150 Prof. T. E. Thorpe and Mr. J. \V. Rodger. [1-Yk -2i>, 



there is little evidence of association of molecules at any temperature, 

 the following conclusions may be drawn : 



1. In homologous series the coefficient of viscosity is greater, the 

 greater the molecular weight. 



2. An iso-com pound has always a smaller viscosity coefficient than 

 the corresponding normal compound. 



3. An allyl compound has, in general, a coefficient which is greater 

 than that of the corresponding isopropyl compound, but less than that 

 of the normal propyl compound. 



4. Substitution of halogen for hydrogen raises the viscosity coeffi- 

 cient by an amount which is greater, the greater the atomic weight 

 of the halogen ; successive substitutions of hydrogen by chlorine in 

 the same molecule bring about different increments in the viscosity 

 coefficients. 



5. In some cases, as in those of the dichlorethanes, substitution 

 exerts a marked influence on the viscosity, and in the case of the 

 dibromides and benzene, it may be so large that the compound of 

 higher molecular weight has the smaller viscosity. 



6. Certain liquids, which probably contain molecular complexes, do 

 not obey these rules. Formic and acetic acids are exceptions to 

 Rule 1. The alcohols at some temperatures, but not at all, are 

 exceptions to Rule 2; at no temperatures do they conform to 

 Rule 3. 



7. Liquids containing molecular complexes have, in general, lar 

 values of dyjdt. 



8. In both classes of liquids the behaviour of the initial member 

 of homologous series, such as formic acid and benzene, is in soi 

 cases exceptional when compared with that of higher homologu.es. 



As regards the influence of temperature on viscosity, it is founc 

 that the best results given by Slotte's formula are in cases whc 

 the slope of the curve varies but little with the temperatui 

 From the mode in which the values of the constants n and 6 

 derived, it cannot be expected that their magnitudes will be relat 

 in any simple manner to chemical nature. With the exception 

 certain liquids, such as water and the alcohols, which are character 

 ised by large temperature coefficients, and in which there is 

 to expect the existence of molecular aggregates, the formula 



obtained from Slotte's expression by neglecting terms in the dent 

 minator involving higher powers of t than t\ gives a close agreemt 

 with the observed result*, and in this formula the magnitude of 

 and 7 are definitely related to the chemical nature of the sul 

 stances. 



In order to obtain quantitative relationships between viscosity 



