1894.] The Viscosity of Liquids and their Chemical Nature. 151 



chemical nature, and to compare one group of substances with another, 

 it is necessary to fix upon particular temperatures at which the 

 liquids may be taken as being in comparable conditions as regards 

 viscosity, and to compare the values of the viscosities at those tem- 

 peratures. 



The first comparable temperature which suggested itself was the 

 Diling point. 



A second comparable temperature was obtained by calculating^ 

 dues of corresponding temperatures by the method of van der 



with such data as could be obtained. 



The third basis of comparison consisted in using temperatures of 

 jual slope, i.e., temperatures at which the rate of change of the 

 iscosity coefficient is the same for all liquids. 



At each of the different conditions of comparisons, the experimental 

 aults have been expressed according to the same system, in order to 

 low at a glance relationships between the magnitudes of the viscosity 

 instants and the chemical nature of the substances. The liquids are 

 inged so that chemically related substances are grouped together, 

 ibles are constructed which give the values of the three different 

 litndes derivable from measurements of the viscosity of the sub- 

 mces. 



1. Values of viscosity coefficients (?/). 



2. Values of r\ X molecular area, i.e., molecular viscosity. 



3. Values of ij X molecular volume, i.e., molecular viscosity work. 

 The coefficient rj is the force in dynes which has to be exerted per 



lit-area of a liquid surface in order to maintain its velocity relative 

 that of another parallel surface at unit distance equal to unity, 

 seemed, however, that relations between viscosity and chemical 

 iture would best be brought to light if, instead of adopting merely 

 lit-areas, areas were selected upon which jLhere might be assumed to 

 the same number of molecules. The molecular viscosity is propor- 

 )nal to the force exerted on a liquid molecule in order to maintain 

 velocity equal to unity under the unit conditions above defined, 

 the units chosen it is the force in dynes exerted on the molecular 

 in square centimetres under unit conditions. The molecular 

 viscosity work may be regarded as proportional to the work spent in 

 aoving a molecule through the average distance between two adjncent 

 lolecules under unit conditions. In ordinary units it is the work in 

 rgs required to move a surface equal to the molecular area in square 

 antimetres through the molecular length in centimetres. 

 In the case of the comparison of the viscosity coefficients at the 

 )iling point, it is found : 



1. As an homologous series is ascended, in a few cases the viscosity 

 efficient remains practically the same, but in the greater number of 

 ries the coefficients diminish. In one series the coefficients increase ; 



M 2 



