ON THE SI'KCIKIC IIKSISTANCB OF MERCURY. 



301 



The micrometer screws were only used to measure very small lengths, never so 

 great as '1 cm., and were tested and found without any error which could be 

 appreciable. 



The tubes were cleaned by passing through them in succession nitric acid, caustic 

 potash, and distilled water, the last being repeated three times, alcohol, and finally 

 ether, redistilled for the purpose. These were followed by air dried with chloride of 

 calcium and passed through cotton wool. While the dry air was being passed 

 through the tubes were heated with a spirit lamp, and then allowed to cool. 



In several cases, following a suggestion of Professor ROWLAND'S, a plug of cotton 

 wool was pushed through the tube with a wire, in order to loosen any small particles 

 of dust which might adhere to the sides. 



To calibrate the tubes, a short thread of mercury was inserted and moved into the 

 various positions required ; its length X being measured with the reading microscopes. 



Table III. gives one set of readings for each tube. For the tubes VI., VIII., V., 

 IX., the calibration observations were repeated, using threads of different lengths. 



TABLE III. 



MDCCCLXXXVIII. A. 



3 A 



