ON '['UK DIFFUSION <>F LIQUIDS. 



n=sin a v N- — siirÖ -f cosa. sind. (4) 



Now if the vessel be filled with the diffusing Liquid, »and there- 

 fore will be different from layer to laver. By measuring 1 V, the 



concentration // at différent heights can be deduced and k calculated. 

 Tt must be owned here that the above method involves one doubt- 

 ful point. As the concentration is determined by the critical angle 

 of total reflection, its value is that at and near the glass wall of the 

 vessel. This may differ from the value at the interior of the liquid 

 mass, owing to the molecular action of the wall, and the value of A 1 

 deduced by this method may be influenced by the nature of the prism 

 used. This is the point which requires further study. As T have 

 not yet hit upon a suitable method of deciding this point, it is left 

 as an open question (see however Wiener's paper). It may, I think, 

 be investigated, by making the thickness of Q very small, sav one- 

 hundredth of a millimeter, and bringing the influence of the opposite 

 wall R into the action. 



This arrangement can be used as a demonstration experiment in 

 diffusion, for the curve traced out by the critical ray EF can be pro- 

 jected on a screen and shown to a large audience. For this purpose, a 

 cylindrical lens is placed in front of the face b in such a way that sun 

 light sent toward it, is focussed at D (fig. 2) and forms there a bright 

 vertical line. In order to avoid the heating of the liquid and the 

 consequent convection current, an absorption trough for heat rays 

 must be used. A long focus lens placed before the face a, will then 

 project a curve on a screen at a proper distance. This curve consists 

 usually of two vertical lines at its two ends connected by a curved 

 portion having an inflexion poinit. This curved portion corresponds 

 to the region where the diffusion is proceeding and the change in its 

 form shows the progress of diffusion. Fig. 3 is from a photograph 

 obtained by projecting the diffusion curve directly on a sensitive plate. 



