EUCLASE

euclase, a very rare mineral, occasionally cut as a gem-stone for the cabinet. it bears some relation to beryl in that it is a silicate containing beryllium and aluminium, but hydrogen is also present, and the analyses of euclase lead to the formula hbealsio5 or be(aloh)sio4. it crystallizes in the monoclinic system, the crystals being generally of prismatic habit, striated vertically, and terminated by acute pyramids. cleavage is perfect, parallel to the clinopinacoid, and this suggested to r.j. hauey the name euclase, from the greek [greek: eu], easily, and [greek: klasis], fracture. the ready cleavage renders the stone fragile with a tendency to chip, and thus detracts from its use for personal ornament. the colour is generally pale-blue or green, though sometimes the mineral is colourless. when cut it resembles certain kinds of beryl (aquamarine) and topaz, from which it may be distinguished by its specific gravity (3.1). its hardness (7.5) is rather less than that of topaz. euclase occurs with topaz at boa vista, near ouro preto (villa rica) in the province of minas geraes, brazil. it is found also with topaz and chrysoberyl in the gold-bearing gravels of the r. sanarka in the south urals; and is met with as a rarity in the mica-schist of the rauris in the austrian alps. euclid [eucleides], of megara, founder of the megarian (also called the eristic or dialectic) school of philosophy, was born c. 450 b.c., probably at megara, though gela in sicily has also been named as his birthplace (diogenes laertius ii. 106), and died in 374. he was one of the most devoted of the disciples of socrates. aulus gellius (vi. 10) states that, when a decree was passed forbidding the megarians to enter athens, he regularly visited his master by night in the disguise of a woman; and he was one of the little band of intimate friends who listened to the last discourse. he withdrew subsequently with a number of fellow disciples to megara, and it has been conjectured, though there is no direct evidence, that this was the period of plato's residence in megara, of which indications appear in the _theaetetus_. he is said to have written six dialogues, of which only the titles have been preserved. for his doctrine (a combination of the principles of parmenides and socrates) see megarian school.