14 COMPRESSIBILITY OF AIR. 



the aperture and strongly blowing into it, the water 

 will be made to sink at b and to rise at c : the air above 

 c is compressed. If the neck of the retort be held first 

 very slightly inclined and filled with water up to a, and 

 then turned so as to become nearly or quite vertical, 

 the water will immediately sink a little below a, because 

 the weight of the water in the neck, which now forms 

 a higher column than before, compresses the air above 

 c. The pressure of the hand can also be made to show 

 the compressibility of air. A medicine bottle is closed 

 by a well-fitting cork ; one end of a piece of glass tube 

 is passed through the cork, the other end into a small 

 bladder(a calf's bladder), which is firmly tied round it. 

 Holding the bladder by the cork, it is filled with water 

 and the bottle then placed upon it in an inverted 

 position (fig. 14). If the bladder be now strongly 

 pressed, some of the water will pass into the bottle, as 

 far as a ; but if the pressure ceases, the air within the 

 bottle, which has been compressed, expands again, and 

 drives the water back into the bladder. The bladder 

 when wet slips easily off the tube ; this may be pre- 

 vented by passing the latter through a cork, and tying 

 the bladder firmly round the cork. 



If a piece of glass tubing is required for any purpose, it may be 

 eut from a longer tube by making- a scratch across it with a 

 triangular file, or a knife-blade of hard steel which has a rough 

 edge. To sharpen such a blade it should be ground on a coarse 

 stone without water. Tubes of less than l cm in diameter require 

 only a single scratch across one side, and may be broken off by a 

 conjoint sharp pull and bend, placing both thumbs upon the tube 

 opposite to the scratch. Stronger tubes must be filed all round 

 and cracked along the scratch by charcoal or pastille. The sharp 

 edges at the fracture must be rounded off, especially in tubes which 

 have to be inserted into corks. In smaller tubes the edges dis- 

 appear if the end is simply heated to redness; but in thick and 



