i40 PHILOSOPHICAL TRANSACTIOMS. [aNNO 1786. 



above stated, any single quantity of clay may be made up into thermometer- 

 pieces, that shall differ very little, if any thing at all, from each other. 



But a new difficulty now arose, more embarrassing than any of the former ; 

 that of procuring fresh supplies of clay, of the same thermometric quality with 

 the first. The quantity of the clay which,, after trial of many others, 1 had 

 made choice of, was small ; but the particular spot it was taken from being 

 known, and having purchased the little estate in which it was raised, I had not a 

 doubt of obtaining more of the same when it should be wanted : for clays in 

 general, when raised from an equal depth, in the same stratum, and near the 

 same place, are found to possess the same properties, with respect to ductility in 

 the hands of the workman, a disposition to assume by fire a porcelain or vitreous 

 texture, singly, or in composition, and all other qualities relative to their use in 

 pottery. In this, however, I was deceived ; for when a fresh supply was wanted, 

 to complete my experiments, though I had some taken from a pit joining to the 

 first, and at the same depth, it was found to diminish differently from the former 

 parcel. I then had some raised from different parts of the same field and bed, and at 

 different depths ; and in various other places in Cornwall, from the spot where 

 this species of clay is first met with to the Land's End ; but all these clays dif- 

 fered so much from the first in the quantity of their diminution by fire, and 

 most of them also from each other, that I despaired of being ever able to find 

 one that would correspond with it, or any natural clays that could be obtained, 

 twice of exactly the same thermometric properties, how similar soever in other 

 respects. 



On a review of the numerous comparisons made of these new clays, in different 

 degrees of heat, from the commencement of redness up to intense fire, the most 

 striking differences of the greatest part of them from the old seemed to ori- 

 ginate in the lower stages of heat ; and of those which were got from the 

 neighbourhood of the old, the variations from it in the higher stages seemed, 

 for the most part, to be only consequences of those differences in the lower 

 ones. I have mentioned, in the first paper, that the original thermometer 

 pieces had their bulk enlarged a little on the approach of ignition ; but that by 

 the time they became visibly red-hot throughout, they are reduced to their 

 former dimensions again ; and at this moment the thermometric diminution 

 begins. The new clays had their bulk enlarged in a much greater proportion, 

 and the enlargement was of much longer continuance : some of them required 

 a heat of 15 degrees to destroy the increase which ignition had produced in 

 their bulk, and bring them back to their original dimensions : after this period, 

 most of them diminished pretty regularly, and uniformly with the old, being 

 nearly so many degrees behind it, in all the succeeding stages of heat, as they 

 required to bring them back from the enlarged state. 



