M. V. Regnault on an Air -Thermometer . 537 



tation of the smelter would be so much increased by the higher 

 degree of purity of the selected lead. It is to be feared that, in 

 the attempt to get the utmost out of the ores at the "first " 

 operation, they have often deteriorated from the quality of metal 

 produced in former times. 



LXXIII. On an Air -Thermometer used as a Pyrometer in mea- 

 suring High Temperatures. By M. V. Regnault*. 



[With a Plate.J 



THE pyrometers which from time to time have been proposed 

 for the measurement of high temperatures in industrial 

 furnaces, have not hitherto received any important application. 

 Those which depend upon the expansion or on the increase in elastic 

 force of air confined in a closed vessel, are apparatus both difficult 

 and expensive to construct, and can only be used by observers 

 well practised in delicate manipulations. 



The pyrometers whose indications are based on the apparent 

 expansions of two metals, or on that of a metal as compared with 

 a rod of baked clay or porcelain, which is supposed to be unalter- 

 able, could in no case be used other than as pyroscopes, to see if 

 the same furnace has been raised to the same temperature in suc- 

 cessive operations. But it is difficult to graduate these instru- 

 ments, and even to compare them with a normal instrument so 

 as to be able to transform their indications into degrees of our 

 thermometric scale. Lastly, the same instrument undergoes, 

 when exposed to high temperatures, permanent alterations which 

 affect the scale and prevent any comparison of its indications. 



Wedgewood's pyrometer, which depends on the contractions 

 which the same clay undergoes at different temperatures, can also 

 only be used as a pyroscope ; but it is still more defective in 

 principle. The contraction which the same clay undergoes for 

 the same increase of temperature depends on the degree of com- 

 pression to which it has been exposed in the crude state, on the 

 more or less rapid increase of temperature, on the more or less 

 prolonged action of heat. 



I proposed in 1846 (Memoir es de V Academie des Sciences , 

 vol. xxi. p. 267) an apparatus easy of manipulation, by which the 

 temperature of any part of a furnace may at any given moment 

 be obtained with sufficient accuracy. The apparatus consists of 

 a kind of flask A, fig. 5, Plate II., either cylindrical or spherical, 

 of from J to I litre in capacity, and which may be either of cast 

 or wrought iron, of platinum or of porcelain ; the mouth a b is 

 closed by a plate cd containing an aperture o. From 15 to 20 



* Translated from the Annuks de Chimie et de Physique, September 1861. 



