4 o6 Mr. Wedgwood’s additional Obfervations on 
5. When the clay has paffed the porcelain ftate, it begins to 
be enlarged again, a fymptom of the vitrefcent ftage being 
commenced ; and in this period it fwells more or lefs, accord- 
ing to the nature of its compofition. 
6. By further heat the fvvelled mafs, becoming fluid, fubfides, 
is converted into glafs or flag, and contra died into lefs volume 
than the clay occupied in any of its preceding ftates. 
It is plain, therefore, that clay can be a meafure of heat na 
further than from ignition, or that point beyond ignition where 
the third ftage terminates, to the beginning of the vitrefcent 
ftage ; and that, as the three firft changes are completely palled 
before the clay is applied to thermornetric purpofes, being 
, ftriftly no other than preparatory procefles, the thermometer- 
pieces, whatever clay they maybe made of (provided it is fuffi- 
ciently unvitrefcible), are to be confidered as pofleffing only 
the fourth ftage. But a lingular property of the compaction of 
clay and alum earth remains to be mentioned, viz. that it has 
really no other than this one ftage : it fuffers no enlargement 
of its bulk at ignition, or in any other period ; but proceeds in 
one uninterrupted courfe of diminution, from the foft ftate ill 
which the pieces are formed, up to the extreme fires of our 
furnaces. Though the diminution, however, is uninterrupted*, 
it is at the fame time fo inconfiderable at the beginning, from 
the heat of boiling water (at which the pieces are adj ufted) up 
to ignition, that the fame point of vifible rednefs is taken for 
the commencement of the fcale, in this as in the original clay, 
without any fenfible error or variation in their progrefs. 
I am inclined to believe,, though experiments have not yet 
enabled me to fpeak with certainty on this point, that the fame 
caufe which enlarges the natural clays on their firft expofure to 
the fire, operates alio in this compofition r but in a much lower 
degree %. 
