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the learned Dr. Beddoes. In the Philofophical Tranfadlions 

 for 1 79 1, Page ^^, &c. he tells us, that "a mixture of different 

 " earths, with more or lefs metallic matter, in retui-ning from 

 " a flate of fufion to a folid confiftence, may affume, fometimes, 

 " the homogeneous bafaltic, and fometimes the heterogeneous 

 " granitic internal ftruflure. No fa(5l is more familiar than 

 " that it depends altogether on the management of the fire, 

 ♦' and the time of cooling, whether a mafs Ihall have the uniform 

 " vitreous fradlure, or an earthy broken grain arifing from 

 " a confufed cryftallization. The art of making Reaumur's 

 " porcelain confifls entirely in allowing the black glafs time to 

 " cryftallize by a flow refrigeration, and the very fame mafs, ac- 

 " cording as the heat is condudled, may without any alteration of 

 " its chymical conftitution be fucceflively exhibited any number 

 *' of times, as glafs, or as flony matter vvith a broken grain. 

 " In the flagg of iron furnaces the fame pieces generally ex- 

 *' hibits both thefe appearances." How far the fame mafs in 

 fufion is capable of alTuming fometimes, the bafaltic and 

 fometimes the granitic, we fhall prefently fee. With refpedl to 

 Reaumur's porcelain, it is certain that the changes of texture', 

 mentioned by the learned author, may be produced in it, not 

 by a flower or more rapid cryftallization (for in fadl there is 

 no cryftallization at all) but by the continuance of a higher 

 or lower degree of heat. This is evident from the experiments 

 of Dr. Lewis and Mr. Delaval. Now the efFedl of the higheft 

 heat of our furnaces, in this cafe, is to rob the glafs of Its fliline 

 part, as Dr. Lewis well obferved ; and hence it is not wonderful 

 that the texmre fliould be altered, and the mafs at laft become 



loofe 



