576 grs. wheat, the Troy ounce has 480 grs. Troy or 640 grs. wheat. How came this augmentation of the ounce?
It is in Apothecaries' weight that we find the key. This standard runs thus
20 grs. = 1 scruple,
3 scruples = 1 drachm,
8 drachms = 1 ounce,
12 ounces = 1 pound.
Now note that there are 24 scruples in the ounce, and 288 scruples in the pound, exactly as in the Roman system. But there is an element foreign to the old Roman system as seen in the drachm of 60 grs. Now Galen and the medical writers of the Empire used the post-Neronian denarius of 60 grs. as a medicine weight. What more convenient weight unit could be employed than the most common coin in circulation? The drachma and denarius had long since been used synonymously in common parlance. But as there were 18 grs. (Troy, 24 wheat grs.) in the old scruple, and there were 60 grs. in the drachm or denarius, they were not commensurable, and accordingly to obviate this difficulty the physicians for practical purposes raised the scruple to 20 grs., in order that it might be one-third of the drachm. The number of scruples in the ounce remaining 24 as before, the ounce became augmented by 48 grs. (24 x 2) and accordingly rose to 480 grs. We saw above that the Troy grain is the barley-corn. Why is the latter so closely connected with 'Troy weight'? When the scruple was raised from 18 grs. Troy, 24 grs. of wheat, to 20 grs. Troy, it no longer contained an even number of wheat grains, for the new scruple contained 26-2/3 grs. wheat. As this was inconvenient, and on the other hand the new scruple weighed exactly 20 barley-corns, the latter henceforth became the lowest unit of this system.
