INVESTIGATION WITH A ROCK-SALT PRISM. 6l 



of this these two substances were examined on the same day, and the 

 apparent shift is simply due to the fact that the CH3CN band is com- 

 plex, with its second component at 7.25 fx. This is an excellent exam- 

 ple of what might have been considered a shifting of the absorption 

 band, with change in molecular weight, if a smaller dispersion had been 

 used. 



In C2H5SCN the CN part is joined to the S atom. The result is 

 a shifting or stretching apart of all the strong bands, beginning at 4.4 /* 

 and extending to 10.4 ;u,, where a transparent region occurs which 

 extends to 12,8 fi, just as in the CgHgCN. 



Methyi, Iodide. CHsI. (Fig. 36.) 



This compound gives a curve conspicuous for three regions of great 

 absorption, viz, 3.4 /x, 7.2 /a, and 11.4/n, while there are two transparent 

 regions, at 9.5 /* and 13 /x, where ethyl alcohol is opaque. At 10.5 /* 

 there is gentle decline in transmission, with a possible band at 10.8 /*, 

 and terminating in a large minimum at 1 1 .3 fi. Methyl iodide is quite 

 transparent, 60 per cent, if considered in height of the transmission 

 minima and thickness of absorbing cell, also if one were to consider the 

 total transmission. It will be noticed that these great absorption bands 

 lie in the region where the radiation from a black body is very weak, 

 while the 3.4 /* band is shallow, so that the great transparency of CH3I 

 observed by Friedel^ (using Tyndall's method of total absorption), as 

 compared with nitrogen compounds, is due apparently to the lack of 

 strong absorption bands in the region of strong radiation. That the 

 transparency of a medium depends upon the radiation employed is 

 shown in a striking manner in Drew's^ work on vacuum-tube radiation. 

 He used CSg as an absorbing medium, and found it as opaque as water, 

 due to the fact that the emission is concentrated in a strong band at 

 4.75 fi, which coincides with a strong absorption band of CS2 at 4.7 jit. 

 No absorption band is to be found at 7.3 [x, where solid iodine has a 

 large band. The 5.95 [x band is only slight. The 4.6/* band is to be 

 noticed in connection with the several of the simpler CH3 compounds 

 mentioned in the following pages. It is found in only a few compounds. 



Ethyl Iodide. C2H5I. (Fig. 37.) 



Like the preceding, this substance is quite transparent, so that a 0.16 

 mm. cell could be used except beyond 13 /x, where it is more opaque. 



The region from 6 to 12 ju, is conspicuous for its regions of great 

 transparency and of opacity. The ii.4ju, band of CH3I is absent, and 



ipriedel ; Ann. der Physik (3), 55, p. 453. 1895. 

 '^Drew : Phys. Rev., vol. xvii, p. 321, 1903. 



