December 30, 1920] 



NATURE 



571 



of oiliness is due to some reaction between the 

 substance and the solid face. Much is still obscure, 

 but certain facts seem to be capable of interpreta- 

 tion in no other way. Thus water and ethyl alco- 

 hol have no detectable lubricating action on clean 

 glass, whilst both are moderate lubricants for clean 

 bismuth. 



The thickness of the layer needed to lubricate 

 is astonishingly small. It is quite invisible, and 

 probably only one or a very few molecules thick. 

 To discuss this adequately would take too long, 

 but the fact may be instanced by an experiment of 

 great beauty. A tiny drop of, say, acetic acid or 

 tripropylamine is placed near one corner of a plate 

 of clean glass 6 cm. square ; nothing detectable 

 by the senses happens ; the drop is there, and that 

 seems to be all. But the whole surface of the 

 plate has, in fact, been changed fundamentally. 

 It is now fully lubricated by an invisible film 

 which has spread rapidly over it from the drop. 

 The presence of this film may be detected by 

 measuring the friction or by following the migra- 

 tion of two drops of fluid over the face of the 

 plate. It will be found that the drops attract one 

 another under conditions which point to the cause 

 being the contractility of the invisible film. 



This brings me to the second part of my sub- 

 ject — namely, the relation of lubricating power to 

 chemical constitution. 



In particular experiments with bismuth, a slider 

 having a curved surface was applied to a plain 

 surface of metal, both surfaces being highly 

 polished, and the force required to initiate move- 

 ment was measured. This force measures what is 

 usually called static friction as opposed to the 

 kinetic friction when the surfaces are in relative 

 motion. The static friction was found to be a func- 

 tion of the weight of the slider. Therefore, the 

 ratio of the weight of the slider to the friction 

 was used as a relative measure. The results 

 :ippear in the following table : — 



Stittir friction o-^ when the faces "were clean. 



Chain Compounds. 



Alcohols. 



Methyl 

 Elhyl 

 Propyl 

 Butyl 



Amy! 

 Ociyl 

 C.tvl 



Formic 

 Acetic 

 F'ropinnii 

 Valeric 

 Caprylic, 

 Caprylir, 

 on plate 



Sl»lic 

 F'icti«tn. 

 0.29 

 0.3a 

 0.34 

 0.30 

 0.27 

 0.2s 

 17 



iioPropyl 



iioButyl 



Allyl 



Glycol 



Glycerol 



Prnterythritol 



Acids. 



fluid 



frozen 



FnclMn. 

 0-45 

 0.40 



0.3' i 

 o.aS ; 



U. II) 



0.0s 



n-Octane 



Stearic 



Olric 



Ricinolic 



a- Lactic 



Gljrcerir 



Sialic 

 Friction. 

 0.3a 

 0.30 

 0.29 

 0.30 

 o.aa 

 0.40 



^lie 

 Pnciion. 

 0.32 

 CIS 

 0.10 

 a. 02 

 0.20 



n.l2 



SUitlC 

 Friciior*. 



.■Xcetone ... ... 0.3a 



Methyl ethyl 



ketone 0.29 



Ethyl acetate ... 0.36 



Ethyl valerianate... 035 



Tristearin ... ... 0.24 



Triolein 0.14 



.\cetone dicarboxyli'" 



diethyl ester ... 0.29 



n-Hexane 0.37 



n-Heptane 0.346 



Ethyl ether 

 B.P. " Paraffin " 

 Solid paraffin, m.p. 



30.5° 



Solid paraffin, m.p. 



46° 



Carbon tetrachloride 



Chloroform 



Amylene 



Octylene 



Butyl xylene 



Static 

 Frictio 



0.33 



0.20 



0.09 



0.07 



0-43 

 0.30 

 0.26 

 0.28 

 0.27 



Ring Compounds. 



Salic 

 Kriclion. 



Ben2»>ne 0.34 > 



Ethyl benzene ... 0.32 j 



lodobenzene ■•• 0.30 1 



Toluene ... ■• 0.28 | 



Xylene 0.30 ' 



/>-Cymcno ... ... 0.31 



Phenol 0.25 ' 



Catechol 0.39 



Quinol ... ... 0.40 



m-Crosol ... ... 0.26 



Benzyl alcohol ... 0.31 



Benzoic acid ... 0.38 



Phthalic acid ... 0.37 



Cinnamic acid .. 0.27 



Benzilic acid ... 0.45 



Salicylic acid ... 0.41 



Ethyl benzoate ... 0.33 



o-Phthalic ester ... 0.27 



Ethyl hydrocinna- 



mate 0.28 



Ethyl cinnatnate . . . 

 Thiophenol 

 Benzyl hydrosul- 



phide 



Pyridine 



Piperidine ... 

 Naphthalene 

 Anthracene 

 /8-Naphthol 

 Naphthoic 

 C.irvacrol ... 

 Thymol 



Menthol 



Dl|)entene ... 



Camphor 



Active ethyl ester 



of camphor oxime 

 »oCholesterol 



Static 

 Kriciioa. 



0.32 



0.22 



acid 



0.23 



0-33 

 0.32 

 0.29 

 0.26 

 0.38 

 0.39 

 0.23 

 0.24 

 0.26 

 0.3. 

 0.24 



0-33 

 0.27 



Cyclic Compounds. 



cyrloHexane 

 Methyl cycJohoxane 

 I : 3-DimithyI cyclo- 



hexane 



cycfoHcxanol 



I : 2-Mclhyl cyclo- 



hexanot 



I : 3-Mcthyl rycJo- 

 hexanol 



.Ammonia fortiss 



Triethylaminc 

 Tripropyl.nmine 



Sialic 



Friction. 



0.31 



0.30 



0.29 

 0.20 



0.28 i 



0.35 



cyrloHcxanone 



t : 2-Mclhyl cyclo- 



hexanone 

 I : 3-Methyl 



hexanone 

 I : 4-Methyl 



hexanone 



Static 

 FrictloB 



0-3S 



cyclo- 



cyclo- 



0.34 ; Castor oil 

 0.30 1 Water 

 0.26 I 



0.3a 



0-3S 



0.33 



0.03 

 0.33 



It will be seen that static friction is a function 

 of the molecular weight of the lubricant, and in a 

 simple chemical series of chain compounds, such 

 as fatty acids and alcohols or paraffins, a good 

 lubricant will be found if one goes high enough 

 in the scries ; but it is not a simple function. The 

 friction, for instance, rises sharply in moving from 

 CUCl, to CC1«, and from phenol to catechol and 

 quinoj. The influence of molecular weight is over- 

 shadowed by the influence of chemical constitu- 

 tion. 



In some simple chemical .scries the relation ap- 

 pears to be a linear one. Examples arc paraffins 



NO, 2670, VOL. 106] 



