ELASTICITY OF AIR. 



WHEN a part of the air enclosed in any vessel is withdrawn, that which re- 

 mains expanding by its elastic property, fills the dimensions of the vessel as 

 effectually as before. Under these circumstances, however, it is obvious that 

 any given space within the vessel contains a less quantity of air than it did 

 previously, inasmuch as while the whole dimensions of a vessel remain the 

 same, the total quantity of air diffused through them is diminished. When the 

 same quantity of air in this manner is caused to expand into a greater space, it 

 is said to be rarefied. 



But on the other hand, when a vessel containing any quantity of air is caused 

 to receive an increased quantity by additional air being forced into it, then any 

 given portion of its dimensions will contain a proportionally greater quantity 

 of air than it did before the additional air had been forced in. Under these 

 circumstances, the air contained in the vessel is said to be condensed, and it is 

 our purpose in the present lecture to describe the mechanical instruments by 

 which these processes of rarefaction and condensation are practically effected. 



THE EXHAUSTING SYRINGE. 



The most simple form of instrument for producing the rarefaction of air, is 

 that which is called the exhausting syringe. In order to comprehend the con- 

 struction and operation of this instrument, let us suppose A, B, fig. 1, a cylin- 

 der, or barrel, furnished with a stop-cock C, inserted in a small aperture in the 

 bottom. Let the end of this tube be screwed upon the vessel R, in which the 

 rarefaction is to be made. 



From the side of the barrel near the bottom, let another tube, D, proceed, 

 also furnished with a stop-cock. Let us suppose the piston P, at the bottom 

 of the barrel, both stop-cocks being closed. Let the piston P be now drawn 

 from the bottom to the top, as represented in fig. 2, this piston being supposed 

 to move air-tight in the barrel. A vacuum will remain between the piston P 



