Prof. T. E. Thorpe and Mr. J. W. Rodger. [Fob. 22, 



to the adoption of temperatnres of equal slope as comparable tempera- 

 tures, and, indeed, apart altogether from considerations such as the 

 above, which refer to the particular case of viscosity, much may 

 said, from a theoretical point of view, in favour of employing sue 

 temperatures for physicochemical comparisons in general. 



To begin with, at the temperature of equal slope, the effect 

 temperature upon the property examined is the same for different 

 substances. In the case of viscosity, for instance, dijjdt, or the rat 

 at which viscosity is being altered by the temperature, has the same 

 value for all liquids, and this equality might be taken as sufficient 

 justification for supposing that at temperatures of equal slope the 

 substances, so far as viscosity is concerned, are in comparable states. 



Another argument which may be advanced in favour of such 

 method of treatment is that the comparable temperatnres are choset 

 by means of a study of the effect of temperature on the propertj 

 actually examined. The main objection which can be urged against 

 the boiling point as a comparable temperature, even when, as in the 

 case of such a property as density, it gives comparatively definit 

 stoichiometric relationships, is that these relationships may not 

 general. If, however, comparable temperatures be chosen, as in the 

 case of slope by a study of the property considered, the generality of 

 the relationships obtained can be ascertained without the study of 

 other properties of the substances. 



Comparisons were made, therefore, at temperatnres at which drjjdt 

 is the same for the different liquids. Or, graphically, the temperatnres 

 may be defined as those corresponding with points on the viscosity 

 curves at which tangents are equally inclined to the axes of co- 

 ordinates. The temperatures are therefore those at which temperatui 

 is exercising the same effect on viscosity, and for shortness may 

 termed temperatures of equal slope. The temperatures were obtain* 

 by means of Slotte's formula. 



It was apparent from the shape of the curves that all the liquids 

 could not be compared at any one value of the slope, because tl 

 effect of temperature on the slope varied so much from substanc 

 to substance. In some cases the whole of the alcohols for example- 

 the slope at the boiling-point was considerably greater than that at 

 in the case of some of the less viscous liquids. A slope was, ther 

 fore, selected at which as many liquids as possible could be com- 

 pared. Another slope was then obtained at which the outstanding 

 liquids could be compared with as many as possible of the liquic 

 used at the original valneof the slope. The relationships between the 

 magnitudes of the viscosities of these liquids which could be com- 

 pared at the two slopes were then found to be the same at either si 

 so that general conclusions regarding the behaviour of all the liquit 

 could be deduced. These are as follows : 



