. 



MKFU8IOX. 



determination* w may draw off a portion from a given stratum of the 

 mixed liquid, and di-tennine it* composition either by chemical methods or by 

 its specific gtmrity, or any other property from which its composition may be 



deduced. 



But as the act of removing a portion of the fluid interferes with the 

 ttrocce* of diffusion, it is desirable to be able to ascertain the composition of 

 any at rat inn of the mixture without removing it from the vessel. For this 

 propose Sir W. Thomson places in the jar a number of glass beads of different 

 dimtfrW, which indicate the densities of the strata in which they are observed 

 to float. The principal objection to this method is, that if the liquids contain 

 air or any other gas, bubbles are apt to form on the glass beads, so as to 

 make them float in a stratum of less density than that marked on them. 



M. Voit has observed the diffusion of cane-sugar in water by passing a 

 ry of plane-polarized light horizontally through the vessel, and determining 

 the angle through which the plane of polarization is turned by the solution 

 of sugar. This method is of course applicable only to those substances which 

 cause rotation of the plane of polarized light. 



Another method is to place the diffusing liquids in a hollow glass prism, 

 with its refracting edge vertical, and to determine the deviation of a ray of 

 light passing through the prism at different depths. The ray is bent doAvn- 

 wards on account of the variable density of the mixture, as well as towards 

 the thicker part of the prism; but by making it pass as near the edge of 

 the prism as possible, the vertical component of the refraction may be made 

 vety small; and by placing the prism within a vessel of water having parallel 

 aides of glass, we can get rid of the constant part of the deviation, and are 

 able to use a prism of large angle, so as to increase the part due to the 

 diffusing substance. At the same time we can more easily control and register 

 the temperature. 



The laws of diffusion were first investigated by Graham. The diffusion of 

 gases has recently been observed with great accuracy by Loschmidt, and that 

 >f liquids by Fick and by Voit. 



Diffusion as a Molecular Motion. 



If we observe the process of diffusion with our most powerful microscopes, 

 we cannot follow the motion of any individual portions of the fluids. We 



