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_.