AND RATIO OF THEIR ATOMIC' WF.KUITS, 



17 



of weio"bt between the cjlobe and its cDiiiitcritoist' would he the tfaa difference of 

 weight as expressed by brass weights in uii-. 



The method was found so convenient that it was deveh)jied and improved for 

 wider use. 



11. AIIMMMI'S AXI> KXIIAI'SI ION. 



The exhaustions of o-hibes and otlier apparatus re(iuired in tliis sei'ies of 

 experiments wei'e mostly eifected with a nu'rcurial aii-pump of tliefoini devised by 

 Toepler. The tube 1)\ which it discharged itself was sliaped as show n in Fig. ;"). 

 The recipient a couhl be itself exliausted with a piston pump 

 when tlie discharge of the Toepler pump diiertly into the air 

 be^au to be incom[>lete owing to the small volume to be 

 discharged. The body of the ])ump had a capacity of l.t) 

 litres; during this series of e.xpei'iments, it would exhaust a 

 volume of twenty litres to two milliouths of an atmosphere. 

 But such a completeness of exhaustion was avoided. Since 

 the tension of the vapor of meicury at oi'dinary tempei'atures 

 is, if my experiments are sufficiently trustworthy, not much 

 more than one millionth of an atnios[)here, it was hoped that 

 we could avoid the diffusion of niercuiy vapor into the globe 

 which is exhausted, by opposing to the diffusion a I'apid 

 current of gas in the other direction. Now, if the tension of 

 the residual gas in the globe is, let us say, ten millionths of 

 au atmosphere, each stroke of the pum[) will draw a current 



of eas towards itself wdth considerable velocity. Unless, therefore, some 

 vapor can pass from the pump to the globe during the up stroke of the jnunp, it 

 will not reach it at all while the pump is in action. But if the tension of the 

 residual air is made equal to that of the mercuiial vapor in the pump, air will 

 enter the pump by diffusion mainly, and then mercurial vapor will enter 

 the globe. It was hoped that if the vacuum produced still contained gas at 

 a tension ten or tw^enty times that of mercuiy vapor, its entrance into the globe 

 would not take place while the pump was working ; l)y nniking the connecting 

 tube some two metres long, it \vas hoped that, even after the stopping the pump 

 to measure the vacuum, not much mercuiy would diffuse into the globe. 



The exhaustion of globes, but not of other a[i[>aratus, was sometimes effected 

 with a Geissler pump whose body has a capacity of "i.-'i litres. When this is 

 uewdy cleaned, it is possiV)le to exhaust a volume of one litre to a ten-millionth of 

 au atmosphere. 



I'lG. 5. — Discharge tube of 

 Toepler pump for obtain- 

 ing a better vacuum. 



