572 



NATURE 



[December 30, 1920 



and the series benzene, naphthalene, anthra- 

 cene. 



The relation of lubricating qualities to viscosity 

 broadly resembles that to molecular weight. In 

 a simple chemical series lubrication and viscosity 

 change in much the same way with molecular 

 weight, but that there is no fundamental relation 

 between viscosity and lubrication is shown by the 

 following figures : — 



Viicosity »t 2u'. S;atic Kriction. 



Carbon tetrachloride 0.0096 0.43 



Chloroform 0.0056 0.30 



Acetic acid 0.0122 0.40 



Octylic acid 0-0575 °-'9 



Benzene 0.0065 0.39 



Toluene 0.0058 0.28 



Benzyl alcohol 0.0558 0.31 



Fluidity of the lubricant has no constant sig- 

 nificance. The curves for acids, alcohols, and 

 paraffins show no break where, with increasing 

 molecular weight, the lubricant becomes a solid 

 at the temperature of observation. Compare also 

 benzene, naphthalene, and anthracene, menthone 

 and menthol, thymol and carvacrol. 



Perhaps the most unexpected result is the dis- 

 tinction between ring and chain compounds. The 

 simple ring compounds, benzene, naphthalene, and 

 anthracene, show the linear relation to molecular 

 weight, and values are much the same as those 

 for paraffins of the same molecular weight. The 

 similarities, however, end here, for any change in 

 the molecular structure produces opposite effects 

 according as it takes place in a chain or a ring. 

 Thus a double bond decreases the lubricating 

 action of a ring compound, but increases that of 

 a chain compound. As examples, compare naph- 

 thoic acid with double-bonded oxygen, with naph- 

 thalene, menthone with menthol, cycZohexanone 

 with cydohexane, benzoic acid with benzene. As 

 examples of double-bonded carbon, compare cinna- 

 mic ester with hydrocinnamic ester, dipentene, 

 having two unsaturated carbon atoms, with men- 

 thol and cycZohexane. Also the more saturated 

 cyclic compounds are better lubricants than the 

 less saturated ring compounds. When a ring and 

 a chain are joined, as in butyl xylene, the result is 

 a better lubricant than either. 



The esters occupy a quite unexpected position. 

 The simple aliphatic esters are worse lubri- 

 cants than their related acids and alcohols. The 

 ring esters, on the contrary, are better lubricants 

 than are their related acids (e.g. ethyl benzoate 

 and benzoic acid). 



Perhaps the most interesting substances are the 

 hydroxy-acids with CH and COOH groups. This 

 conjunction produces a remarkable increase in the 

 lubricating power of a chain compound (lactic acid 

 and ricinolic acid), and almost destroys lubricating 

 action in the case of the ring compounds (salicylic 

 and benzylic acids). 



In the ring compounds the replacement of hydro- 

 gen decreases lubricating power in the case of 

 N, :0, or .COOH, and increases it in the case of 

 other groups in the order C„H5<CH<OH. 



NO. 2670, VOL. 106I 



The eftect of a second group of the same or of 

 a different kind is to decrease the effect of the 

 first. Compare, for instance, toluene with xylene ; 

 catechol, quinol, and cresol with phenol ; and 

 methyl cyc/ohexanol with cycZohexanol. The 

 simpler the group, the more effective it is. Com- 

 pare cymene with toluene or xylene, and benzyl 

 alcohol with phenol. 



When the atoms are disposed with complete 

 symmetry about a carbon atom, the result is a 

 very bad lubricant, as we see in carbon tetra- 

 chloride and the alcohol penterythritol 



C(CH2-OH)4. 



It will be noticed that no ring compound is a 

 good lubricant. Even cholesterol with the mole- 

 cular weight 366 is no exception. 



The group SH acts much as the group OH, 

 thiophenol, CgHs'SH, and benzyl hydrosulphide, 

 CsHjCHj-SH, resembling phenol and benzyl 

 alcohol respectively. 



Concerning one matter — and that the most 

 fundamental — some conclusion must be reached, 

 even though it be upset later. What is friction 

 due to? The " Encyclopa;dia Britannica" is in no 

 doubt as to this. Friction, it says, is due to in- 

 equalities of the surface. This conclusion cannot, 

 I think, be accepted. Why, if it be true, should 

 clean burnished faces of glass or bismuth refuse 

 to slide over one another? It does not even accord 

 with such simple facts as we now know. For in- 

 stance, the friction of an optical face of glass was 

 found to be the same as that of ordinary plate 

 glass within the limits of accuracy aimed at; and 

 both the optical face and the ordinary plate 

 were found to give higher values than ground 

 glass. 



The subject cannot be fully discussed here, but 

 I think we may conclude with some confidence 

 that the friction both of lubricated and of clean 

 faces is due to true cohesion — to the force, that 

 is, which binds together the molecules of a solid 

 or of a fluid. If there were no seizing, there would 

 be no friction. The function of the lubricant is 

 to diminish the capacity for seizing by saturating 

 more or less completely the surface forces of the 

 solid. In some cases it seems to abolish it com- 

 pletely, so that static friction vanishes. 



The subject of lubrication is of interest to the 

 engineer, but it is perhaps of more interest to the 

 physicist, for it offers a means of exploring the 

 most difficult regions of the physics of boundary 

 zones — namely, the surface energy of solids. It 

 will, for instance, I believe, enable us to prove that 

 the simplest chemical change at the surface of a 

 metal takes place onlv when the surface energy 

 is decreased thereby. The film of oxide of sulphide 

 which forms on copper acts as a very effective 

 lubricant, and it acts also like a grease film in 

 preventing water from wetting the surface ; and 

 from both of these facts we may conclude that the 

 presence of the film lowers the surface energy of 

 the metal. 



