io3 Dr. Falconer on the Knowledge 



a mill. It was then mixed with three parts of 

 the foflil alkali, either by weight or mcafure ; 



and being fufed, was conveyed in a liquid ftate 

 into other furnaces, where it was formed into a 

 mafs called ammonitrum (or fand combined 

 with the foffil alkali), which mafs was melted, 

 and became then pure glafs, and a mafs of white 

 vitrified matter. The fame method of making 

 jt, prevailed in Spain and Gaul. Glafs was like-' 

 wife made to imitate the lapis obfidianus, a fub- 

 flance found by a perfon of the name of Obfidius, 

 in Egypt and i^thiopia. It was of a very black 

 colour, yet obfcurely tranfparent, and often 

 placed among fpecula, in the walls of rooms, to 

 refledl the fliadow of objedls. It was alfo ufed 

 for the fame purpofe as gems (I apprehend for 

 engraving upon) and even for ftatues, Pliny men- 

 tioning, that he faw folid (latues of the Emperor 

 Auguftus, made of this material j and the fame 

 Emperor dedicated four elephants made of the 

 fame fubftance in the Temple of Concord. It 

 appears to have been ufed from great antiquity, 

 Ijnce Tiberius Csefar, when he governed that 

 country, found a ftatue of Menelaus, made of it. 

 In the time of Pliny, the artificial imitation of it 

 by glafs, feems to have been in ufe inftead of the 

 native material. Pliny feems to intimate, that 

 the black colour given to the glafs that was 

 made to imitate the lapis obfidianus, was pro- 

 duced 



