COLLOIDS A name derived from Greek kolla (glue) and eidos (appearance), and given by Graham to those non-crystalline substances which do not diffuse through porous membranes. The chief organic colloids are cellulose, starch, dextrine, tannin, gelatine, caramel, and albumen. The inorganic colloids are hydrated oxides of iron, hydrated silica, alumina, etc.
Graham, in 186i, discovered that many sub stances, particularly those which readily crystal lise, diffuse through animal membranes, whilst other substances, such as gelatine, which do not crystallise, do not so diffuse. (Modern researches have shown that Graham's conclusions must be modified somewhat.) The latter class of bodies he called " colloids." The diffusion of the crystal line salts through a membrane he termed " dialysis," and the vessel in which the solution was placed a " dialyser." There are a great many natural or organic substances, such as starch, dextrine, gums, albumen, caramel, rubber, resin, etc., which are colloids and behave pre cisely in the same way as the first-mentioned gelatine ; but there are also many inorganic chemicals, such as ferric hydrate, silicic acid, etc., which act similarly. Apparently these dissolve in water, but when submitted to the test of dialysis prove themselves to be true colloids. The apparent solutions of such sub stances are called " pseudo-solutions," to differ entiate them from the so-called true solutions.
Graham also discovered that water was not unique in forming colloidal solutions, but that alcohol, benzole, glycerine, and sulphuric acid, as well as other solvents, were capable of acting in the same way ; and the term " sol " is used to designate these. Thus, hydrosol indicates a water sol, alkosol an alcoholic sol, and glycerosol a glycerine colloidal sol. Generally, when the solution is of an organic nature, it is termed an " organosol." The scientific student may here be told that, practically, a sol or colloidal solution consists of two ingredients, a liquid and a solid, the latter being in an extremely finely divided state, dis tributed or suspended in the liquid. The sharply separated parts of the sol are said to be its phases, and in colloidal solutions there are several multiple-phase or heterogeneous forma tions, and the one phase, being in an extremely finely divided state, naturally presents to the second phase a very large surface, and with normal examination the sol appear perfectly homogeneous. This is called " microhetero
geneity." Many substances, particularly those which form jellies or " gels," do not, however, show this particular form of heterogeneity, par ticularly when coagulation is effected, and then it is termed " macroheterogeneity." In con tinental literature, the term " disperse-hetero gene " is used for the former, and a generic term of " dispersoids " is used for all microhetero geneous systems. Other colloid solutions take another form, and this has been likened to a sponge, that is, they practically form a network distributed throughout the dispersion medium.
The density of colloidal solutions cannot be calculated from the densities of the disperse phase and the dispersion medium, or the sub stance and solvent; for instance, a solution of a given quantity of gelatine in a given quantity of water is not the sum of their respective volumes, but less, a small but marked contraction taking place. Their osmotic pressure is very low, and in many cases not to be detected, and their boiling and freezing points vary but very slightly from those of the liquid, water, alcohol, etc.
It has been already stated that colloids would not diffuse through an animal membrane, but recent researches have shown that this is only partially true, and that some colloids will diffuse as well as crystalloids, but at a much slower rate, so that the fundamental difference is in their rate of diffusion.
Provided that the size of the particles of the disperse phase are sufficiently small, they exhibit under a powerful microscope peculiar vibratory motions, which were first discovered by Brown in 1827, and are therefore called " Brownian movements." This motion is approximately a zig-zag or to-and-fro motion, and has been ascribed to the contrary pull of gravity and the viscosity or thickness of the liquid. Particles which are larger than 3 to 5 /.4 (i µ = •oor milli metre = in.) do not show this movement. Many hydrosols appear perfectly dear and homo geneous, but others exhibit the phenomena of fluorescence or opalescence when illuminated by suitable light, and Tyndall's phenomenon is often apparent with light of very small wave length, that is to say, the particles are sufficiently large to reflect violet or ultra-violet light of extremely short wave length, and polarise it. This is the foundation of ultramicroscopy.