320 INFRA-RED EMISSION SPECTRA. 



iron wire through which an electric current was passed. The walls of 

 the tube were heated to the temperature they generally had during the 

 electrical discharge at very low pressure. The tube was filled with air 

 and with COo, and the radiation was found for all pressures up to 

 atmospheric. The deflections were thrown entirely off the scale for 

 all pressures, for the region of 4.75 fi, hence this region of the spectra 

 was not explored for emission bands. This indicates an intensity of 

 radiation several times that from the vacuum-tube during the passage 

 of the discharge. Although the ends of the tube were not hot, when 

 heated externally, the tube is analogous to the black body. Whether 

 or not it is entirely analogous is not of the chief importance. Its radia- 

 tion was several times as intense as that from the electrically excited 

 tube, which was the question to be answered. Whether or not the emis- 

 sion band at 4.75 /i is due to thermal excitation, due merely to a rise in 

 temperature, or to electrical excitation, is undetermined. 



The experiment shows that since its intensity is less than that of a 

 black body, it is not untenable to consider it to be due a rise in tempera- 

 ture of the gas. 



But this is not a sufficient criterion for judging the quality of the radia- 

 tion. One objection is that the selective emission at 4.75 fi ceases im- 

 mediately after the electrical discharge ceases, which is not true of the 

 externally heated tube. Warburg (loc. cit.) has shown from theoretical 

 considerations that after the discharge ceases it requires only a small 

 fraction of a second for the gas to assume its original temperature. 



In this connection it is well to notice Paschen's' work on the emission 

 of COo, when heated in a metal tube, also when passed through a coil of 

 sheet platinum 4 cm. long, 3 mm. internal diameter, heated electrically. 



He found the radiation of the 4.4 /j. band of CO2 for columns 7 cm. 

 and 33 cm. long. The 7 cm. column behaved like a layer of infinite thick- 

 ness, i. e., the intensity of the emission of this wave-length is propor- 

 tional to the intensity of radiation of a black body for the same wave- 

 length and temperature. In figure 152, curve b represents the emission 

 of the 4.4 fi band of COg, for a tube 7 cm. long when heated by means 

 of a Bunsen burner from 100° to 500°. In this same figure curve e, also 

 due to Paschen, shows the emission of the 4.4 fi band of CO2 when 

 heated by passing the gas through a coil of platinum which was heated 

 electrically. 



The temperature of the gas was observed by means of two thermo- 

 piles, the first one being placed at the point where the gas issued from 

 the orifice ; the second one being placed about one centimeter above this 



^Paschen ; Ann. der Physik (3), 53, p. 26, li 



