80 INFRA-RED ABSORPTION SPECTRA. 



cell was i6 times as thick, so that no comparison can be made for total 

 absorption. According- to Magini/ the absorption of isomeric com- 

 pounds in the ultra-violet increases in the order meta, ortlio, para. The 

 present curves do not show this, except a possible indication at 4/*. 

 Mesitylene is isomeric with cumene. Their absorption spectra are en- 

 tirely different. 



VARIOUS OTHER BENZENE DERIVATIVES. 

 MonochlorbEnzene. CeHsCl. (Fig. 85.) 



This compound is of about the same transparency as benzene. How- 

 ever, the substitution of a CI atom for an H atom has wrought a 

 marked change in the general outline of the benzene curve. The lower 

 curve was made with a wide slit and a 0.19 mm. cell, hence serves only 

 for a g-eneral comparison with benzene. The upper curves were made 

 with a narrow slit, while the parts of curves were made with the large 

 spectrometer. They show that the original bands of benzene at 3.25 ju, 

 and 6.75 )u, still exist. There are new bands at 6.27 /* and at 6.94 /i. 

 The 6.27 ju, band is a characteristic of benzene derivatives, and strength- 

 ens the belief that it exists in benzene, since that curve has a slight 

 depression in this region. It adds weight to the belief expressed fre- 

 quently in this paper, that the substitution of a new element is not so 

 much the cause of new bands as it is the cause, in some manner, of a 

 greater freedom among the original benzene ions, whereby those bands 

 are intensified. 



Comparing same thicknesses of cells, the benzene curve has been 

 radically changed, which seems to show that the limited chemical rela- 

 tion of benzene to its derivatives is also to be found in the absorption 

 curves. 



MONOBROMBENZENE. CoHsBr. (Fig. 86.) 



In this substance we have a striking similarity to the preceding com- 

 pound. In fact, the curves for the same thickness of cell seem almost 

 superposable, excepting at 8 /a. The region of 5.7 /t shows great simi- 

 larity, as does the 6.9 /a, ii.I/x, and 12.2 /x band. The general trans- 

 mission seems to progress in two steps ; the first, of 75 per cent, extends 

 to 5 /x; the second, of 40 per cent, extends to 13 /x, where the substance 

 becomes opaque. The same facts are to be noted in CeHjjCl, and the 

 discussion of that substance applies to this one. It is unfortunate that 

 time did not permit a further examination of this compound, using a 

 larger dispersion. As in C0H5CI, the 3.25 /x band of benzene still exists. 



^Magini, loc. cit. 



