Dr. A. Fick on Liquid Diffusion. 33 



saturation ; and for a certain other value of x which coi-respouds 

 to the base of the funnel^ y becomes =0. In an experiment 

 made with this apparatus, the following numerical values were 

 obtained : — 



Depth of Stratum behw the Surface. 

 27-7 milUms. 55-5 721 88-8 105-4 1221 1387 155-4. 



Observed excess of Sjiecijic Gravity above unity {proportional to the 

 concentration). 



0-000 millim. 0008 0019 030 0010 0055 0075 0105. 



Calculated excess of Specific Gravity above unity. 

 0006 millim. 0-015 0023 0-031 0043 057 0078 0107. 



The determination of the constants for different salts could 

 now be proceeded with, — a determination which measures their 

 diffusibility, and is expressed by k in equation (1). It may be 

 defined as the quantity of salt v/hich, during a unit of time, 

 passes through the sectional-unit, out of one stratum into the next 

 adjacent one, when the rapidity of the diminution of concentra- 

 tion \-j-) ^^ equal to unity. Let the sectional-unit be the sec- 

 tion of our tubes, i. e. the surface of a circle of 1 millim. radius. 

 The diminution of concentration ( -/ ) will then be equal to unity, 



if, through a column of liquid assumed permanently constant, 

 and the height of which is equal to the linear-unit i millim., it 

 has for its result a difference of concentration of both terminal 

 surfaces of such a character, that the one possesses a concentra- 

 tion corresponding to absolute saturation, the other a concentra- 

 tion = 0. Let one day be a unit of time. 



Three tubes of different lengths were now arranged exactly as 

 the cylinder and funnel above described, and after the establish- 

 ment of the stationary condition, the amount of salt was deter- 

 mined which diffused out of the terminal section of the tube in 

 measured spaces of time, and which therefore also, in the same 

 time, passed through any single section of the cylindrical 

 tube. In this case the whole of the values required for the de- 

 termination of k were known, and the rapidity of the diminution 



of concentration ( -^ I was especially determinable, inasmuch as 



it produced permanently, throughout the entire length of the 

 tube, the difference between perfect saturation below and above. 



It is easy to perceive, that if our law be correct, the amounts 

 of salt, passed through the tlircc tubes in the same time, must be 

 inversely proportional to the length of the tubes; and that if 

 these amounts be divided by the time and multiplied by the 



Phil. May. S. 4. Vol. 10. No. 63. July 1855. D 



