60 



ELASTICITY OF AIR. 



munication with the atmosphere may be permitted or intercepted. The ex- 

 tremity A communicates by a tube with the receiver in which the air is to be 

 condensed. At the commencement of the process, before any condensation has 

 taken place, the stop-cock C is closed, and the air included between it and the 

 surface S 7 has then the same pressure as the external atmosphere. The air 

 in the receiver having also that pressure, the two surfaces S and S 7 necessa- 

 rily stand at the same level. When the condensation of air in the receiver 

 commences, the pressure on the surface S is increased, therefore that surface 

 falls, and the surface S' rises. The pressure of the air condensed in the re- 

 ceiver will thus be balanced by the weight of the column of mercury between 

 the levels S and S 7 , together with the pressure of the air enclosed between S' 

 and C. The pressure of the air enclosed in S 7 C is increased in the same 

 proportion as the space S' C has been diminished. Now, as the original pres- 

 sure of the air contained in this space was equal to the pressure of the atmo- 

 sphere, it is always easy to find the pressure of the air reduced in bulk by in- 

 creasing the amount of atmospheric pressure in the same proportion as the 

 space S 7 C has been diminished. Thus if the air enclosed in the tube be re- 

 duced to half its original bulk, then the pressure it exerts will be double the 

 atmospheric pressure. If it be reduced to two thirds of its bulk, then the 

 pressure of the enclosed air will be the atmospheric pressure in the proportion 

 of three to two, and so on. The pressure thus computed being added to the 

 pressure arising from the column of mercury between the levels of the surfaces 

 S and S 7 , will give the whole pressure of the air condensed in the receiver. 



Although the condenser is not without its use in experimental physics, yet 

 it is an instrument far less important than the air-pump, to which it is so 

 analogous. The cases are innumerable in which it is necessary to inquire 

 what effect would take place in the absence of the atmosphere ; but they are 

 comparatively few in which it is necessary to investigate what effects would 

 be produced under increased atmospheric pressure. We do not, therefore, 

 think it necessary to enter into further details concerning the condenser. 



