66 



Dr. H. J. S. Sand on the Concentration 



and then screwing down the glass tube on it. By always 

 proceeding in this manner, all risk was avoided of any 

 vaseline being spread on the surface of the electrode. The 

 method of obtaining metallic contact through the cap by 

 means of a drop of mercury will be understood from the 

 figure. On the top of the cap a piece of plate glass was 

 laid and on this a small spirit-level, it having been previously 

 ascertained that it was possible to level the end of the 

 polarimeter-tube by this means. The rest of the apparatus 

 will be sufficiently understood from the figure. As seen, it 

 was jacketed with water. The surface of the cathode was 

 measured by accurately determining the length of the polari- 

 meter-tube, and tben weighing it out with mercury. Jt was 

 found to be 0'699 sq. cm. 



Fijr. 3. 



IT 



The apparatus 3 was only employed in the series of 

 experiments on pure copper-sulphate solution, and only the 

 last three values of Table VI. have been obtained with it. The 

 cathode here again consisted of an engravers' plate, which 

 reached beyond the surface of the tube and allowed a small 

 level to be placed on its projecting part. In this apparatus 

 the cathode could be easily observed, when suitably illu- 

 minated. Its surface was calculated as that of an ellipse 

 from the length of two diameters of the tube, taken at right 

 angles to each other with a screw-caliper. Its value was 

 ,3*782 sq. cm. 



For the experiments the tubes were tightly clamped to a 



