Day and Shepherd — Lime-Silica Series of Minerals. 287 



The Holborn-Kurlbaum pyrometer is a very simple tele- 

 scope consisting of two cheap lenses, in the focus of the first 

 of which (eye-piece) is placed a small incandescent lamp in 

 which the current and therefore the brightness of the filament 

 can be varied. The operation of measuring then consists 

 merely in focussing the telescope upon the hot body of which 

 the temperature is required and changing the current in the 

 filament until the latter can no longer be seen against the hot 

 object observed. The current then passing through the fila- 

 ment is a measure of the temperature. Monochromatic light 

 (preferably red) only is used. The calibration of this instru- 

 ment is arbitrary. It requires merely to be directed at a hot 

 object of which the temperature is known, and the current 

 observed. The relation between current and temperature for 

 several temperatures can then be elaborated into a curve for 

 purposes of interpolation or extrapolation. One condition 

 must not be overlooked in the use of such an instrument, — 

 the radiant energy sent out at a particular temperature is differ- 

 ent for different substances unless they are enclosed within a hol- 

 low body of uniform temperature, in which case all bodies radi- 

 ate alike and perfectly. Such a hollow body with a small open- 

 ing has been called by Kirchhoff a tk black body " or perfect 

 radiator, and the radiant energy passing through the opening, 

 black radiation. A fair working test for the " blackness " of 

 radiation within a furnace, for example, is obtained by observ- 

 ing whether objects within can be distinguished. When all 

 detail disappears within a furnace, its radiation is reasonably 

 black. This is approximately true at the center of nearly all 

 enclosed electrical resistance furnaces in which no combustion 

 is going on, but if the temperature is high it is usually not 

 entirely uniform even for small areas, and the radiation is con- 

 sequently not black. 



For purposes of comparing the thermal constants of dif- 

 ferent substances of unknown radiating power, therefore, we 

 first obtained small incandescent lamps which had been cali- 

 brated upon a black body at the Eeichsanstalt and verified the 

 calibration upon a similar body in our own laboratory for the 

 purpose of comparing standards. We did not deem it safe to 

 assume the approximate blackness of charges within the fur- 

 nace, although the conditions sometimes appeared sufficiently 

 good to warrant it. We took two trustworthy fixed points, 

 the melting temperature of anorthite (1532°) and platinum 

 (1720°), both being in the region in which we proposed to 

 apply the method, and observed the radiation from a tiny 

 fragment of iridium ribbon at those temperatures. In this 

 way we obtained two points slightly below the black body 

 curve and passed through these an empirical curve parallel to 



