28 MEASUREMENT OF HIGH TEMPERATURES. 
early as 1700. . Guyton-Morveau, 1 in whose thermal researches the dila- 
tation of solids and the specific heat of platinum were discussed with 
reference to their availability in thermal measurements, also proposed 
gases for that purpose. Prinsep, 2 however, appears to have been the 
first to construct an air thermometer and to apply it as an instrument 
of research. Prinsep's bulb was of gold. This was in pneumatic con- 
nection with a reservoir of olive-oil provided with a sensitive manome- 
ter. As the air in the bulb expanded it displaced the oil which exuded 
through a cock at the bottom of the reservoir. Pressure being main- 
tained constant the amount of olive-oil discharged is equal in bulk to 
the amount of air which enters the receiver at the given temperature. 
Hence by weighing the oil the temperature of the bulb may be calcu- 
lated. Prinsep's apparatus is unique, and his absolute thermal data 
are very much nearer the truth than those of his predecessors. Indeed 
they compare well with the known data of the present day. Prinsep's 
chief data refer to the melting points of alloys of gold, silver, and pla- 
tinum which bear his name. To these I shall recur. 
Leaving Davy, 3 who constructed an air thermometer in which the air 
expansion was weighed in mercury, and Mill 4 and Petersen, 5 to whom 
also forms of air thermometers are due, the next observer seems to be 
Pouillet. 6 Pouillet's researches are of prime importance. Having con- 
structed a bulb of platinum, which enabled him to reach the highest tem- 
peratures, he then took the first definite steps in radiation-pyronietry 
by investigating the temperature at which solids glow, in calorimetric 
pyrometry by determining the specific heat of platinum between 
0° and 1,200°, and in thermo-electric pyrometry by carefully calibrat- 
ing a thermo-couple of iron and platinum. As these apparatus will be 
referred to again, I need only remark here that to Pouillet the form of 
constant pressure manometer is due very nearly as it is to be used in 
pyrometric work to-day. This apparatus was perfected by Eegnault, 7 
ami afterwards accurately figured by Pouillet 8 himself. Some time 
after this Silbermanu and Jacquelin 9 described a platinum air-thermom- 
eter, of variable capacity, operating like a constant volume thermome- 
ter, but it does uot seem to have led to practical results. Erman and 
Herster, 10 in their endeavor to measure the amount of permanent ex- 
^oc. cit. 
2 Prinsep : Philos. Trans. Roy. Soc. Lond., 1827. Ann. ch. et phys., 2d series, vol. 
41, 1829, p. 247. 
3 Davy: Dingler's Jonr., vol. 46, 1832, p. 249. 
4 Mill : Gehler's Physikal. Worterbuch, 2d series, vol. 7, p. 997. 
6 Petersen: Ibid., pp. 993, 1004. 
6 Pouillet : Recherches sur les hautes temperatures et sur plusieurs pmSnomenes qui 
en dependent; Comptes Rendus, vol. 3, 1836, pp. 782-790. 
7 Regnault : Relation des Exp6riences, Paris, vol. 1, 1847, p. 163. 
8 Pouillet: Physique, vol. 1, 9th ed. 1853, p. 233. 
9 Silbermann et Jacquelin: Bull. Soc. d'Encouragement, 1853, p. 110; cf Becquerel, 
loc. cit. 
10 Erman and Herster : Pogg. Ann., vol. 97, 1856, p. 489. 
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