47 225 



the tube, so that the small gas bubbles collected in this spot may be pushed down 

 the lube together with them ercury. The lower part of the tube becomes wider again, 

 so that the mercury can flow with sufficient rapidity out of the tube. The width 

 of this part of the tube is so adjusted that the mercury in the lower part is still 

 moving downward when the consecutive portion coincides with it, provided that 

 the tube is a vacuum, and that (he intervals between the two bodies of mercury 

 are suitable. In this way the fall-tube is exempted from violent concussions of the 

 mercury, and (he spol in which the portions of mercury coincide still remains 

 covered by mercury when the succeeding body of mercury closes the upper end of 

 the tube. Thus the pump is entirely without detrimental space. The theory of 

 the working of this pump is the same as that of the Geisler pump. In both cases 

 a known volume of the gas is cut oiT and then expelled. The sole difference is 

 that the gas volume cut off is greater in the Geisler pump than in the pump just 

 described, but on Ihe other hand the strokes of the pump are much more frequent. 

 With the dimensions above mentioned, the gas volumes cut off in the tube by 

 the falling mercury were about 4 ccm., and the strokes of the pump were, as a rule, 

 about 80 per minute. 



The working of the pump is probably as efficient at high as at low pressure, 

 and although it does not perhaps work so quickly as a large Geisler pump, yet 

 I think it may be used with advantage especially in analytical work; amongst 

 other reasons, because it is very cheap and it does not require specially cautious 

 handling. 



After working a long time, however, the tube becomes more fragile and at 

 last breaks, but in my experience it is sufficient to attach the fall-tube to the 

 glass bulb g, by means of indiarubber tubing, rendered airtight by mercury, the 

 fall-tube can then be easily replaced. I have, however, used the same fall-tube 

 for a year, and many times during this period the pump has worked incessantly 

 for 7 hours. On the whole, the same tube has sustained the working of the pump 

 during many hundred hours without breaking. By fastening the fall-tube to the 

 wood-work of the pump special care must be taken that no tension arises in the 

 glass for it will inevitably cause a premature fracture of the tube. It is sufficient 

 to fasten the tube in two places to the wood-work. 



A great advantage is that this air pump with falling mercury is especially 

 suitable for automatic working. I have used a water jet pump for raising the 

 mercury to the height required. The mode of proceeding is the same as usual, 

 and is illustrated by Fig. 11. The air sucked by the jet pump enters the tubing r 

 through the side tube a, but as the air passage through a is made narrow, which 

 is most conveniently done by means of an indiarubber tube and a screw clip, the 

 air in r, is rarified so that the mercury in the vessel N is raised into r. The air 

 entering at a divides the mercury into small drops which are carried by the air 

 current into the reservoir 0, where the mercury is collected, while the air is passed 

 on to the jet pump. The mercury is conducted from the reservoir through the 



