486 Professor F. T. Trouton [March 11, 



WEEKLY EVENING MEETING, 



Friday, March 11, 1904. 



His Grace The Duke of Northumberland, K.G. D.C.L. F.R.S. 

 President, in the Chair. 



Professor Frederick T. Trouton, M.A. D.Sc. F.R.S. M.B.I. 



The Motion of Viscous Substances. 



Viscosity is one of the more familiar phenomena. An excellent ex- 

 ample of the behaviour of a viscous substance can be afforded by- 

 treacle or honey. For instance, on helping oneself to honey, the 

 slow and leisurely way in which the substance leaves the spoon is 

 only too patent. This is in marked contrast to liquids such as water 

 which flow freely. 



If we stir such a liquid as treacle, we experience a resistance, a 

 force opposing the movement; on diluting the treacle with water 

 less resistance is afforded by it to stirring — the liquid does not feel 

 so thick. 



Liquids of any desired thickness or viscosity intermediate between 

 treacle and water may be prepared by mixing them in the proper 

 proportions. 



Another convenient series of substances can be easily made, 

 beginning with benzine, which is a particularly mobile liquid, and 

 ending with pitch, ordinarily viewed as a solid. 



The substance next to benzine is just a little less easily stirred 

 than it. Next to this we have a material a little thicker, and so on 

 by slight increases until we arrive at pitch. 



In considering this set of substances we see the difficulty there is 

 in defining exactly what a liquid is. The members of the series, at 

 one end, would be said by all to be liquid, while at the other end 

 they appear at first sight equally entitled to be considered solid; 

 but if we examine them carefully we find that as we go up the 

 series each one can flow just like its neighbour only not quite so fast. 

 Even the seemingly solid pitch flows when given time. 



The question naturally suggests itself. How much is each one 

 thicker or more viscous than the one next it in the series? This at 

 once leads us to the further question. What is the precise meaning we 

 propose to attach to the term viscosity ? 



Comparative measurements might be made of the rate at which the 

 substances flowed under similar circumstances, and these made the 

 basis of a scale of viscosity, but a clearer and more precise definition 

 is got by having recourse to the more mechanical ideas used in the 

 well-known definition of viscosity. 



