1876.] Sprengel Air-pump and Vacuum-tap. 399 



After having removed the mercury, the stopper A. is replaced and con- 

 nected with the pump. When the exhaustion of the latter is complete 

 the tap may be turned on without admitting any air into the apparatus. 



Where this tap is used in connexion with the pump itself, the mercury 

 covering the little aperture d need not be removed, as it merely runs 

 into the pump when the tap is turned on. 



There are three of these taps used in the instrument (fig. 1, t, u, v) : — 

 V, placed at the exhaust arm of the pump, is useful to turn off after 

 the exhaustion of an instrument, to prevent air entering the pump when 

 another apparatus is blown on (a certain amount of time is thus saved) ; 

 t and u connect with the pump the two instruments having the largest 

 cubic contents, viz. the radiometer and McLeod's apparatus, enabling 

 them to be cut off when not in use, thereby greatly diminishing the space 

 within the pump. 



The air-trap fig. 1) is the same as used and described by Mr. 

 Crookes, in his first papers on " E/adiation ; " it is enlarged in fig. 4. The 

 tube a is blown into h at the point c, and passes some distance down 

 inside, the end being covered by a small glass cap (d). This cap must 

 have sufiicient room to entirely fall off the end of the tube descending 

 inside 6, in order to enable one to empty the trap of air into the pump 

 when necessary, which is done by opening the pinch-cock M (fig. 1) 

 Avhen the reserv^oir A is down, thus allowing the mercury to fall in the 

 tube a. The cap then drops off the end of the inner tube, and the air 

 rushes into the pump, the latter of coarse having been previously 

 exhausted. It will be easily seen how any air carried mechanically up 

 the tube a (fig. 1) is caught by this trap and collects round the joint c 

 (fig. 4). ^ 



Following the mercury up from the trap, we come to the sulphuric acid 

 tap, used to lubricate and clean the fall-tubes when required. This opera- 

 tion, and also that of admitting air when necessary, used to be done by 

 raising a simple stopper, kept covered with mercury and sulphuric acid ; 

 but on account of the many accidents happening through the unsteady 

 raising of the stopper, since devising the vacuum-tap I have adapted a 

 modification of the latter to this purpose. Tig. 5 shows the arrange- 

 ment on a large scale : a is a bulb for the reception of sulphuric acid ; 

 b, a stopper and funnel, drilled and grooved as for a vacuum-tap, but 

 the stopper having, on the opposite side to the aperture, a groove cut 

 so far down the stopper that it shall slightly overlap with the groove in 

 the funnel when turned so that they meet. 



Now if the stopper is turned to bring the aperture opposite the groove 

 in the funnel, sulphuric acid runs from the bulb a into the pump, and 

 is carried with the mercury to the jet, where it is distributed to the three 

 fall-tubes. On the other hand, when it is so turned that the two grooves 

 come together, the mercury in the funnel first runs in, followed by air, 

 of which the rate of influx and amount admitted is, by this means, under 



