ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 347 
of vibration of the molecules, with the result that it absorbs rays which are 
within the limits of visibility of the human eye. Instances of chromo- 
phores are afforded by oxygen and nitrogen, as in hydroxyl and nitroxyl. 
Or more rarely in the case of oxygen, when united to carbon, as in uric 
acid. Two benzene molecules doubly linked by two nitrogen atoms 
—N=WN-—as in azo-benzene, have their mode of vibration so modified that 
a colour as low down in the scale as the yellow rays is the result. These 
nitrogens are chromophores. Again, when two atoms of hydrogen in 
benzene are replaced by two atoms of oxygen as in quinone, a golden colour 
results. In this case the oxygen atoms are chromophores, “The chromo- 
phoric properties of oxygen atoms when united in a certain manner to 
carbon, and especially to benzene nuclei, are strikingly illustrated by such 
bodies as resorcin-phthalein, ce. 
When methyl is substituted for hydrogen in benzene, three of the 
absorption bands are merged in one, and the oscillation frequencies of the 
rays absorbed are reduced, or in other words the general absorption is 
increased. The OH, NH,, and NO, radicals invariably act in this manner 
whether absorption bands are shifted or not. 
Triphenylmethane, 
The absorption curve of this substance has the same general character as 
that of benzene, with the following modifications. There is one broad 
absorption band with just an indication of a second one being merged in 
this. The amplitude of the vibrations, as shown by the intensity of the 
absorption bands, is very largely increased, and the rate of vibration of the 
absorbed rays is very greatly reduced. A milligram molecule of benzene, 
for instance, begins to transmit rays with oscillation frequencies 3760 and 
4330, while ,3;th of a milligram molecule transmits all rays as far as 
4650. With triphenylmethane a milligram molecule transmits nothing 
beyond H or oscillation frequency 2510 ; ,,',;th of a molecule transmits 
rays as far as 4600. 
Rosaniline base, 
/C,H,NH, 
C(OH)—C,H,NH, 
XC sH,(CH,)NH, 
Here, the introduction of the methyl and amide groups to form rosaniline 
causes the molecule to absorb allrays beyond C or !/A 1250 ; ;3,,th causes 
an absorption near D!/) 1670, extending to near H !/\ 2450, while ,3;th 
of a molecule transmits all rays to 1/A 4600, 
Aurin, 
HO-C,H,.\ of | CoH 
HO-C,H, i 
The introduction of the hydroxyls causes greater absorption than the 
introduction of the amide groups. };th of a molecule absorbs everything 
beyond C !/X 1520; while =1,th absorbs as far as D!/X 1670. goth 
absorbs from !/\ 1780 to between Q and R or } RS \ 3100; and +,),5th 
practically transmits the whole spectrum. 
