262 DIFFUSION, OSMOSIS, AND FILTRATION. 



which this approaches on increasing dilution, is a measure of the 

 coefficient of activity of the solution. According to this view, then, a 

 very dilute solution of sodium chloride consists of positively -charged 

 sodium and negatively-charged chlorine ions moving amongst the water 

 molecules, but unable to part company by virtue of their charges of 

 opposite sign, and only separable by the application of energy from 

 without (electrolysis). Other substances which do not conduct elec- 

 tricity in aqueous solution are believed to be in a simpler state of 

 solution, the molecules moving among those of the solvent not being 

 known to be in a different condition to those of the undissolved sub- 

 stance, but simply capable of freer motion. 



It is further probable that in the case of certain non-electrolytes in 

 solution, instead of single molecules wf deal with aggregates of mole- 

 cules, and such substances are said to be in colloidal solution (x&XXa, 

 glue). As instances of organic substances the aqueous solutions of 

 which are colloidal, may be mentioned albumin, gum-arabic, starch, 

 haemoglobin. 1 



It must at once appear likely that the ease with which the 

 " molecules " of different substances can move among those of the 

 solvent in a solution is different in the case of different substances, i.e. 

 that the power of diffusibility must be very variable. 



Graham z gives the following table : 



Equal weights had diffused to the same extent in the following times : 



Hydrochloric acid . 1 

 Sodium chloride . 2 '3 3 

 Cane-sugar 7 



Magnesium sulphate . 7 

 Albumin . . .49 

 Caramel 98 



Substances in solution tend to diffuse from places of higher to those 

 of lower concentration, and in the law of Tick 3 it is stated that the 

 quantity of dissolved substance so diffusing is proportional to the rate 

 of fall in concentration. 



Thus, if a is the quantity of substance passing section q of a diffusion 

 cylinder in time z, when at x the concentration in the section is c, and at 

 x ~h dx is c + dc ; then 



do 



where & is a constant peculiar to the substance and known as the 

 coefficient of diffusion. 



From the law of Fick, Stefan 4 calculated for a special case the 

 following formula : 



/kz 



a = cq*/ - 



(T 



1 Picton and Linder (Journ. Chem. Soc., London, 1892, vol. Ixi. p. 148; 1895, vol. 

 Ixvii. p. 63) have prepared solutions of arsenious sulphide of various " grades." Thus one 

 may have (a) aggregates visible by microscope ; (/S) no visible aggregates, but the substance 

 not diffusible ; (y) the substance .diffusible but not filterable ; (2) the substance both 

 diffusible and filterable, but the aggregates still large enough to scatter light. They 

 consider that in matter in solution one can pass by grades from obvious suspension, to 

 colloidal solution, to non -electrolytic crystallised solution, and so to the first grade of 

 electrolytic solution. 



2 Phil. Trans., London, 1861, vol. cli. p. 183. 



3 Ann. d. Phys. u. Chem., Leipzig, 1855, Bd. xciv. S. 59. 



4 Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1879, Bd. Ixxix. S. 161. 



