Diffusion of Liquids, 317 



whose density varies from below upward, as is the case in a 

 diffusing salt or sugar solution, must act as a prism whose 

 refracting angle is turned upward, and hence that a ray of 

 light which enters such a medium in a horizontal direction 

 cannot pass through it in the same straight line, but must ex- 

 perience a refraction downwards. This conclusion of Stefan 

 is strengthened by the fact that the values found by means of 

 the optical methods are much smaller than those required by 

 Fick's theory. Any further discussion of these methods is 

 unnecessary in this place. 



Marignac* published the next investigation on diffusion. 

 He followed exactly the process of Graham ; and his work 

 differed from that of the latter only in this, that he observed 

 the simultaneous diffusion of two salts, instead of that of solu- 

 tions of single salts, as did Graham in most cases. We shall 

 have occasion to refer to his work again. 



About a year ago H. F. Weber published a very interesting 

 paper, under the title " Untersuchungen iiber das Elementar- 

 gesetz der Hydrodiffusion "f . For a description of the unique 

 method employed by him I refer to his paper, and will mention 

 here only some of the results. He experimented with solu- 

 tions of ZnS0 4 , and found the constant k for this substance 

 corresponding to several different concentrations and tempe- 

 ratures of the solution. From his results it appears that k 

 very slowly decreases in value as the concentration increases, 

 and that, on the other hand, it increases very rapidly with the 

 temperature, as shown bv the following figures : — 



* = 0-1252 when T= l°-2 C. 



& = 0'2421whenT=18 o -5 



& = 0'4146 when T=44°-7, 

 which results can be expressed in the equation 



&=0\L187[1 + 0"0557T]. 



Weber was led to conclude from his experiments that Fick's 

 law is approximately correct. 



Since Weber's still another investigation of the subject has 

 appeared. In the Philosophical Transactions for 1861 % is an 

 article by Graham, " On Liquid Diffusion applied to Analysis," 

 which seems to have escaped the notice of succeeding investi- 

 gators. 



Stefan §, however, has quite recently reviewed this work, 

 and has deduced therefrom the values of k for a number of 



* Arch. Phys. et Nat. 1. p. 79. Compt. Rend, lxxviii. (1874). 

 t Vierteljahrsschrift der Ziiricher naturf. Gesett. Nov. 1878. [Phil. Mag. 

 December 1879.] 



X Also Liebig's Annalen, cxxi. p. 1. 



§ Wiener Sitzungsber. Jan. 1879. [Phil, Mag April 1879.] 



