VARIOUS SOLIDS. 95 



The resistance placed in the primary circuit of the transformer to regulate 

 the current, and the low capacity of the transformer, acted as a "ballast" 

 to the radiating substances. The rods were provided with platinum ter- 

 minals, sealed in the ends, and were securely mounted in incandescent 

 lamp sockets. After heating them until they became conducting, they 

 were mounted securely before the spectrometer slit. The substances that 

 could not be melted and formed into rods were made into a paste and 

 spread upon the "heater tube" of a Nernst lamp. The "heater tube" 

 consisted of a hollow porcelain tube about 5 cm. long and 8 mm. diameter 

 covered with a coil of fine platinum wire, and was used in preference to a 

 platinum strip on account of its rigidity and ease in handling. It gave 

 the same results as the same material on a platinum strip. The spectrom- 

 eter, the fluorite prism, and the bolometer were previously used in ex- 

 amining the Nernst glower. It is important to notice, however, that by 

 inclosing the optical parts of the instrument it was possible to entirely 

 eliminate the atmospheric absorption bands, which had not been done 

 successfully by previous experimenters. The emission curves are, there- 

 fore, within experimental errors, an exact portrayal of the distribution of 

 the energy emitted in different parts of the spectrum. Unfortunately it is 

 not possible to determine the temperature of the radiating body, the thick- 

 ness of which would have to be specified, in many cases. A thermocouple 

 can not be applied on account of the loss of heat by conduction. It is, of 

 course, absurd to attempt to measure temperatures with an optical py- 

 rometer. For example, the rod of oligoclase was a perfectly transparent 

 glass, and emitted no light other than that due to sparking of the platinum 

 terminals, although at a temperature of 11 00 to 1200 C. Nevertheless, a 

 substance such as iron oxide would have emitted light when at the same 

 temperature, while both emit strongly in the infra-red. A further example 

 is the rod of topaz, which was an opaque white mass. On withdrawing it 

 from the oxy-hydrogen flame, it was accidentally stroked with the iron 

 forceps, when the parts so stroked emitted a dull red light, due to the 

 greater emissivity of the iron oxide, while the untouched parts retained 

 their usual white color. 



Instead of temperatures, the energy consumption is given; also the 

 dimensions of the rods, the lengths being the distances between the platinum 

 terminals. The diameters are in some cases not very accurate, owing to 

 the irregularity of some of the filaments. The voltage was measured with 

 an electrostatic voltmeter, joined to the terminals of the rod. The current 

 was measured with a milliammeter, of a suitable range to insure accuracy. 



It is manifestly impossible to have the smoothness of the surface and 

 the thickness of the radiating layer the same for all substances. The 

 distance (about 2 cm.) of the radiator from the slit and the bolometer 

 sensibility will also vary. No attempt has therefore been made to obtain 

 the emission curves of the various substances at the same temperature, 



