or lowest pressures-which the vnpoius of such liquids will produce when used ns the opernting media in condensntioii pumps. Assuming that nil ionizntioii gnuge is uscd to mensure the vncuuni produced mid thnt the cooling wnter of the pump is held n t normnl room teinpernture, then the pressure indicntcd depends on n number of rnrinble firctors ns hns been observed by Hickinon." Included niiioiig these firctors we may cite the following :-( a ) Tiine to coildition the gnuge. (6) The ioiiicirtioii coefficient of tlic pump fluid. (c) Tiine to condition thc pump fluid. ((1) The degree of frnctionntioii of tlic pump fluid during pumping. (c) Tlic ninouut of theriiinl decomposition of the pump fluid during pumping.
As would be expected, thcrcfore, the published values of ultimntc vncun sttniiirrblc wlicii usiiig a given punip fluid differ widely.
Siiicc soiiie degrce of frnctioiiation occurs in the columii of vnpour in tlie inniii boihg-point tube of tlie nppnrntus described in this pnper, the exnmples of published figures of ultininte vncun listed in Tnbh 111 rclnte to the frnctiountiiig type of pump. The vnlues in Tublc 111 show the diversity which exists ninougst tlie published figures rclnting to siinilnr puiiip fluids. On bnlniice, however, it will be noted that the rnrious estrnpolnted values of vnpour pressure a t room tempernture obtniiied from Fig. 6 tend, in gciieral, to vnry in syinpnthy with the published vnlues of the vnpour pressures of the different pump fluids. There is renson to believe, therefore, thnt the extrnpolntcd rnlucs may be at lenst as nccurntc nn indication of the actual vnpour pressures as other correspondiug figures which hnve so far beeii published. -~CRNOWLEDQEMENTS The nuthor wishes to thniik Sir Arthur P. 11. Fleming, C.B.E., D.Eng., Director of Reseirrcli nud Educntion, ~Ietropolitnii-~'ickers Electricnl Co. Ltd., for permission to publish this pnper. The liclpful ndricc given by Dr. C. R. Bureh, F.R.S., duriiig the prepnrntion of this pnpcr is also gratefully ackuowfedged.