CHAPTER II. 



SELECTIVE RADIATION FROM VARIOUS SOLIDS. 



INTRODUCTION. 



Our knowledge of the emission of radiant energy from various sub- 

 stances with change in temperature is extremely limited, being confined 

 to platinum in the case of metallic electrical conductors, to several gases 

 in vacuum tubes, to water-vapor and carbon dioxide in the Bunsen flame, 

 to carbon, to the oxides of copper and iron, and to the radiation from a 

 uniformly heated cavity or complete radiator. The emission spectra of the 

 Bunsen flame and of gases in vacuum tubes were found to be composed 

 of sharp emission bands superposed upon a weak continuous spectrum. 

 The solids were found to have smooth continuous emission spectra, and it 

 seems to be the general expectation to find (see Kayser's Spectroscopic, 

 vol. 2, pp. 135 and 284, also Rudorf, Jahrb. Radioakt. und Elektronik, 4, 

 385, 1908) that all solids emit continuous spectra. 



To Paschen is due the credit for the first systematic study of the spectral 

 distribution of radiant energy from various solids, and from the Bunsen 

 flame. Subsequent work by others has been but little more than the 

 establishment of the so-called radiation constants to a greater number of 

 significant figures than was possible by Paschen, with the facilities at his 

 disposal. Great credit is due to Lummer and Pringsheim for establishing 

 the limits within which the radiation laws, notably Wien's law, hold. It 

 must be said, however, that Paschen had these limits partly established; 

 but he insisted that the discrepancies between theory and experiment were 

 due to errors of observation. In this brief summary it is not possible to 

 present the subject fully, but after working over the data, one can not help 

 feeling that it is extremely unfortunate that the results of these investiga- 

 tions are beclouded with controversies as to whom belonged the credit for 

 doing or suggesting this, that, or some other thing in connection with the 

 work. 



The best proof of Kirchhoff's law of the proportionality of emission and 

 absorption is due to Paschen, 1 who found that the intensity of the emission 

 of the C0 2 band at 4.4 /*, when using a column of gas 7 cm. long, was as 

 great as for a column of gas ^ cm. long. In other words, the intensity 

 of the radiation was as great as that of a complete radiator for the same 



1 Paschen: Ann. der Phys. (3), 53i P- 26, 1894. 



93 



