1Q91 Mr Stewart on the Causes of Obstrndion in 



fion. A portion of the carbonic gas contained in the liquor con* 

 tinues to disengage itself, until, accumulating under the cork, it 

 causes such an increase of pressure on the surface of the fluid, 

 that the farther evolution of gas is stopped, the liquor becomes 

 tranquil, and the rest of the gas remains suspended in it. But 

 when, by the drawing of the cork, the accumulated pressure is . 

 suddenly reduced to that of the atmosphere, the suspended gas is 

 instantly disengaged with violent effervescence. 



The fact of the natural tendency of fluids, under the ordinary 

 atmospherical pressure, to absorb a certain portion of the air, 

 and to allow it to escape on the removal of that pressure, is ex- 

 hibited with great distinctness and precision in the well-known 

 experiment of the Toricellian tube. If a glass tube, three feet 

 in length, and with one of its ends hermetically closed, be filled 

 with unprepared quicksilver, and then inverted in a perpendicu- 

 lar position with its open end plunged in a basin of the same 

 fluid, the mercury in the tube will suddenly sink to the height 

 of thirty inches (the mean height of a column of mercury equi^ 

 yalent to an atmospherical column of equal diameter), leaving 

 in the upper end of the tube a perfect vacuum. This is proved 

 by bringing the tube to an inclined position ; for then the mer- 

 cury will suddenly spring upwards, until it strike sharply against 

 its upper extremity, falling to its former height when the tube 

 resumes its vertical position. But the surface of the mercury 

 in the tube being no longer pressed by the weight of the atmo- 

 sphere, the air contained in the fluid will begin to evolve itself; 

 and, rising to the surface, will diffuse itself in the vacant portion 

 of the tube. If, after a few days'* repose, the tube be again ex- 

 amined, it will be found that, on being inclined from its vertical 

 position, the mercury, although it will indeed still rise in the 

 tube, will no longer reach the summit; but, on approaching, it 

 will rebound with repeated oscillations, shewing at once the 

 presence and the elastkity of the air that has been disengaged. 



The same fact, of the absorption and disengagement of air, 

 may be shewn in the case of water, although in a much less con- 

 venient form, inasmuch as the column of water equivalent to 

 the atmospherical column is no less than 32 feet in height. If 

 a tube of iron or lead, about 34 feet in length, closed at one end, 

 and having (for the convenience of observation) a long slip of 



