16 PROF. OSBORNE REYNOLDS ON THE 



air, raise water by suction to a height of more than sixty- 

 feet. At first sight it cannot but appear remarkable that 

 such a fact should for so long have escaped notice ; but a 

 little consideration 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 sufl&cient to cause the column to break and 

 prevent 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 disen- 

 gages 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 mercuiy, so tliat on the gauge being shaken the mer- 

 cury falls with a sudden drop. Although it does not seem 

 to have attracted any special notice, this phenomenon is 

 clearly due to the same cause as that which I have found 

 capable of maintaining thirty inches of mercuiy 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 prac- 

 tical limits to the height of the column of mercury in the 

 barometer or the column of water in the common pump, 

 they show that these limits are owing to the presence of 

 air or some other minor disturbing cause, and are not, as 

 seems to have been hitherto supposed, owing to the want 

 of cohesion of the liquid. And it seems to me that the 



