INVESTIGATION WITH A ROCK-SALT PRISM. 6/ 



that of Julius, which extends to about lo fi. Carbon disulphide is simi- 

 lar to carbon dioxide (fig. 22), but we find no such similarities in their 

 absorption spectra. 



THE MUSTARD OILS AND SULPHOCYANATES. 



Of all compounds studied, this group is the most conspicuous for 

 demonstrating the fact that absorption depends upon the arrangement 

 of the atoms in the molecule. The mustard oils are unique in having 

 an enormous absorption band in the region of short wave-lengths, this 

 side of 5 fi. In carbon disulphide the first strong band occurs at 6.7 /x. 

 Just as the fatty acids and terpenes were noticed to have similar absorp- 

 tion spectra, so the mustard oils have a characteristic group of absorp- 

 tion bands; and if we knew nothing of their chemical relations to the 

 sulphocyanates, their absorption spectra would lead to a classification 

 by themselves. The characteristic band of the mustard oils is at about 

 4.8 fx, which happens to be close to a somewhat less prominent one in 

 CS2 at at 4.6 (i. 



Methyl Sulphocyanate. CH3 — S — C=N. (Fig. 51.) 



In the discussion of the simple methyl (CH3) and ethyl (CoHg) com- 

 pounds it was noticed that a marked change was brought about in the 

 absorption spectra by adding a CN or NOo group to the methyl or ethyl 

 radical. In the present compound the substitution of the SCN for the 

 CN group is just as marked. The bands at 3.4 /x and 7 /x show some 

 similarity, but beyond this point we have an entirely new spectrum. 

 Usually the region from 4 to 5 /x shows no maxima, but in some of the 

 simpler compounds like this one, and in CH3I, there is quite a marked 

 absorption band. 



Methyl Mustard Oil (Methyl Isosulphocyanate). CH3— N=C=S. (Fig 51.) 



This compound is isomeric with methyl sulphocyanate, and the effect 

 upon the absorption spectrum is very marked. Unless we can show 

 that every compound has its own characteristic absorption spectrum, 

 just as is true of the elements, we have here an excellent demonstration 

 of the effect of structure of the molecule upon the transmission of heat- 

 waves. There is plenty of evidence against the belief of an individual 

 spectrum for each compound, as will be noticed in phenyl mustard oil 

 and other benzene derivatives, in which traces of the original benzene 

 spectrum are still present, and we must conclude that the effect observed 

 is due to structure. Whether it is due to a specific group of atoms like 

 CS or the manner of the bonding of these atoms, or whether it is due 

 to both, will be discussed elsewhere. It will be sufficient to notice that 



