34 



Dr. G. Gore. 



[Dec. 18, 



application of a hot plate of metal to the locality of the adhesion, 

 rarely fails to remove them. Notwithstanding, however, all these 

 precautions, the meniscus often subsequently sticks at a particular 

 place without any apparent cause, and without the presence of any 

 body visible by the aid of a microscope magnifying from 30 to 50 

 diameters. Free use of the instrument reduces many of these ad- 

 hesions ; it also gives rise to fresh ones. Immovable adhesions of the 

 mercury frequently occur, or are subsequently produced by use of the 

 instrument which interfere with the accurate measurement of so 

 feeble an electric power as that mentioned above, and appear to be 

 due to irregularities of smoothness in the glass itself ; the best precaution 

 to take in such cases consists in selecting as the sphere of the move- 

 ment of the meniscus a portion of the capillary tube known to be free 

 from such a source of error ; but even such a space does not perma- 

 nently remain free. 



By employing a suitable tube, dilute sulphuric acid as the liquid, 

 and taking all the above precautions, I have constantly found that the 

 mercurial column, instead of moving from point to point with a jerk 

 and stopping suddenly, moved with a gradually diminishing speed 

 until it stopped. The momentary movement and sudden stoppage 

 occurs more readily with a short capillary tube and currents of greater 

 power than that above mentioned, but with a long tube, as free as ob- 

 tainable from irregular adhesion, a movement of from 10 to 20 millims. 

 not unfrequently occurs, requiring in some cases as much as 80 millims. 

 variation of height of mercury pressure to bring the meniscus back to 

 its original point. If the tube happens to be one of parallel bore, and 

 especially if its lower part enlarges towards its lower extremity, such 

 a current as the one mentioned will cause the meniscus to descend as 

 much even as 5 or 6 centims., and the mercury to run out at the end 

 of the tube. The influence of gravity no doubt operates in all these 

 cases, but with the same electric power, and whilst using a tube the 

 diameter of which does not diminish too rapidly towards its point, 

 the column usually moves more readily towards the end of the 

 capillary than the reverse, whether the instrument be in a vertical or 

 horizontal position. The mercury in all cases tends also to pass from 

 the form of a cylinder to that of a sphere, and if it once gets beyond 

 the end of the tube, it forms a globule which draws the narrower part 

 rapidly into itself by its great cohesion. Similarly if the tube is too 

 taper the mercury tends to draw itself upwards towards the wider 

 cavity. 



A good capillary tube is essential ; the only way I know of obtain- 

 ing it is to make a number of tubes and try them until a suitable one 

 is found. Glass tubing, of an external diameter of about 6 to 8 

 millims., and having a bore of about one-third or one-fourth of a milli- 

 metre, is of a very convenient size for making the capillary, because 



