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14 GENERAL AND PHYSICO-CHEMICAL.
the colloids and also analogous forms of stannic acid, titanic acid,
molybdic acid and tungstic acid, aluminium hydroxide and analogous
metallic oxides, when they exist in the soluble form, and also starch, dex-
trins, the gums, caramel, tannin, albumin and gelatin.
Some colloids are characterized by the fact that under certain con-
ditions they solidify into a gelatinous form containing considerable water.
In the case where water is the solvent then Graham called the soluble
form hydrosol and the gelatinous form hydrogel.
By diffusion through a membrane (called dialysis by Graham) colloid sub-
stances can be separated from crystalloids. Colloidal silicic acid as well as
corresponding forms of certain other bodies are obtained by treating the soluble
alkali salt with hydrochloric acid, then removing the excess of hydrochloric acid
as well as of chlorides, by means of dialysis. Colloidal alumina was obtained by
Graham by dissolving aluminium hydroxide in aluminium chloride. This last salt
was removed by dialysis and the hydroxide remained with more or less HC1 com-
bined in solution.
Various metallic sulphides can be obtained in colloidal solution. Such solu-
tions of As2S3 and Sb2S3 can be obtained by passing H2S into dilute solutions
of the respective metallic oxide, 1
and colloidal CuS can be prepared by washing
the precipitated compound with water, by which treatment thej,CuS finally becomes
soluble in water. 2
The metals can be obtained as hydrosols, and indeed in two ways:
1. By treating a salt with various reducing agents (for example formaldehyde,
hydrosulphurous acid, hydrazine, Irydroxylamine) the various metals are obtained
in colloidal solution. 3
As the solutions thus obtained are often very unstable,
it has been found advisable to help their stability by the addition of organic
colloids (gelatin). We will discuss the mode of action of these so-called pro-
tective colloids on page 23.
2. Bredig 4
has discovered a method which makes possible the production
of pure metallic sols by the cathode spraying of metallic wires under water.
Svedbeug 5
prevents the heating of the fluid in this spraying by using the
induction current. This makes the spraying also possible under organic fluids
and sols of the light metals have also been prepared. Practically sols of all
metals and metalloids can be prepared in this way.
Among those bodies which can be obtained in the colloidal state
we have acids as veil as bases, and the chemical elements are also known
as colloids, as well as bodies of more complex molecular structure like
the proteins and starches. The colloid bodies, therefore, have from a
chemical standpoint nothing in common. More likely the colloidal con-
dition is due to physical properties, and this follows from the researches
of Graham. The crystalloids and the colloids are therefore not to
be considered as chemically different classes of bodies, but rather only
as different physical conditions of matter and the boundary between
1
II. Schulze, Journ. prakt. Chem. (N.F.), 25, 431 (1882), and 27, 320 (1883).
•Spring, Ber. d. d. chem. Gesellsch., 16, 1142 (1883).
’ Miillr-r, Allg. Chemie d. KoHoide. Leipzig (1907), 6.
* A&arganiflcfae l ermente. Leipzig (1901), 24.
* Ber. d. d. chem. GeBellach., 38, 3616 (1905); 39, 1705 (1906).
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