VOL. LXXVt.] PHILOSOPHICAL TRANSACTIONS. 143 



tenacious of water, insomuch that, though apparently dry, the water and air 

 amount to near as much as the pure earth, and are not to be completely driven 

 out without a full red heat. When divided by the admixture of other earthy 

 bodies, it parts with its water easier indeed than before ; but a mixture contain- 

 ing so much of it as the thermometric composition does, is far more retentive of 

 water than common clay, and requires to be kept for some time in a heat equal 

 to that of boiling water, before it is to be considered as dry, that is, before the 

 adjustment of the pieces in the gage. If they are adjusted when only apparently 

 dry, or of such a degree of dryness as they can be brought to by a heat that the 

 hand can bear, the heat of boiling water will diminish them 2 or 3 degrees ; and 

 the greatest part of what they have thus been deprived of, they gradually recover 

 again on being exposed to the atmosphere, so that the adjustment must be made 

 immediately after the boiling heat. 



By the same expedient to which I have thus been obliged to have recourse for 

 procuring to the porcelain clay of Cornwall the standard degree of diminution, 

 and resistance to fire, the same qualities may probably be communicated to any 

 other clay that is tolerably pure from calcareous earth and iron ; so that the 

 thermometer clay is no longer to be considered as the produce of any particular 

 spot (which was the principal inconvenience originally imagined to attend it,) but 

 may be procured and prepared in all parts of the world where good common clay, 

 and alum, are to be found ; and corresponding thermometers may consequently 

 be constructed, without any standard lo copy from. For, if a converging canal 

 be formed, of any convenient length, with the widths at the two ends in the 

 proportion of 5 to 3, with the sides perfectly straight, and divided into 240 

 equal parts, numbering the divisions from the wider end ; * — and if a clay be ob- 

 tained of such quality, that when formed, in the manner already mentioned, 

 into pieces of such size as to enter to O in the gage or canal, these pieces shall 

 just begin to diminish, or go a little further in the canal, by a heat visibly red ; 

 — go to 27, by the heat in which copper melts ; — about 90 by the welding heat 

 of iron ; about 1 60, by the greatest heat that can be produced with coked pit- 

 coal in a well-constructed common air-furnace, about 8 inches square, still con- 

 tinuing bibulous, so as to stick to the tongue : such gages, and pieces of such 

 clay, so adjusted, will always compose correspondent thermometers. 



Having mentioned occasionally several alternate periods of dilatation and eon- 

 traction in clay, it may be proper to state, and bring into one view, the whole 

 succession of changes which I have observed in this curious material ; as other- 

 wise they might create some confusion in the minds of those who have not had 



* Or the divisions on the side may be continued to 300 j and in that case, instead of the widths of 

 the two ends being in proportion of the odd numbers 5 and 3, the one will be just double to the 

 Other. — Orig. 



