ELASTICITY OP AIR. 59 



as the external atmosphere. This arises from the valve V requiring some 

 definite force, however small, to open it. When the air which has passed into 

 the chamber A, B, requires a pressure wfcch is less than the atmospheric 

 pressure, by an amount equal to the tension of the valve V, then the excess 

 of the pressure of the atmosphere over the resistance of the air contained in 

 A B will be insufficient to open the valve V 7 , and no more air can pass into 

 the cylinder. It should also be observed, that the valve V being pressed up- 

 ward by the elastic force of the air condensed in the receiver, requires a still 

 greater pressure than this to open it, and therefore, before the valve V can be 

 opened, the air enclosed below the piston P must always be condensed by the 

 pressure of the piston in a higher degree than the air is condensed in the re- 

 ceiver. The observations which have been made respecting the limit of the 

 operation of the exhausting syringe, arising from mechanical imperfections and 

 other causes, will also be applicable here. However nicely the piston P and 

 the cylinder in which it plays may be constructed, there will still be some 

 small space remaining between it and the silk-valve V, when it is pressed to 

 the bottom of the cylinder. Into this space the air contained in the cylinder 

 may finally be condensed, and when the pressure of the air contained in the re- 

 ceiver becomes equal to the pressure of the air condensed into the space be- 

 tween the piston at the bottom of the cylinder and the silk-valve, the operation 

 of the instrument must necessarily cease ; for then the utmost degree of con- 

 densation which can be produced above the silk-valve V will be insufficient 

 to open the valve, and therefore the syringe cannot introduce more air into the 

 receiver. 



THE CONDENSER. 



The condenser has the same relation to the apparatus just described as the 

 air-pump has to the exhausting syringe. The condenser consists of a receiver 

 firmly and conveniently fixed, communicating by a tube with one or two con- 

 densing-syringes, which may be worked in the same manner as the exhausting- 

 syringe described in the air-pump. 



In the use of such an instrument, it is convenient to possess the means of 

 indicating the degree of condensation which has been effected. For this pur- 

 pose a mercurial gauge is used analogous to that which is applied to the air- 

 pump. A bent tube, A, B, C, fig. 14, contains a small quantity of mercury, 



Fig. 14. 



S, B, S', in the curved part. When the ends of the tube are open, and in 

 free communication with the atmosphere, the surfaces S, S 7 , will stand at the < 

 same level. The extremity C is furnished with a stop-cock, by which a com- 



