Thomas Graham (1861) pointed out that soluble substances could be divided into two classes, the Crystalloids and the Colloids. He designated as crystalloids all those substances, such as sugar, urea, common salt etc. which could be obtained in a crystalline form and could diffuse through animal or parchment membrane. The colloids, on the other hand, were substances, like protein, starch, gum, clay etc. which could not diffuse through animal membrane, and were amorphous.

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