398 
MESSRS. T. E. THORPE AHD J. W. RODGER OH THE REEATIOHS 
densities, the attempts which have been made to establish similar relationships 
between the magnitudes of other physical constants and chemical composition have 
shown that probably all physical constants are to be regarded as functions of the 
chemical nature of molecules, and that the variations in their magnitude observed in 
passing from substance to substance are to be attributed to changes in chemical 
composition. 
The physical properties first investigated from this point of view were naturally 
those either often measured or at least capable of being easily measured. To this 
class belong such determinations as density, boiling-point, refractive index, &c., &c. 
On the other hand, properties not so clearly understood, or less readily perceived, 
received little or no attention. An example of this kind occurs in connection wdth 
the viscosity of liquids. 
When a liquid flows, or when its form is altered, forces are called into l^lay within 
the liquid which offer resistance to the force causing flow or change of form. The 
viscosity of the liquid may be taken as a measure of these internal forces, but, 
although the common use of the terms “viscid,” “oily,” “mobile,” “limpid,” &c. 
shows that the endeavour has not been wanting to indicate the different character of 
liquids in respect to this property, it is only within quite recent times that the 
attempt to obtain quantitative measures of the viscosity, or viscosity-coefficients, for 
a large number of liquids has been made. This is due to a variety of reasons. To 
begin with, physio-chemical inquiries have been almost exclusively carried out by 
chemists, who have hitherto had little cause to study such a property as viscosity, the 
conception and mode of quantitative expression of which have been developed b}^ 
physicists. Moreover, even from a pm-ely physical point of view, the accurate deter¬ 
mination of absolute coefficients of viscosity has been beset with difficulties, both in 
the theory and practice of the methods employed. Viscosity is, no doubt, the nett 
result of at least two distinct causes. When a liquid flows, during the actual collision 
or contact of its molecules, a true friction-like force will be called into play, opposing 
the movement. But, in addition to this force, even after the actual collisions, mole¬ 
cular attractions will exercise a resistance to forces wdfich tend to move one molecule 
past another, and hence it may have been surmised that, even if accurate values of 
the coefficients of viscosity could be obtained, they might not exhibit simple relation¬ 
ships to chemical composition.^'" 
Although few absolute measures of viscosity have been hitherto published, several 
researches have been made which may be regarded, in certain cases at least, as being 
concerned with the relations of viscosity to the chemical characters of substances. 
That they may be so regarded arises from the circumstance that the observers have 
incidentally made use of one of the methods for obtaining the viscosity-coefficient, 
which consists in noting the time which a definite volume of liquid takes to flow 
* Comp. Gkaetz, ‘ Wiedem.vnn’s Annalen,’ 24, 2.5, 1888; Mutzel, ‘ Anualen,’ 43, 35, 
1891. 
