Static Friction. 205 



The upward trend of the first part of the curve for the 

 aliphatic alcohols is in agreement with the fact that methyl 

 alcohol is abnormal in some of its physical properties such 

 as specific gravity. 



In their qualities as lubricants of bismuth, ring compounds 

 are the converse of chain compounds : thus the effect of a 

 double-bonded atom is to decrease the lubricating value of 

 the former and to increase that of the latter. As examples, 

 compare naphthoic acid with double-bonded oxygen, with 

 naphthalene, menthone with menthol, cyclohexanone with 

 cyclohexane, benzoic acid with benzene. As examples of 

 double-bonded carbon compare cinnamic ester with hydro- 

 cinnamic ester, dipentene, having two unsaturated carbon 

 atoms, with menthol and cyclohexane. On the other hand, 

 the presence of unsaturated atoms increases the lubricating 

 qualities of chain compounds, whether it be the double- 

 bonded oxygen of ketones or acids, or carbon of olefines 

 and alcohols ; but this rule is departed from (in the case of 

 acids) when the chain becomes much elongated. 



Whatever view be taken of the structure of the benzene 

 ring, it must be admitted to be less saturated than cyclo- 

 hexane, and we find consistently that the more saturated 

 cyclohexane and its derivative are the better lubricants. 



When ring and chain are joined as in butylxylene, the 

 result is a better lubricant than either, 



When the atoms are disposed with complete symmetry 

 about a carbon atom the result is a very bad lubricant, 

 as we see in carbon tetrachloride and the alcohol pent- 

 erythritol C(CH 2 OH) 4 . 



In the ring compounds the replacement of hydrogen 

 decreases lubricating power in the case of N, : 0, or 'OOOH, 

 an J increases it in the case of other groups in the order 

 C 2 H 5 <CH 3 <OH. 



The effect 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 cyclohexanol with cyclohexanol. 

 The simpler the group the more effective it is. Compare 

 cymene with toluene or xylene ; and benzyl alcohol with 

 phenol. 



The esters occupy a quite unexpected position. The 

 simple aliphatic esters are much worse lubricants than their 

 related acids or alcohols. On the contrary, the ring esters 

 are better lubricants than are their related acids (e.g., ethyl 

 benzoate and benzoic acid). 



Perhaps the most interesting substances are the hydroxy 



