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or by using apparatus more suitable to the purpose, much 
greater heights might be attained. But however this may 
be, we have proof that mercury and water will by their 
cohesion resist a tension of at least one atmosphere, or that 
the common pump would, if the water were free from air, 
raise water by suction to a height of more than 60 feet. At 
first sight it cannot but appear remarkable that such a fact 
should for so long have escaped notice, but a little considera- 
tion removes the difficulty. 
Water is almost always more or less saturated with air, 
which separates into bubbles as soon as the pressure is 
relieved, and in the common pump a single minute bubble 
would be sufficient to cause the column to break, and pre- 
vent it being raised to a greater height than that due to 
the pressure of the atmosphere. 
In the case of barometers it is the custom to fill the tubes 
full and boil the mercury, so as to get rid of the air ; but the 
column falls to the usual height not by the rupture of the 
mercury, but by the separation of the mercury from the 
glass, for which it has but little adhesion. Whether the 
ordinary method of boiling the mercury really disengages all 
the air is, I think, an open question. In vacuum gauges of 
small diameter it is not uncommonly found that the mercury 
sticks to the glass until the pressure has fallen considerably 
below what is represented by the height of the mercury, so 
that on the gauge being shaken the mercury falls with a 
sudden drop. Although it does not seem to have attracted 
any especial notice, this phenomenon is clearly due to the 
same cause as that which I have found capable of main- 
taining 30 inches of mercury suspended in a comparatively 
large tube. 
It would seem then that although the facts which I now 
bring before the Society have little bearing on the practical 
limits to the height of the column of mercury in the bar- 
ometer or the column water in the common pump, they show 
