400 Mr. Wedgwood’s additional Olfervatlom on 
tion, it was hoped that, by fuch a procefs, part of its com- 
bined air might be detached. 
But experiments made on this idea have proved, that thefe 
clays, kept moift for a twelvemonth, — kept for a confiderable 
length of time ill a heat juft below vifible rednefs, — boiled in 
tvater for many hours, — alternately, and repeatedly, moiftened 
and dried,— fuffer no alteration in their thermometric proper- 
ties, and continue to differ from the ftandard clay juft as 
much as they did at firft. 
Some of thefe new clays differed from the old in a property 
ftill more effential, and by which I was much more difconcerted ; 
for though they continued diminifhing with tolerable regu- 
larity, keeping only fome degrees behind it, up to a certain 
period of heat, about that in which call: iron melts ; yet many 
of the pieces, urged with a heat known to be greater than 
that, were found not to be diminilhed fo much as thofe which 
had fuffered only that lower heat. Further experiments 
fhewed, that, after diminilhing to a certain point, they begin, 
upon an increafe of the heat beyond that point, to fwell again : 
and as this effedl is conftant in certain clays, and begins earlieft 
in thofe which are moft vitrefcible, and as clays are found to 
fwell upon the approach of vitrification, I look upon this fe- 
cund enlargement of bulk, however inconfiderable, as a fure 
indication of the clay or compofition having gone beyond the 
true porcelain ftate, and of a difpofition taking place towards 
vitrification ; which ftage is always, fo far as my experience 
reaches, attended with a new extrication of air; and in fome 
inftances, this air being unable to make its efcape from the 
tenacious mafs that envelopes it, the burnt clay is thereby fo 
much increafed in bulk as to fwim on water like very light 
wood. The degree of heat therefore, at which this enlarge- 
ment 
