LABORATORY WORK 



173 



The effect of a solute in lowering the vapour pressure of the 

 solvent may be tested by allowing two vessels, such as flat porcelain 

 capsules, one containing water, the other 10 per cent, sodium 

 chloride, to remain for some time side by side under a small bell 

 glass, closed by resting the rim, greased with vaseline, on a piece 

 of plate glass. The two vessels are weighed to begin with, and \ 

 again after a few days. Water will be found to have passed from the / 

 vessel containing it in the pure form to that containing the solution. 



Electrolytic Dissociation. To make measurements of electro- 

 lytic conductivity with accuracy requires somewhat complicated 



B 



FIG. 10. 



A, dry cell or storage cell. 



B, key. 



C, high resistance slide wire to obtain a fraction of the E. M. F. of the battery. For 



the present purpose, it need not be graduated, but this may be necessary for other 

 work. 



D, sliding contact. 



E, galvanometer. 



F, small beaker with the platinum electrodes. 



apparatus. For our purpose it will suffice to use a galvanometer, 

 and to observe the deflections produced by the same potential 

 difference through the various solutions to be compared. The 

 single-pivot galvanometer (one scale division = one micro-ampere), 

 made by Paul, will be found to be convenient. The current is sent 

 through the solution by means of two platinum plates, about I cm. 

 square, immersed therein. They should have platinum wires welded 

 to them, and the wires then fused into glass tubes passing through 

 holes in a flat rod of ebonite, about 2 cm. apart. The circuit is 

 arranged as in the diagram (Fig. 10). The wires to the electrodes 

 should be fine copper wires and inserted into mercury in the glass 

 tubes, in order to make contact with the platinum plates. 



