in the Receiver of an Air-pump. 105 



by ebullition. This is best observed by inclining the tube till 

 the mercury conies into contact with the upper end, when any 

 air that may have been diffused through the vacuum will be 

 seen collected in a small bubble, but greatly rarefied. It is easy 

 to calculate approximately the depression of the mercurial column 

 produced by this residual air. For this purpose the tube must 

 be inclined till the bubble is exposed to a pressure of a few inches 

 cf mercury, measured in a vertical direction. In this position 

 its apparent diameter is measured, as also the pressure to which 

 it is exposed. For the object in view, the volume of the bubble 

 may be calculated on the assumption that it is a sphere. The 

 space occupied by the vacuum must also be estimated ; and with 

 these data, the depression of the mercurial column may easily 

 be calculated. 



Let V be the volume of the space above the mercury when 

 the tube is vertical ; 



p, the pressure under which the diameter of the bubble 

 of air has been measured ; 



r, the semidiameter of the bubble ; 



x, the depression of the mercurial column. 



Then 4 3 1 



z=-r 3 .irpY- 



If the diameter of the bubble 2r be 0*02 inch, the pressure p 

 2 inches, and the space V 1-2 cubic inch, the value of x is nearly 

 0-00001 inch ; or the depression of the mercury, in consequence 

 of the vacuum not being absolutely perfect, amounts only to 

 -i-dth of an inch. It is easy in actual practice to realize this 



lCKi,(J00 •> mi 



close approximation to a perfect vacuum. The quantities now 

 stated apply, in fact, to a barometric tube employed in an expe- 

 riment which will be subsequently described. 



The Torricellian vacuum leaves therefore scarcely anything to 

 be desired in point of completeness; but it is unfortunately 

 applicable to very few physical investigations. No instrument 

 of any kind can be introduced into it, nor even any substance 

 which is acted on by mercury. The vacuum obtained by the 

 exhausting pump is not liable to these objections ; but even with 

 machines of the most perfect construction, and in the best order, 

 a very imperfect approach can be attained to a complete exhau- 

 stion. A good ordinary pump with silk valves seldom produces 

 an exhaustion of 02 inch. ; and it is very rare indeed, if the 

 manometer is properly constructed, to have it carried to 0-1 inch. 

 In his "Etudes Hygrometriques " (Ann. de Chim. 3rd Series, 

 vol. xv. ]>. 190), M. Regnault has given the following method 

 for pushing the exhaustion further after the valves have ceased 

 to act. In a large glass globe of from 20 to 25 litres capacity 



