Page:Theophrastus - History of Stones - Hill (1774).djvu/87

 But if this be the Cae in foile Subtances in general, it is much more particularly o in this Clas of them, the Gems; the Differences of which are owing to the Ditribution of a certain kind of Particles in their Maes; which are o very uncertain, both in Quantity and Manner of placing, and in their various Effects upon the Mas, that carce any thing abolute is to be determined from them.

The Gems are naturally angular, as are the Crytals: but like them, from various Accidents in their Formation, they are found ometimes in rude or hapeles Maes; and when angular, they have till all that Variation of Figure which we ee take place in Crytal and Spar; from the different Diturbances of their Crytalization. In all thee Caes a various Number of Angles may be occaioned, as we ee in Salts, from the Accidents of their Concretion. In thee, as well as in thoe, we have the ame Kind in different Figures; and as we can crytalize them under the Eye, we can determine the Caues of thoe Alterations. The round, or pebble Gems, eem not to have been original in that Form, but worn to it by rolling about in a Fluid.

The Hardnes and the Lutre of the Gems, mut ditinguih them from all other Stones; for if we conidered their Form, as their eential Character, many Crytals would aume the Name: and Crontedt has well determined, that a certain Spar he had een in Figure of the mot regular Diamond, mut then be called, a Diamond.

No peculiar Contruction, no Form of contituent Parts is viible in the Gems: they appear as Maes of uniform Nature; and they break irregularly and indeterminately; yet there is in all a really plated Structure. The Lapidaries find this in ome, and can plit them; the Burning Glas dicovers it in the ret; and when turned to it in a right Direction, tears them to pieces: they plit into the thinnet Plates that can be conceived, and eem to have been compoed in the Manner of the Talcs, only more compact. 'Tis pity this Character is not more obvious: for it affords a real ditinctive Mark between the Gems, and all other Stones: Crytals, which eem to come nearet to them, have it not.

Their Colours are les eential, for they can in mot be driven away by Fire; and Nature ometimes gives the Gem without them; they are evidently owing to the Metals; for we can by means of Metals, give the ame to Glas; our artificial Gem.

The Salt Sytem of Linnæus appears here almot ludicrous. To a truly philoophic Eye, the Difference of Etimation and Price are nothing; but the common Reader will hardly they engrave Seals. Its colour is red,