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666. Process of Making Medicines.

To Powder Substances.--Place the substance in the mortar, and strike it _gently_ with direct perpendicular blows of the pestle, until it separates into several pieces, then remove all but a small portion, which bruise gently at first, and rub the pestle round and round the mortar, observing that the circles described by the pestle should gradually decrease in diameter, and then increase again, because by this means every part of the powder is subjected to the process of pulverization. In powdering substances, making emulsions, and whenever using a mortar, the pestle should always travel _from the right to the left_.