1873.] Double Refraction in a Viscous Fluid in motion. 47 



of a viscous fluid has been hitherto employed ; but it appears capable of 

 furnishing important information as to the nature of viscosity in different 

 substances. 



Among transparent solids there is considerable diversity in their action 

 on polarized light. If a small portion is cut from a piece of unannealed 

 glass at a place where the strain is uniform, the effect on polarized light 

 vanishes as soon as the glass is relieved from the stress caused by the un- 

 equal contraction of the parts surrounding it. 



But if a plate of gelatine is allowed to dry under longitudinal tension, 

 a small piece cut out of it exhibits the same effect on light as it did 

 before, showing that a state of strain can exist without the action of 

 stress. A film of gutta percha which has been stretched in one direc- 

 tion has a similar action on light. If a circular piece is cut out of such 

 a stretched film and warmed, it contracts in the direction in which the 

 stretching took place. 



The body of a sea-nettle has all the appearance of a transparent jelly ; 

 and at one time I thought that the spontaneous contractions of the living 

 animal might be rendered visible by means of polarized light transmitted 

 through its body. But I found that even a very considerable pressure 

 applied to the sides of the sea-nettle produced no effect on polarized 

 light, and I thus found, what I might have learned by dissection, that the 

 sea-nettle is not a true' jelly, but consists of cells filled with fluid. 



On the other hand, the crystalline lens of the eye, as Brewster ob- 

 served, has a strong action on polarized light when strained either by 

 external pressure or by the unequal contraction of its parts as it becomes 

 dry. 



I have enumerated these instances of the application of polarized light 

 to the study of the structure of solid bodies as suggestions with respect 

 to the application of the same method to liquids so as to determine 

 whether a given liquid differs from a solid in having a very small 

 " rigidity," or in having a small " time of relaxation"*, or in both ways. 

 Those which, like Canada balsam, act strongly on polarized light, have 

 probably a small " rigidity," but a sensible " time of relaxation." Those 

 which do not show this action are probably much more " rigid," and owe 

 their fluidity to the smallness of their " time of relaxation." 



The Society then adjourned over the Christmas Recess to Thursday, 

 January 8, 1874. 



* The " time of relaxation " of a substance strained in a given manner is the time 

 required for the complete relaxation of the strain, supposing the rate of relaxation to 

 remain the same as at the beginning of this time. 



