642 Professor T. E. Thorpe [March d, 



whether the isomerism is in the alcohol or the acid radicle. The 

 normal aldehydes have invariably a greater viscosity than the iso- 

 compounds, whilst the alcohols have a greater viscosity than the 

 correspondicg aldehydes and ketones. The introduction of the 

 hydroxyl group into the molecule greatly increases the viscosity of 

 the liquid. This is strikingly illustrated by the instances of propyl 

 alcohol, propylene glycol and glycerin. Indeed the high viscosity 

 of solutions of carbohydrates, e.g. the sugars, gums, &c., is probably 

 dependent on the relatively numerous hydroxyl groups in the mole- 

 cule. The manner in which tlie hydroxyl group is combined seems, 

 however, to have considerable influence on the viscosity. Thus in 

 the cases of the isomeric substances, benzyl alcohol and metacresol, 

 it is found that in the first-named substance, in which the hydroxyl 

 group occurs in the side chain, the viscosity is very much less than 

 that of the second, in which the hydroxyl group is attached to a carbon 

 atom in the benzene ring. 



Whilst the broad fact of a connection between the viscosity of a 

 liquid and the chemical nature of its molecules is established, it 

 cannot be said that the numerical results hitherto obtained ajSbrd any 

 accurate means of determining the quantitative character of this con- 

 nection. This is owing partly to the imperfection of observational 

 methods, and partly to the uncertainty of the basis of comparison. It 

 seems futile to expect that any definite stoichiometric relations should 

 become evident by comparing observations taken at one and the same 

 temperature. Hitherto few attempts have been made to ascertain 

 the influence of temperature upon viscosity, and hence the law of the 

 variation is unknown. It seemed therefore, obvious, that in order to 

 investigate the subject with reasonable hope of discovering stoichio- 

 metric relations, one essential point was to ascertain more precisely 

 the influence of temperature on viscosity, and then to compare the 

 results under conditions which have been found to be suitable in 

 similar investigations in chemical j)hysics. Unfortunately, the accu- 

 rate determination of absolute coefiScients of viscosity is beset with 

 difficulties, both in the theory and practice of the methods which can 

 be employed. Moreover, it is quite possible that even if accurate 

 values of the coefficients of viscosity were obtained, their relationships 

 to chemical composition might not be simple. 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 is called into play which opposes the movement, whilst at 

 the same time molecular attractions exercise a resistance to the forces 

 which tend to move one molecule past another. 



After indicating the meaning of viscosity and the principles 

 involved in measuring it, the lecturer proceeded to point out how 

 the coefficient of viscosity may be defined. It is the force which is 

 necessary to maintain the movement of a layer of unit area past 

 another of the same area with a velocity numerically equal to the 

 distance between the layers when the space between them is con- 



