Experiments on Whvifione and Lava, - 13 



were cryftallized, by being kept long in a ftationary temperature, between 28 and 30. 

 This laft operation was beft performed in a long and narrow muffle, wholly, furrounded 

 with burning coals, according to a pradice long followed by Dr. Kennedy, by which the 

 heat could be maintained with fo great fteadinefs as to render the refult almoft certain. 



The fufibilities were determined in an open muffle, in which a fragment of the fubftancc 

 under trial was placed contiguous to a pyrometer piece. As foon as the fragment, in con- 

 fequence 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 Wedgwood's fcale *, have been Hated in a table, to which I would be under- 

 ftood always to refer. I have diftinguilhed the cryftallized fubftances, obtained from the 

 glafles, by the name of cryjiallite, a term fuggefted by Dr. Hope. It may be obferved in 

 this table, that the original whins foften in a range from 38 to 55;^ the glafTes from 15 to 

 24, and the artificial cryftallites from 32 to 45. 



No. I. Whin of Bell's Mills ^tarry. 



This ftone was the fubje£t of all the foregoing experiments, which were frequently re - 

 peated with fuccefs on a large fcale. 



In trying the fufibility of the glaft obtained from it, a curious circumftance occurred, 

 which accounts for the unexpe£ted refults already mentioned. I had placed in the muffle 

 a long and flender fragment of this glafs, with its extremities refting on two fupports of 

 clay, and its middle unfupported. Having then increafed the temperature by flow degrees, 

 I expefted to difcover the loweft point of emollefcence, by obferving when the fragment 

 funk by its own weight. The muffle having attained a moderate heat, I obferved the glafs 

 to lofe its fhape a little. Wifliing to fee it completely melted, the fame heat was continued, 

 but no further change took place The heat was then raifed feveral degrees, but without 

 efFea. At laft, being urged ftill further, the glafs funk down completely between its fup- 

 ports. The pyrometer being then withdrawn, denoted a temperature above 30. 



It occurred to me, that, on this occafion, the glafs, by the firft application of heat, had 

 foftened, and then had cryftallized, fo as to become hard again; that, in cryftallizing, it 

 had acquired fuch infufibility as to yield to no heat under 30. I immediately confirmed 

 this conjeflure by the following experiment. 



A piece of the fame glafs, placed in a cup of clay, was introduced into the muffle, heated 

 to 21. In one minute it became quite foft, fo as to yield readily to the pieflure of an iron 

 rod. After a fecond minute had elapfed, the fragment, being touched by the rod, was 



• 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 exaaiy ; and Dr. Kennedy's had, at 

 hisrequeft, been carefully examined by the prefent Mr. Wedgwood, who found them true by his father's 

 original ftandard, 



found' 



