48 THE DIFFUSION OF GASES THROUGH 



c being closed. The jar is then inverted as in the figure, pp being closed. 

 Air is now introduced through pp in small bubbles, which are caught 

 within the Cartesian diver k, until the requisite amount of air is conveyed. 

 During this operation it is expedient to connect the air pump at c {d being 

 closed throughout) and to exhaust the air above w to the identical partial 

 pressure subsequently to be used in the experiment. Air is then allowed to 

 enter k through p, until the diver just floats. On closing p and restoring 

 atmospheric pressm^e through k the diver will sink as far as its supports. 



Clearly the diver \vill be in equilibrium at a higher artificial presstue in 

 the air above ww when the condenser is charged than when it is uncharged, 

 because of the attraction of electrical forces. This difference of pressure 

 is the basis of the measurements. 



In the course of the experiments it was found that means had to be 

 provided to secure parallelism between the disks hh and gg of the condenser. 

 This was done satisfactorily by placing hh on three suitable set screws, 

 held at mm, together with three corresponding downward-tending springs. 

 In such a case the disk hh, in the absence of the lid ee and the disk gg, may 

 also be removed on loosening the springs, an operation frequently necessary, 

 as, for instance, for the insertion of different divers k. The supports / are 

 to be adjustable for this purpose. 



In constructing the instrument for definite purposes a metal vessel JJJ'f 

 should be used, provided with opposed plate-glass windows, through which 

 the disk hh and the upper part of the diver may be seen during measurement 

 in order that the time of drop may be ascertained. The disk hh would 

 then be practically a horizontal partition in the vessel, though the disks 

 should still be adjustable for parallelism. The pipe pp is to be soldered to 

 the bottom for efflux of oil and influx of air. The form of apparatus given 

 in fig. 15, however, sufficed very well for experimental purposes. 



Since the guard ring and diver, the vessel, and lid are all put to earth, 

 while the movable disk gg is charged, convection may be produced in the 

 oil between the plates in case of high potential. Such a reaction on the 

 disk of the diver may tend to modify the equilibrium and the amount of 

 such an error will have to be shown by experiment; but nothing serious of 

 the kind was detected. If a solid plate of mica or glass is interposed a 

 variety of complications enter, which will be referred to below. 



34. Equations. — If we return to equation (i), Chapter I, and neglect m/M, 

 the ratio of the mass of the air contained in the Cartesian diver and the 

 weight of the solid vessel (here of brass), in comparison with /, 



h-\-E -rz 7- (l) 



p„ Mg i-pJPe 



where h is the difference between the level of the liquid within the diver 

 and in the bell jar (viv, fig. 15), H the corrected pressure in centimeters of 

 mercury of the artificial atmosphere above w, p^, p„, Pg, the densities of 

 mercury, of the liquid floating the diver, and of the solid walls of the diver. 



