The term "crystalloid" was introduced to describe the differences between water-soluble substances in terms of their ability to form true solutions and pass, while in the dissolved form, through vegetable or animal membranes. Some substances percolate much faster than others, which makes it possible to separate them by dialysis. Most salts and acids can crystallise and diffuse quickly (chlorinated metals, magnesium sulphate, hydrochloric acid and arsenous acid), others do not crystallise (soluble silica, gelatin, albumen, caramel, tannin). Representatives of the first group are referred to as crystalloids, and of the second group, colloids.