166 Prof. R. Bunsen on the Condensation of 



gas used in the experiments is passed in a perfectly dry con- 

 dition from the exit-tube, fitted on by an india-rubber stopper, 

 to the end B of the measuring-tube; and this is continued for 

 a considerable time until that passing out by stopcock c proves 

 to be quite pure. As soon as this is the case, the india-rubber 

 stopper as well as the stopcocks b and c are closed, and the 

 instrument placed in the mercury-trough for reading off the 

 level, in such a way that the lower level of the mercury in the 

 tube stands somewhat higher than that in the trough. To 

 accomplish this, a short thick glass rod of such a size as to 

 displace the necessary amount of gas is placed in the tube 

 beforehand and attached to the india-rubber stopper by means 

 of a thread ; then, on the removal of the cork from the tube 

 under mercury, the rod will be drawn out and the necessary 

 displacement effected. Whilst the stopcock b remains closed 

 the vessel A is put in communication with a mercury-pump 

 and exhausted as far as possible: and after closing the stopcock c 

 the pressure and temperature of the gas remaining behind in 

 A is estimated. The total weight of the gas still present in 

 the apparatus is in three separate parts — (1) the part in the 

 closed space A, (2) that in the passage of the stopcock b, 

 and, lastly, (3) that contained in the closed measuring-tube. 

 Measured in cubic centimetres, millimetres, and centesimal 

 degrees, we have : — 



V a) that of the space in A not occupied by the glass threads; 



V m , the space occupied by gas in the measuring-tube ; 



Yh, the volume of passage of the stopcock b ; 



P a and t a , the pressure and temperature of the residual gas 

 in V a ; 



P w and t m , the pressure and temperature of the gas in V m ; 



V h and t h , the pressure and temperature of the gas in V h ; 



s, the density of the gas (air = 1) ; 



«, its expansion coefficient ; 



then the total weight W of the gas enclosed in the apparatus, 

 expressed in grammes, is 



w= i / p«v. . P»V»^PaVA 



0-76x773\l+a* a l + *t m ' 1+octJ' 



The condensation of the gas of the weight W com- 

 mences at the moment when, by turning the stopcock b, it is 

 brought into contact with the surfaces of the glass threads in 

 vessel A. If, now, after the time T (reckoned from the opening 

 of 6), the pressure P and temperature t, to which the gas in the 

 instrument is exposed, be estimated and the volume of gas V^ 

 read off, we have from these three observations the weight g 



