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 we 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 2 gives the following table :— 
Equal iceiglrfs had diffused to the same extent in the folloicing times: — 
Hydrochloric acid 
. 1 
Magnesium sulphate 
7 
Sodium chloride 
. 2-33 
Albumin 
49 
Cane-suo-ar 
7 
Caramel 
98 
Substances in solution tend to diffuse from places of higher to those 
of lower concentration, and in the law of Fick 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 + dx is c + dc ; then — 
de 
a = -^ z Tx 
where k is a constant peculiar to the substance and known as the 
cot I'icient of diffusion. 
From the law of Fick, Stefan 4 calculated for a special case the 
following formula : — 
Ihz 
a = cqx/ - 
1 Picton and Linder (Journ. Chem. Soc, London, 1S92, vol. lxi. p. 14$ ; 1S95, vol. 
lxvii. p. 63) have prepared solutions of arsenious sulphide of various "grades." Thus one 
may have (a) aggregates visible by microscope ; (£) no visible aggregates, but the substance 
not diffusible ;. {y) the substance diffusible but not filterable ; (S) 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. 
-Phil. Trans., London, 1861, vol. cli. p. 183. 
3 Ann. d. Plnjs. u. Chem., Leipzig, 1855, Bd. xciv. S. 59. 
4 Sitzungsb. d. Je. Akad. d. Wissensch., Wien, 1879, Bd. lxxix. S. 161. 
