165 
tube was full of water the column would be higher than the 
pressure of the atmosphere would maintain, and hence could 
only be maintained by the cohesion of the water. The 
difficulty of such an experiment, however, appeared to be 
great. It was clear that if mercurv could be substituted 
o ^ 
for water this difficulty would be much reduced, but then 
mercury does not readily adhere to glass, and the ordinary 
method of making barometers seemed to disprove the pos- 
sibility of making it adhere. 
It was only on the 2nd of this month that an accidental 
phenomenon at once afforded me the experimental proof for 
which I had been looking. 
First Experiments. 
The phenomenon was observed in a mercurial vacuum 
gauge (a syphon gauge, which allowed of a column of mer- 
cury 31 inches long). Before the mercury was introduced 
the tube had been wetted with sulphuric acid, a few drops 
of which covered the mercury on both ends of the column. 
The gauge had been in constant use as a vacuum gauge 
for three weeks, and, probably owing to the action of the 
acid on the mercury, a little gas had generated between 
the mercury and the closed end of the tube, sufficient to 
cause the column to sink to 27 \ when the barometer stood 
at 29. To get rid of this air the tube was removed from its 
situation and placed in such a position that the bubble of 
air passed along the tube and escaped, the open end of the 
tube being entirely free. Before the tube was tilled in 
this way the unbalanced column was 27 J inches long. 
When tilled the mercury ran back right up to the end of 
the tube as the bubble of air passed out. On erecting 
the tube again the mercury remained up to the end 
of the tube, except about one-eighth of an inch which 
filled with sulphuric acid. The unbalanced column of 
mercury was therefore 31 inches long. At first the full 
significance of this phenomenon was not recognised, 
