carried by the Ions produced by Rontgen Rays. 535 



The test-tube slides freely up and down the larger tube and 

 serves as a piston. Its lower end is always below the surface 

 of the water which fills the lower part of the outer tube. A 

 glass tube passing through the indiarubber stopper puts the 

 inside of the test-tube in connexion with the space E. This 

 space is in connexion with an exhausted vessel, F, through the 

 tube H. The end of this tube is ground flat and is closed by 

 an indiarubber stopper which presses against it; the stopper is 

 fixed to a rod, by pulling the rod down smartly the pressure 

 inside the test-tube is lowered and it falls rapidly until the 

 test-tube P strikes against the indiarubber stopper. The 

 tube T, which can be closed by a stop-cock, puts the vessel E 

 in connexion with the outside air. The tubes R and S are 

 for the purpose of regulating the amount of the expansion. 

 To do this, the m ercury- vessel R is raised or lowered when the 

 test-tube is in the lowest position until the gauge G indicates 

 that the pressure in A is the desired amount below the atmo- 

 spheric pressure. The clip S is then closed, and air is admitted 

 into the interior of the piston by opening the clip T. The piston 

 then rises until the pressure in A differs from the atmospheric 

 pressure only by the amount required to support the piston, 

 this is only a fraction of a millimetre. 



If II is the barometric pressure, then the pressure of the 

 air before expansion is _ 



where it is the maximum vapour-pressure of water at the 



temperature of the experiment. The pressure of the air after 



the expansion when the temperature has risen to its former 



value is 



P 2 =P 1 ~p, 



where p is the pressure due to the difference of level of the 

 mercury in the two arms of the gauge. 



Thus if v 2 is the final and v L the initial volume, 



V A _V^ H-7T 



II — ic—p' 



A is the vessel in which the rate of fall of the fog was 

 measured and the electrical conductivity of the gas tested. 

 It is a glass tube about 36 millim. in diameter covered with an 

 aluminium plate ; a piece of wet blotting-paper is placed on 

 the lower side of the plate and the current of electricity passed 

 from the blotting-paper to the horizontal surface of the water 

 in this vessel. The blotting-paper was placed over the alu- 

 minium plate to avoid the abnormal ionization which occurs 

 near the surface of a metal against which Rontgen rays strike 



