42 CHANGES OF CONDITION.
are of so specific a nature (for example, iodo-starch) that it has been only
after the investigation of the quantitative relations between their com-
position and the conditions under which they are formed that their
difference from true chemical compounds has been established. Such an
exhibition of adsorptive power as this has also been designated as affinity of
condition.
Different dissolved colloids can also mutually combine to form adsorption
compounds: gold hydrosol, which is so exceedingly sensitive toward elec-
trolytes, can, by the addition of a non-sensitive colloid such as gelatin, be
itself made stable toward electrolytes; this would tend to show that a com-
bination had taken place between the two kinds of dissolved colloids (cf.
No. 25). Substances which act as gelatin does in this case are known as
protective colloids.
Again, other colloids are capable of mutually precipitating one another
out of solution.^1 Thus arsenic-sulphide-hydrosol and iron-oxide-hydrosol
when mixed in the right proportions are both precipitated as a common
adsorption compound (cf. No. 24, 4). This precipitating power arises from
the same cause as another characteristic property which pseudo-dissolved
substances have in common with suspensions', if suspensions or colloids are
subjected to the action of a strong electric potential, a passage of the suspended
material through the solution occurs, but this is of a very different nature
from ionic migration. (Convective transference.) While, with electrolytes, the
dissociated parts possess opposite electrical charges, here the opposite charges
reside upon the pseudo-dissolved material and the solvent itself, respectively.
It is a rule that two such colloids in order to precipitate each other must
have charges of opposite sign when referred to that of the common solvent.
(Example, zircon-gold-purple, No. 25.)
Finally, adsorption compounds can be produced by precipitating two col-
loids out of a common solution by the addition of an electrolyte. (Example,
Purple of Cassius, No. 25.)
- Colloidal Platinum, according to Bredlg.
Connect two platinum wires of 1 mm. diameter, whose upper
ends are insulated by glass tubes, with the terminals of the 110-volt
lighting circuit, and insert an ammeter and a suitable resistance.
Clamp one electrode so that its lower end dips into 100 to 150 c.c.
of distilled water in a glass dish; hold the other electrode in
the hand, and while it is immersed in the water, touch it to the
first one and remove it to such a distance that a small arc can be
maintained. The resistance should be regulated so that a current
of 6 to 10 amperes will flow.
2
From the cathode the disintegrated
> W. Biltz, Ber. 37, 1095 (1904).(^2) A technical ammeter with large capacity is used here. Before begin-
ning the experiment, see that the wiring will safely bear the current required.