346 Mr. R. T. Glazebrook's Comparison of 



Experiment showed that mercury from different sources, 

 even when it had been rendered impure by the admixture of 

 copper, lead, or zinc, after being treated by this process, gave 

 the same results. The process, however, does not free the 

 mercury from silver or from the less oxidizable metals. 



[In my experiments described below the mercury was 

 freshly distilled by the aid of the admirable piece of apparatus 

 designed by Weinhold, and introduced into England by Mr. 

 W. N. Shaw.] 



5. Method of Cleaning the Tubes. — The tubes should be 

 cleaned by passing through them in succession: — 



1. Distilled water. 



2. Strong nitric acid. 



3. Distilled water. 



4. Ammonia. 



5. Distilled water. 



They must then be dried by a current of dry air. 



To clean the tubes it is best to separate them from the cups. 

 To effect this, the pieces of india-rubber tubing which connect 

 them should be cut. To make the glass slip into the india- 

 rubber tubing, it should be moistened with a drop of benzine. 



In preparing the tubes for my measurements, I endeavoured 

 to carry out the above instructions as carefully as possible. 

 The resistance comparisons were made by Carey Foster's 

 method, using the wire bridge of the British Association as 

 designed by Dr. Fleming. 



In order to compare the resistance of one of the tubes with 

 a standard coil, it was necessary either to know accurately or 

 to eliminate the resistance of the contact-pieces. It was also 

 important to determine within what limits the resistance of 

 the contact-pieces remained constant, and whether they could 

 be repeatedly filled with mercury in such a way as to retain 

 the same resistance. 



For this purpose they were compared with a short piece 

 of copper rod. The cups D (fig. 2), one belonging to each 

 contact-piece, were immersed in the same beaker of mercury, 

 being placed close together and covered to the same depth 

 as when actually in use; the other ends of the copper rods 

 E connected in the usual way to the bridge. The contact- 

 tubes had been filled by being placed in a large test-tube 

 which was exhausted, and into which mercury was then 

 admitted by means of a tap attached to the test-tube. The 

 resistance of the contact-pieces was thus found in terms of 

 the divisions of the bridge-wire before and after a set of 

 comparisons of the mercury-tubes and standard coils. 



