26 INFRA-RED REFLECTION SPECTRA. 



observed with accuracy. In the case of the silicates (as compared with 

 quartz), where the height of the reflection band at 8 to 10 p is always 

 considerably less than 50 per cent, after three reflections the galvanometer 

 deflections would be only 1 or 2 mm. at 18 to 20//. It does not follow, 

 therefore, because no residual rays of a substance are to be found in the 

 region of 18 to 20 p, as was the case in the present examination, that no 

 bands exist, but that they are too weak to be measured. From the simi- 

 larity of the spectra, throughout the infra-red, of great groups of chemically 

 related compounds, and from the fact that quartz and mica have bands of 

 residual rays in the region of 18 to 20 p, it is to be assumed that the sili- 

 cates, in general, are selectively reflecting in this region. The fact that 

 no bands were found is to be attributed to the weakness of the reflection 

 bands. 



As a whole, however, the reflecting power of some of these bands is 

 very high. One has really no conception of the state of affairs until he 

 examines a substance like quartz. The phenomenon is so easily observed 

 with quartz that it might well serve as a general laboratory experiment. 

 In regard to the ease of observing in the infra-red, the writer's experience 

 has extended from the optical region into the most remote infra-red, and 

 it may be said that more difficulty was experienced in the region of 12 to 

 15 p, on account of the absorption of the rock-salt prism, than in the 

 region of greater wave-lengths. The actual intensity in the emission 

 spectrum differs greatly throughout the spectrum. For example, Rubens 

 and Aschkinass (loc. cit.) compute that for a temperature of 2000 the 

 maximum emission at 1.5 p is 800,000 times as great as at 60 p. 



APPARATUS AND METHODS. 



In the present examination the usual methods of procedure were 

 employed. The spectrometer was the one used in previous work. The 

 spectrum was produced by means of a wire grating G, fig. 18. The grating 

 was made by the well-known method of winding two copper wires of the 

 same diameter on a brass frame. One wire was then unwound and the 

 remaining one was fastened to the frame by means of an electrolytic deposit 

 of copper. The strands were then cut from one side of the frame. The 

 wire was not of uniform thickness, so that the grating was far from perfect. 

 Such a grating has the well-known property of producing only the odd 

 order of spectra. On account of its imperfections it was not possible to 

 make accurate measurements on orders higher than the seventh, using a 

 Bunsen sodium flame. Two gratings were employed, the one (No. 2) 

 having a constant of .=0.2120 mm., for the other = 0.3279 mm., 

 determined with the sodium flame. The coarser grating was the more 

 uniform in its individual windings, but in one region the wires were more 

 closely but very regularly wound. The magnesite band at 29.4 p was 

 apparently double, at least very wide, just as though this grating spectrum 



