds to the bomb 

 being almost completely full. This limiting ratio can be determined cold 

 at any time by measuring the resistance of the lower cell and then invert- 

 ing the bomb and measuring that of the upper cell. Finally, the conduct- 

 ance-ratios are plotted as abscissas and the corresponding volumes as 

 ordinates, whereby a straight line is obtained. 



The computation of the volumes is made with the help of the following 

 data : Zepernick and Tammannf have found that equal volumes of a 

 0.52 normal potassium chloride solution and of water at upon heating 

 from that temperature to 140 become different from each other by only 

 0.1 per cent. It is therefore perfectly safe to assume that the expansion of 

 the 0.02 normal potassium chloride solution used by us is the same as that 

 of pure water. From Hirn'sJ results the specific volume of water at the 



*The reasons for taking this solution instead of pure water are that it makes 

 the conductance at the upper electrode high enough to give a good minimum, and 

 that the solution is so strong that contamination can not possibly make any trouble. 



tZ. phys. Chem., 16, 665 (1895). 



JG. A. Hirn, Ann. chim. phys., (4), 10, 32 (1897). His series of observations 

 covers the range of temperature up to 180. Between 110 and 143 his values 

 differ from those found by Zepernick and Tammann by only 0.02 per cent. 



