Having mentioned the virtues of the simples, we are next to shew in what manner they are to be mixed, and what compositions are made from them. Now they are mixed variously, and there is no certain method for it, since of similar ingredients some are taken away, and others are added; and though the very same simples are all used, the proportion of their weights may be changed. And therefore though the number of medicinal simples be not so very great, there are innumerable kinds of mixtures; which it would be needless to enumerate, though it were possible: both because the same effects are found within the compass of a few compositions; and because it is easy for any person, who is acquainted with their virtues, to change them. I shall therefore confine myself to the most esteemed. Now in this book I shall give an account of those, which may either have been wanted in the preceding, or are employed in the cures, which I am to treat of, only throwing together these compositions, which are of more general use. Such as are accommodated to one particular disorder, or even to a few, I will insert in their proper places. But before I proceed, I would have it understood, that in an ounce is contained the weight of seven denarii. Next, that I divide each denarius into six parts, that is sextantes, so that I have the same quantity in the sextans of a denarius, that the Greeks have in their obolus[DM]. That being reduced to our weights makes a little more than half a scruple. [DM] ὀβολὸς. [Sidenote: _Of the difference betwixt malagmas, plaisters, and troches._]