The disperse phase carries a positive or nega tive electric charge, which is dependent to some extent on the dispersion medium—that is, the water or other liquid—and sometimes on its alkalinity or acidity. Colloidal solutions can exhibit a change of condition under mechanical action, or the application of heat, and the dis solved substance may separate in an insoluble form or be converted into jellies by the addition or certain substances, such as electrolytes. When the substance separates out in an insoluble form it is known as a " gel," and if formed from an aqueous solution it is known as a " hydrogel," an " alkogel " from alcohol, and a " sulfogel " from sulphuric acid, etc. The process of the formation of the gel is called pectinisation or coagulation. When the residue left after coagu lation is soluble in water, the process is said to be reversible ; if insoluble, it is irreversible. Frequently an insoluble and otherwise irrevers ible colloid is precipitated in the presence of a reversible colloid; it also becomes reversible ; and the colloid that produces this state is known as a protective colloid, or, to use the German word, a " schutz-kolloide." Lottermoser has also pointed out that by certain precipitating agents a colloid may be precipitated from the hydrosol in such a condition that it will again form a hydrosol, and therefore suggests the terms " solid " and " liquid " hydrosol, and confines the term hydrogel to the insoluble amorphous substance. The law is that sols with opposite electric charge precipitate one another, but those of like charge do not.
Gels or jellies may be considered as colloid solutions in which the disperse phase is in a higher concentration, and molecular and col loidal solutions can diffuse through a gel more or less rapidly, according to the concentration of the disperse phase of the gel.
Colloidal solutions differ from true solutions in that the latter are perfectly homogeneous under the most critical visual examination that can be applied, whilst the former show the particles under sufficiently high power as already pointed out. They differ also from suspension liquids or mixtures in that in the latter the particles or disperse phase are sufficiently large to be seen with the naked eye or a weak power. There is, however, some evidence to prove that these divisions are but arbitrary, and that so called true solutions may be of a colloid nature.
P. Weimarn and Wolfgang Ostwald (" Grun driss der Kolloidchemie") considering that, as sus pensions, colloidal and true solutions are merely varying degrees of dispersion, have proposed the name of " dispersoids," and the latter divides them into (i) coarse dispersions, such as suspensions and emulsions ; (2) colloidal solu tions ; (3) molecular dispersoids ; and (4) ion dispersoids, assuming that free ions exist. The crystalloid solutions belong to classes (3) and (4). The above classes merge one into the other, and colloidal solutions are divided into suspension colloids and emulsion colloids, which are also termed suspensoids and emulsoids. The occur rence of colloidal silver and gold is assumed in many photographic processes, and many reactions can only be satisfactorily explained on this assumption. There is, however, an increas ing tendency to drag the phenomena of colloids into every obscure photographic process, and there is the grave danger that it may be used merely to cloak our ignorance of the true state of affairs.