VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 285 



case has been found to heat sooner than the matter in the larger vessel; but in 

 small ones, as the crucible and case are nearly alike in bulk, there is little danger 

 of error from this cause. 



10. These thermometer-pieces possess some singular properties, which we 

 could not have expected to find united in any substance whatever, and which 

 peculiarly fit them for the purposes they are here applied to. 1st. When 

 baked by only moderate degrees of fire, though they are, like other clays, of a 

 porous texture, and imbibe water; yet, when saturated with the water, their 

 bulk continues exactly the same as in a dry state. 2d. By very strong fire, they 

 are changed to a porcelain or semi-vitreous texture; yet their contraction, on 

 further augmentations of the heat, proceeds regularly as before, up to the 

 highest degree of fire that I have been able to produce. 3d. They bear sudden 

 alternatives of heat and cold; may be dropped at once into intense fire, and, 

 when they have received its heat, may be plunged as suddenly into cold water, 

 without the least injury from either. 4th. Even while saturated with water in 

 their porous state, they may be thrown immediately into a white heat, without 

 bursting or suffering any injury. 5th. Sudden cooling, which alters both the 

 bulk and texture of most bodies, does not at all affect these, at least not in any 

 quality subservient to their thermometric uses. 6th. Nor are they affected by 

 long continuance in, but solely by, the degree of heat they are exposed to. In 

 3 minutes or less, they are perfectly penetrated by the heat which acts on them, 

 so as to receive the full contraction which that degree of heat is capable of pro- 

 ducing, equally with those which had undergone its action during a gradual 

 increase of its force for many hours. Strong degrees of heat are communicated 

 to them with more celerity than weak ones: perhaps the heat may be more 

 readily transmitted, in proportion as the texture becomes more compact. 



These facts have been ascertained by many experiments, the particulars of 

 which are omitted, because they would swell this paper much beyond the bulk 

 intended. 



1 1 . The use and accuracy of this thermometer for measuring, after an opera- 

 tion, the degree of heat which the matter has undergone, will be apparent. 

 The foregoing properties afford means of measuring it also, easily and expedi- 

 tiously, during the operation, so that we may know when the fire is increased to 

 any degree previously determined on. The piece may be taken out of the fire 

 in any period of the process, and dropped immediately into water, so as to be fit 

 for measuring by the gage in a few seconds of time. At the same instant, 

 another piece may be introduced into the place of the former, to be taken out and 

 measured in its turn; and thus alternately, till the desired degree of heat is 

 obtained. But as the cold piece will be 2 or 3 minutes in receiving the full heat, 

 and corresponding contraction ; to avoid this loss of time it may be proper, on 



