HEAT OR CALORIC. 63 



(/) Wedgwood original pieces are not now attainable, at least 

 not the same that Mr. Wedgwood used, the bed of clay being ex- 

 hausted. 



Mr. Wedgwood connected his pyrometer with the common ther- 

 mometer, by the expansion of cylindrical pieces of silver measured 

 in a groove of earthern ware similar to his scale. 



Henry states that the greatest degree of heat observed was 1 85- 

 W. 25127Fahr. Appendix. 



(g.) Wedgwood? s pyrometer is the only one for measuring high fur- 

 nace heat. 



(h.) The highest degree of Wedgwood corresponds to 32.277 

 Fahr. 



(i.) The greatest range of observations made by Fahrenheit's ther- 

 mometer does not exceed the -^ part of that ascertained by Wedg- 

 wood. 



There is no measure for the highest heat ; Dr. Hare's compound 

 blow pipe readily melts all porcelains and other earthy compositions, 

 more refractory than Wedgwood's clay pieces. 



(j.) Artificial clay pieces may be made, but little dependence is 

 now placed upon these earthy compositions for pyrometers ; for, Sir J. 

 Hall has ascertained, that a mild heat long continued, has a similar 

 effect in causing them to contract, with a sudden and violent one. 

 Mr. Faraday* considers Daniell's pyrometer as the best.f 



II. DISTRIBUTION OF TEMPERATURE AND COMMUNICATION OF 

 HEAT. 



1. CALORIC CONSTANTLY TENDS TO AN EQUILIBRIUM. 



This tendency is never effectual on a great scale, because of the 

 operation of numerous disturbing causes ; the equilibrium is, to a 

 good degree, attainable in a limited and confined space, as in a close 

 room - y J in such a situation, a thousand bodies of different temperature 

 will ultimately assume nearly or quite the same temperature, and the 

 thermometer, when applied to them severally, ascertains the fact. 



(a.) Radiation and actual contact both contribute to the effect. 

 At high temperatures, radiation is the most effectual, but actual contact 

 is most efficient at low degrees of heat. 



(b.) Caloric radiates through a vacuum. 



Therefore a medium is not necessary to its transmission, a body in 

 a vacuum cools about half as fast as in the air. 



2. THE ATMOSPHERE IS VERY UNEQUALLY HEATED. 



* Chem. Manip. pa. 146. 



t See Quarterly Jour, of Sci. XI. 309. 



t Even in such circumstances there is generally a sensible difference between 

 the temperature of the floor and of the ceiling of the room. See Mr. Marcus Bull's 

 account of his experiments on the heat afforded by different kinds of fueL 



