144 PHILOSOPHICAL TRANSACTIONS. [aNNO l/SS. 



occasion to think attentively on this subject, and lead them to ask how a body so 

 variable, and liable to such opposite changes from different degrees of heat, can 

 yet be a just measure of tho*e degrees. The changes which take place in all the 

 natural clays that have come under my examination are 6. 



1 . The first is, the shrinking of the moist clay in drying, from the mere loss 

 of its water. The purer the clay is, the more water it requires to soften it, and 

 V the more it diminishes in bulk by the loss of that water. 2. The dry clay, gra- 

 dually heated, preserves its bulk unvaried up to the approach of ignition. At 

 this period it is enlarged a little ; probably, as already observed, from its com- 

 bined air endeavouring to escape. 3. When this air has made its escape, the 

 clay begins to diminish, or to lose the bulk it had before acquired ; and returns 

 back, sooner or later, to the same dimensions which it was of when dry. It is 

 at this point that the thermometric diminution commences. 4. From this 

 point the clay continues to diminish more and more in proportion as the heat is 

 increased. This I call the thermometric stage of diminution : it is of greater or 

 less extent, terminating at different periods of heat, according to the nature of 

 the clay : in the standard thermometer clay, it commences with visible ignition, 

 and continues so doubtless far beyond the extreme heats of our furnaces, an in- 

 terval consisting of 1 6o degrees of the scale : in others, it begins 4, 6, and in 

 some even 15 of those degrees later, and terminates also much sooner : and in 

 some its whole extent is not above 20 of the same degrees. Throughout the 

 greatest part of this stage, the clays are found to retain their property of stick- 

 ing to the tongue and imbibing water : between this bibulous state and the 

 vitrescent there is an intermediate one, distinguished by the name of porcelain ; 

 and to the higher term of this porcelain state the stage of thermometric dimi- 

 nution seems to continue. 5. When the clay has passed the porcelain state, it 

 begins to be enlarged again, a symptom of the vitrescent stage being com- 

 menced ; and in this period it swells more or less, according to the nature of its 

 composition. 6. By further heat the swelled mass, becoming fluid, subsides, is 

 converted into glass or slag, and contracted into less volume than the clay occu- 

 pied in any of its preceding states. 



It is plain therefore, that clay can be a measure of heat no further than from 

 ignition, or that point beyond ignition where the 3d stage terminates, to the 

 beginning of the vitrescent stage ; and that, as the first 3 changes are com- 

 pletely passed before the clay is applied to thermometric purposes, being strictly 

 no other than preparatory processes, the thermometer pieces, whatever clay they 

 may be made of, provided it is sufficiently unvitrescible, are to be considered as 

 possessing only the 4th stage. But a singular property of the composition of 

 day and alum earth remains to be mentioned, viz. that it has really no other 

 than this one stage : it suffers no enlargement of its bulk at ignition, or in any 



