50 EXPERIMENTS on WHINSTONE and LAVA. 



ed with burning coals, according to a practice long followed by 

 Dr Kennedy, by which the heat could be maintained with 

 fo great fleadinefs as to render the refult almoft certain. 



The fufibilities were determined in an open muffle, in which 

 a fragment of the fubftance under trial was placed contiguous 

 to a pyrometer piece. As foon as the fragment, in confequence 

 of the gradual rife of heat, had fo far foftened as to yield to 

 the touch of a bent iron rod, the pyrometer was removed and 

 meafured. The fufibilities, thus obtained, in degrees of Wedg- 

 wood's fcale *, have been flated in a table, to which I would be 

 under flood always to refer. I have diftinguifhed the cryftal- 

 lized fubftances, obtained from the glaffes, by the name of 

 cryjlallite, a term fuggefted by Dr Hope. It may be obferved 

 in this table, that the original whins foften in a range from 38 

 to 5$ ; the glaffes from 15 to 24, and the artificial cryflallites 

 from 32 to 45. 



No. 1. Whin of Bell's Mills Quarry. 



This ftone was the fubject of all the foregoing experi- 

 ments, which were frequently repeated with fuccefs on a large 

 fcale. 



In trying the fufibility of the glafs obtained from it, a cu- 

 rious circumftance occurred, which accounts for the unexpected 



refults 



* The meafurement of the temperatures may be relied upon as accurate ; they 

 were determined by two fets of pieces, one purchafed by me during the lifetime 

 of the late Mr Wedgwood, and the other likewife made by him, belonging to 

 Dr Kennedy. The two fets correfpond exactly; and Dr Kennedy's had, at his 

 requeft, been carefully examined by the prefent Mr Wedgwood, who found them 

 true by his father's original ilandard. 



