130 INFRA-RED EMISSION SPECTRA. 



trum is due to the presence of a large number of free electrons, this is to 

 be expected. At low temperatures the electrical conductivity is small, and 

 the emissivity is confined to particular bands caused by certain groups of 

 electrons. With rise in temperature more electrons, extending over a wider 

 range of wave-lengths, are excited to activity and the separate emission 

 bands become merged into a continuous spectrum. This is not always 

 true, however, in the present work. For example, the sharp emission band 

 of zirconium oxide at 4.3 tx seems to retain the same intensity, superposed 

 upon a continuous background of increasing intensity, irrespective of the 

 temperature. In fact, many of the emission bands are as sharp as those 

 found in gases. 



Many of these oxides have an emission band in common in the region 

 of 2.85 fi, from which it would appear that this band may be due to the 

 oxygen atom which is in common with all of them. Furthermore, the 

 emission spectra, when smooth, generally show a depression at 3.2 ll, for 

 which no satisfactory explanation has been found. There are no atmos- 

 pheric absorption bands in this region, and fluorite is not known (see 

 fig. 29) to have absorption bands at this point (Paschen's absorption curve 

 for a 4 mm. plate shows an increase in absorption of 2 per cent from 2.75 

 to 3.2 ll, maximum at about 3.1 ll). The emission spectrum of platinum, 

 carefully examined at the same time, showed no depression. The emission 

 curve of the bare "heater-tube," which consists of a porcelain tube wound 

 with fine platinum wire, likewise gave a smooth emission curve similar to 

 that of the platinum strip. On the whole, the present data indicate that 

 the depression is a characteristic of the oxides. In other words, the oxides 

 have a small (sometimes large) characteristic emission band at A=2.8 to 

 A=3 ll and a second group of bands at 4.5 to 5 ll. If this be true, then it 

 will be necessary to assign the cause of the selective emission to the element 

 common to all the oxides, viz, to oxygen. It is well known that groups of 

 atoms have characteristic bands; but, heretofore, no data has been at 

 hand which, in any way, indicated the possibility of the characteristic 

 bands being due to a particular atom in the group. 



The tentative conclusion that these emission bands in the oxides are 

 due to oxygen atoms is perhaps to be expected, for the presence of traces 

 of the universal impurity, silica, will not explain matters. The oxides are 

 the most important and the most stable of all the important groups of 

 chemical compounds; and the spectra of substances belonging to a particu- 

 lar group are similar. But few substances are inert to the action of oxy- 

 gen. The spectrum of oxygen (see Carnegie Publication No. 35, p. 49) 

 shows absorption bands at 3.2 and 4.75 ll. The emission spectra of CO 

 and C0 2 show bands in the region of 2.7 and 4.75 ll (Carnegie Publication 

 No. 35, p. 313). Oxygen in a vacuum-tube showed a strong emission band 

 at 4.75 li, which, at the time the research was made, was ascribed to CO 

 or C0 2 supposed to be formed by the electrical discharge. In view of the 



