A colloid may be defined as a substance which forms a jelly-like solution, incapable of passing through a membrane of parchment or similar material. Colloids are thus contrasted with crystalloids which are able in solution to pass through colloidal membranes. A colloid does not crystallize, and has no definite melting point; in fact, the physical changes induced in it by heat or by solvents are perfectly continuous so long as the chemical molecule of the substance remains intact. The isolation of a particular colloid from a mixture containing other colloids offers great technical difficulties, and such a substance is exceedingly difficult to obtain in a chemically pure condition. All the ordinary chemical tests of identity are baffled by the consistency of such a material. The colloidal condition is believed to be generally associated with the existence of a very large chemical molecule.