But all the standards of mediaeval Europe (with the exception of the Irish) were based on the gold _solidus_ of Constantine the Great[239]. The _solidus_ (itself weighing 72 grains Troy or ⅟₇₂ of the Roman pound) was divided into 24 _siliquae_. The _siliqua_, or as the Greeks called it _keration_ (κεράτιον, from which comes our word _carat_), was the seed of the _carob_, or as it is often called, _St John’s Bread_ (_Ceratonia siliqua_ L). Thus the lowest unit in the Roman system, as it is usually given, is found to be the seed of a plant. The same holds of the Greek system, for the _drachma_ is described as containing 18 _kerata_ or _keratia_, whilst according to others “it contains three _grammata_, but the _gramma_ contains two _obols_ and the _obol_ contains three _kerata_, and the _keras_ contains four _wheat grains_[240].” From this we see that the _keration_ or _siliqua_ was further reduced to 4 _sitaria_, or grains of wheat, whilst from another ancient table of weights[241] we learn that the _siliqua_ likewise equals 3 barley-corns (_siliqua grana ordei_ iii). Hence it appears that 3 barley-corns = 4 wheat grains. Thus both Greek and Roman systems just like the English and Irish take as their smallest unit a grain of corn. This also throws important light on the origin of that mysterious thing, the Troy grain. We saw above (8 Edward I.) that at the time of its introduction into England that 24 grains Troy = 32 grains of wheat, that is the Troy grain stands to wheat grain as 3:4. But as we have just seen that the _siliqua_ = 3 barley-corns, and also = 4 wheat-corns, it follows that 3 barley-corns = 4 wheat-corns. And as 3 Troy grains = 4 wheat-corns, it likewise follows that 3 Troy grains = 3 barley-corns, or in other words, the barley-corn and Troy grain are the same things. It thus appears that the Troy grain is nothing more than the barley-corn, which was used as the weight unit in preference to the grain of wheat in some parts of the Roman empire. Furthermore this relation between barley-corns and wheat-corns can be proved to be a fact of Nature. In September, 1887, I placed in the opposite scales of a balance 32 grains of wheat “dry and taken from the midst of the ear,” and 24 grains of barley taken from ricks of corn grown in the same field at Fen Ditton, near Cambridge, and I thrice repeated the experiment; each time they balanced so evenly that a half grain weight turned the scale. The grain of Scotch wheat weighs ·047 gram, the Troy grain = ·064, ·047 × 4 = 188, ·064 × 3 = 192. Practically 4 wheat grains = 3 Troy grains.