40 CHANGES OF CONDITION.
which are unable of themselves to pass directly into colloidal solution, but
can by suitable means be brought into that state; if such substances are
separated from the solution, they do not possess the power of dissolving
again unaided (irreversible colloidals).
Solutions of irreversible colloids can be obtained by disintegrating the
substance in the electric arc under water (Bredig's method; see No. 20), by
forming the substance in aqueous solution by double decomposition and
preventing its precipitation by suitable means (Nos. 21; 24, Note on zircon
oxide hydrosol; No. 25), or by previously imparting to the solid the ability
to dissolve by treatment with small quantities of alkali or acid. The last
method was compared by Graham with the process of digestion and called
by him peptonization (No. 25, tin-oxide hydrosol).
By means of optical methods (ultramicroscopic investigation), it has been
proved that many of these colloidal solutions consist really of suspensions of
extremely small particles the size of which can be estimated down to a diam-
eter of about 6 MM- In this manner the formerly-very-puzzling fact of the
existence of solutions of substances that are ordinarily insoluble, such as gold,
platinum, and the sulphides of the heavy metals, finds its explanation.
Modern views concerning the nature of colloids indicate that any substance
under proper conditions can be obtained in the colloidal state. There is no
sharp distinction between mechanical suspensions of a finely powdered sub-
stance in a liquid, a colloidal solution and a true solution. It seems to be
merely a matter of size of the individual particles of the substance. By
employing micro-photographic methods and working with the short waves of
ultra-violet light, the limit of microscopic vision with an ordinary microscope
is reached at diameters of about 0.001 mm. or 1 micron (symbol M). The
pores of the best grades of hardened filter paper are about 1 M in diameter.
Clay and porcelain filters have pores about 0.2-0.4 M in diameter. The size
of the average molecule varies from 0.0001 to 0.001 M or, using the term
micro-micron or amicron to represent one-millionth of a millimeter (symbol MM),
the diameter is 0.1-1 nfx. A very large molecule such as starch has a diam-
eter of about 5 ij.fi..
On the basis of the size of the particles which a powdered substance assumes
when it is shaken with a liquid, it is possible to distinguish arbitrarily between
suspensions, colloidal solutions and true solutions. If the particles have diam-
eters of 0.1 M or more, so that they may be removed by filtration and can be
seen under the microscope, the solid substance is said to be in suspension. If
the diameters of the particles of solid lie between 0.1 M and 1 fn/j., the mixture of
solid and liquid is said to be a colloidal solution. If the substance dissolves,
as we say, in the liquid and assumes dimensions corresponding to those of
individual molecules, having diameters of 1 fj.fi. or less, we say that the sub-
stance is in a state of molecular dispersion, or true solution.
Colloidal solutions, as a rule, appear clear when viewed by transmitted
light, i.e., looking straight through the solution in a test tube, but appear
turbid when viewed by reflected light, i.e., with the test tube held above or
below the level of the eye. The particles can be seen with the aid of the ultra-
microscope and show a lively movement to and fro (Brownian movement).