Another fundamental idea of colloid chemistry is that the great extension of surface involved in a high dispersity causes the surface energy to be no longer a negligible fraction of the total energy of the system, and that the recent advances in knowledge respecting surface phenomena may be called in to assist in the explanation of the special properties of the colloid state. Particles which exhibit the Brownian movement, about 10^(-5) cm. diameter, down to the limit of microscopic visibility (10^(-3) cm.) are termed _microns_. Particles less than this, but just visible in the ultra-microscope (5×10^(-7) cm.) are termed _submicrons_. Particles still less, approximately 10^(-7) cm., have been shown to exist, and are termed _amicrons_. The dimensions of molecules such as may exist in true solutions are of the order of 10^(-8) cm. A colloid sol may contain particles of various sizes. Thus a gelatine sol (like other lyophile systems) contains chiefly amicrons, but submicrons are also observable. 1. THE CONTINUOUS PHASE Owing to the contractile force of surface tension, it is concluded that the surface layer of a liquid is under very great pressure, much greater than the bulk of the liquid. Any extension of the surface of the liquid naturally causes a corresponding extension of the proportion of liquid which is thus compressed. If in a beaker of water there be placed a porous substance, such as animal charcoal, there is a great extension of the surface of the water, and a corresponding increase in the amount of compressed water. If instead there be substituted a large number of very small particles of a substance, a still further increase in the amount of compressed water is involved. As the specific surface of the substance inserted is increased, and its amount, the proportion of compressed and denser water increases also, until it is a practically appreciable percentage of the total volume. It is clear also that the extent of the zone of compression will be determined also by the nature of the substance with which the water is in contact at its surface, _i.e._ by the extent to which it is hydrophile, and this indeed may be the more important factor.