Mr. Daniell on the Barometer. 83 



inch. I made a contrivance to hold a glass tube perpendicularly 

 immersed in a basin filled with mercury ; and when one of the 

 steel points was made to touch the surface of the fluid in the tube, 

 and the other the surface in the basin, the depression of the for- 

 mer was accurately measured by the nonius. In this manner I 

 determined the capillary action of several tubes, varying in their 

 diameters from one to six-tenths of an inch. The results agreed 

 as nearly as possible with Dr. Young's table, calculated from the 

 experiments of Mr. Cavendish. The end of the tubes, opposite to 

 those at which the measures had been taken, were then hermetically 

 sealed, in such a manner as to be readily reopened under mercury : 

 they were immediately filled with mercury, and carefully boiled. 

 I expected to be able to ascertain the differences of depression by 

 opening them in the basin of mercury, and proceeding as before. 

 The experiment was performed as soon after the operation of boil- 

 ing, as the mercury in the tube had cooled down to the tempera- 

 ture of that in the basin. At first the attraction between the mer- 

 cury and the glass appeared to be perfect, and no depression could 

 be perceived. When, however, the tubes were left some time 

 exposed, either before or after they were opened, the air and mois- 

 ture insinuated themselves between the metal and the glass, and 

 an immediate depression was the consequence. This depression 

 increased gradually, till at length it became fixed at the exact 

 point of that of the unboiled tube. The progress of this effect 

 was easily perceptible with a magnifying glass, and was rendered 

 still more visible fby heating the tube, when air-bubbles were 

 immediately detached. This is obviously the same effect as that 

 described by Sir H. Davy in his paper before alluded to, in which 

 he says that, ** on keeping the stop-cock of one of the tubes, used 

 in the experiment on the mercurial vacuum, open for some hours, 

 it was found that the lower stratum of mercury had imbibed air, 

 for when heated in vacuo it emitted it distinctly from a space of 

 a quarter of an inch of the column ; smaller quantities were dis- 

 engaged from the next part of the column, and its production 

 ceased at about an inch high in the tube." Now, I believe that 

 I should not be too presumptuous if, stopping here, I were to 



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