Tlio considcrntions which wo wish to bring forwnrd in this pnpcr nro tho results of many p n r s ' expcricnco of tlic problcnis of lubricntiiig oil and lubricntion, studied from n physical mid physico-clicniical stnndpoint, assisted by a n intirnnte iind dnily contnct ivitli tho lubricntion of all typcs of mnchinery rind primo movers. Our primary object w i s to olucidnto tho rcnson for tho obviously supcrior lubricnting cllicioncy of fntty glyccridcs over strniglit mmcral oils in n largo

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... of prncticnl caws mhicli lind como to our notico. As n corollnry to this \vc had. to rovicm tho physicnl priiiciplcs upon which lubricntion doponds and to scck nn oxplnnntion of the pecnlinr property which hns bccn called oiliness," " body," otc., by niithoritics in tho pnst. It mny not bo out, OF plnco hero to roviow t1i.o position of our knowlcdgo of lubricnnts and lubrlcation. Profcssor c. V. Boys, in liis Prcsidcntinl ndtlrcss to tho Physical Socicty in 1008, crystalliwd tho position vory happily iii thcso words: -'I It ivm found t h a t tho lubricnting proporty of oil dcpcndwl on soinotliing which nt prcsont is unknown-lt is not viiicosity-nniiiinl nnd vegetnblo oils lubricnto bottor, i.c., tlioy nro moro slippory ' tliiin niinornl oils of tlic snnio viscosity, mid though tlio oil trndo hus known how to lllnko good slippery ' niisturcs, no 0110 a t present knows whnt oilines' is, nnd this is :it tho prcsciit tinio an iniportaiit pliysicnl qucat of tho cngincor." Ayniii, Professor J . S. Ilrnrnc 11:is snit1 tlint" ('tlio propcrty of loilincss' wne o m of tho most puzzling of tho proporties of oils. lly soxno i t wits rognrdccl as unnccessiiry to connect i t with viscosity dircctly, sinco i t wns posscsscd in I L niucli grontor dcgrco by sonio liscd oils tlinn by ninny ininoral oils wliich lind pr:icticrilly tho snmo viscosity .' It is ncccssnry to distinguish clcrirly h t a c c n two distinct clrisscs of lnhricnting prnctico. On tlic olio hnnd wo lirivc tho lubricntion of fnst-moving sliiifts otc. supplied with n largo c x w s of oil frocluontly under prcssuro, niid in this cnso tho frictionnl vnlncs iiro dopontlcii t prininrily upon tho viscosity of tho oil; tlio iiinthoinnticnl nnd oxporiinoiitnl invcstigrition of t1ic.m cmos hnvo bocn nniply trcnted by Royrioldr, Towcr, Jmcho, nnd otlicrs. On tho otlior liniid IVO linvc to don1 with elorv spccde nt high ~~iring-:-prcsu~~rcs, frequently with n vory limited supply of oil, and it is i n thoso cnscs tlint tlio npccinl proporty of " oilincsv I' or (' body " is roquisito to rnnintiiin tho film, nnd i t is lioro tlmt viscosity niciisiircinuiits 110 lorigor nssist 11s in tho clioico of tho lnbricnnt. J. Inst. rut. IWI., ioi8,4. 210. This viorv rccoircd unnnimous support at tho reccnt discussion on lubrication a t tho Physical Socicty. J \ h t :iro tlic possible physicnl propcrtice of n liqnid which inllucnco its chnrnctcr ns n lubricant? They nro viscosity dcnsity, cnpillnrity or fiiirfnco tcnsion, compressibility, nnd tensile strongth. It is truo t l i n t very littlo work lins been dono on comprcssibility nnd tensile strength, but from tho obsorr:itions of %'orthington nnd othcre it K O U I~ appear ~o~i b t r l l i miiothcr they rvoiild play a &Jf i i i ! / i i i . d i i q / rido in tilo cliffcrcntintion of oils; nnd, furtlicr, 1~9 wc shall point out, tliero is what np c n y ncss without cnlling upon thcso propcrtics. So fnr ns dcnsity is concerncd thcrc cxists n wid0 rnngc of petrolcum rninernl oils posscssing spccifio grnvitics idcnticnl with tlioso of tho fntty or fixed oils, henco i t i s clcnr thnt iicnsity plays no dotermining part. Viscosity, is, of courso, of groat significanco in tho cnses of high s p d ctc. just rcfcrrcd to. Now i t rciiinincd for UbFlolido t o point out tlint only n liquid aliich wots " or " spreads over " tlic solid cnii constitrita n truo lubricnnt, I1ccaiiso i n ordor for tho liquid to forco itself into tho narrower spnces of highor pressure i t is osscntinl on cnpillnry grounds t h a t tlie said liquid shnll " w t " the solid surfaces. Consider tho ctlso of two cccontric glnss fiurfnccs which aro Iwing forced togothor with n drop of mcrcury or oil bctwxiiCFhcrn;, Now sinco tho inerciiry ( fig. 1) clocs not wet or eprend over tho dnss, tho meniscus in tlris cnsa will be convex to tho liqnid, while in tho caso of tho oil ( fig. 2) , which wcts tlio surfnccs, tho meniscus will bo ooncnve. .to us RII ndcqiinto cxplnnntion of tho nnturo o P 0 1 1~-Fro. 1. Fro. 2. In tho cnso of fig. 1 (niorcury) tho tondency on cnpilliiry grounds will bo for tho liquid to gather itself up iiito n drop iind to pull *tho liquid film in tho dircction of a nwny from tho nnrrowor constricted iirc:L of grontcr prcssuro nt b. In tho cnso of oil ( fig. 2. ) tho opposita will bo tho cmo. Tho oil will, owing to its nicniscus tend to bo pulled tomnrds b or, in otlior words, mih form itciolf into tho iinrrow spncos. Tliis is cxnctly whnt is rcqnircd in a lubricant, nnincly, t l i n t it slinll pcnctrnta into tho nnrrow spiiccs botwoon joiirnnl nnd bonring, and from tho iibovo considorntioiis one clonrly socs tlint liquids wliicli do .riot " wet" fiolid surfaces cannot be doscrilicd ns Iiibricnnts. Only thoso liquid8 which " wot " n solid snrfnm possoss lubricating poncr in tho gcnernlly nccoptd sonso. Altliough thcso considorntions npponrd to indicnto tho conncctioii bctwccn lubricntion nnd cnpil-Inrity, it will bc uocn tlrnt tlioy only go so.fnr ns to oiinblo 11s to sny tliiit niorcury is not n lubricnnt nnd tliiit oil is n lubricnnt. This is (I conclusion OF consitlcrnblo tlieorcticnl intorost, but i t is not vory 0