basus.] METHODS OF PYROMETRY. 43 
nally devised by Peclet, 1 Schiuz first made a number of measurements 
of the beat conductivity of the material composing the furnace walls, 
devising special apparatus for that purpose. Having duly tested the 
method, however, he abandoned it because of the irregularity of the 
conduction phenomenon within the walls and because of its want of 
sensitiveness as compared with electric methods. 
All the above methods have failed in practice. On the other hand, 
the circulating water-pyrometer due to Boulier 2 and others, in which 
the heat passing by conduction into the explorer or measuring part 
of the instrument is carried off by a current of water flowing between 
known levels, seems to be gaining in favor. The thermal estimate is' 
made by measuring the temperature of the water before entering and 
after leaving the furnace. The indications are, of course, wholly em- 
piric. In Boulier's compact and ingenious apparatus the explorer is a 
cylindrical box, with internal cylindrical partitions so adjusted as to 
secure a flow of water in cylindrical sheets. Water enters the outer 
compartment and leaves the inner, thus avoiding loss by radiation. 
According to Brown (loc. cit.) these apparatus, which are used with great 
success in connection with porcelain furnaces (Lauth), are due to Sain- 
tignon. Carnelley prefers a spiral explorer. 
Radiation. — I have mentioned that the first temperature scale pro- 
posed was that of Newton 3 (1701) derived immediately from his law of 
cooling. A piece of red hot iron was experimented upon. Long after 
this M'Sweeney 4 proposed to catch the heat radiated from a furnace by 
a concave mirror, at the focus of which he placed a thermometer. Fol- 
lowing close upon Govi's 5 photometric comparison of spectra, Becquerel 6 
published his large memoir on high temperature pyrometry. Using the 
red copper glass, he investigates an exponential relation in which the 
photometric intensity of red light is expressed in terms of the tempera- 
ture of the source of radiation. Green and blue glasses were also used. 
In addition to many results which must be omitted here, Becquerel 
proves that although all bodies have not the same power of radiation, 
truly opaque bodies like platinum, lime, magnesia, carbon, differ but 
little in this respect as far as the melting point of platinum. Oxdiz- 
able substances like iron and copper are not superficially opaque when 
covered by layers of oxide. Exterpolating by aid of his equation Bee-* 
querel finally concludes the 2,100° is probably the highest temperature 
electrically obtainable. The identity of emissive power accepted for 
1 Peclet : Trait6 element, de Physique, 4th ed\, vol. 1, 1847, p. 418. 
3 C. F. Amagat : C. R., vol. 97, 1883, p. 1053 ; Lauth : Bull. Soc.chimique Paris, n. s., 
vol.40, 1883, p. 108; Carnelley: Jour. Chem. Soc. London, vol. 45, 1884, p. 237 ; Lauth: 
Bull. Soc. chimique, Paris, vol. 46, 1886. p. 786, and others. 
3 Newton : Scala graduum caloris ; Philos. Trans., vol. 22, 1701, p. 824. 
4 M'Sweeny: Pogg. Ann., vol. 14, 1828, p. 531. 
6 Govi: C. R., vol. 50, 1860, p. 156. 
6 Becquerel: C. R., vol. 55, 1862, p. 826; Ann. ch. et phys., 3d series, vol. 68, 1863, 
p. 49. Also Draper: Fundamental researches, Philos. Mag., vol. 30, 1847, p. S45. 
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