82 Mr. W. Crookes on Attraction and 



being inconvenient to have the apparatus in one piece of tubing, 

 and not always possible to seal the different portions together 

 by fusion, e e is a barometer dipping into the same vessel as 

 the gauge-barometer P, the two thus forming a differential 

 system by which the rarity of the atmosphere in the apparatus 

 undergoing exhaustion can be easily estimated, dd is a milli- 

 metre-scale with pointed end, attached to the gauge and capable 

 of being raised or lowered so as to make the point just touch 

 the surface of the mercury, b is a reservoir of strong sulphuric 

 acid, exposing as much surface as possible, but allowing the air 

 to pass across it without resistance. The mercury joint c 1 may 

 either be closed with a piece of glass rod ground in, or it may have 

 either of the two pieces of apparatus i and k fitted to it. k is a 

 mercurial siphon gauge, which is useful for measuring very high 

 rarefactions in experiments where difference of pressure equal 

 to a tenth of a millimetre of mercury is important, i is for still 

 higher rarefactions; it is simply a small tube having platinum 

 wires sealed in, and intended to be attached to an induction- 

 coil. At exhaustions beyond the capabilities of the mercurial 

 gauge I can still get valuable indications of the nearness to a 

 perfect vacuum by the resistance of this tube. I have frequently 

 carried exhaustion to such a point that an induction-spark will 

 not strike across the small distance (J inch) separating the wires 

 of the vacuum-tube, h is the mercury-tap usually employed for 

 letting air into the apparatus, and also for moistening the inte- 

 rior of the pump with oil of vitriol. / is a spiral of glass for 

 attaching the various pieces of apparatus requiring exhaustion. 

 As blown or fused joints are indispensable, this form of con- 

 necting piece is adopted to ensure the necessary flexibility. 

 m is a trap to catch any air which might leak in through the 

 platinum tap/, or the various joints in the lower part of the 

 tubing g. 



The reservoir A being filled with mercury, the tap I is turned 

 off and the reservoir is raised to the top of the slide where it is 

 supported by the detent T. On opening the tap /the mercury 

 rises in the tube/7*, and, falling through the chamber N, carries 

 with it the air contained in the tube R, and in the apparatus 

 attached to the tube I, as in the ordinary Sprengel pump. At N 

 the tubing is enlarged in order that the mercury may not be 

 forced up the tube R, as otherwise frequently happens if the 

 tubes or the mercury gets soiled. 



J, J are iron brackets supporting the apparatus. S is a large 

 inverted glass receiver, to collect the small portions of mercury 

 which are unavoidably and constantly being spilled; it should 

 contain a little weak alkaline solution. 



The part of the tubing g, /, h, N forms a barometric siphon 



