ON ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 



221 



A similar series of experiments was made on substitution in the ineta- 

 position, the results being as follows : — 



1. 120 3. 122 



2, 136 4. 131 



displacement + 2 

 - 2 



substitution of CH3 

 CH, 



Conclusions. — a. When a tertiary nitrogen is replaced by a primary 

 nitrogen, the minimum of transparency (maximum of absorption) is dis- 

 placed towards the more refrangible end of the spectrum, whatever the 

 solvent may be — alcohol, ether, carbon disulpliide, or chloroform. 



It is remarked that this law differs entirely from that indicated by 

 M. Lemoult, who studied the apparent displacement of the band of red 

 rays transmitted by indophenols. 



b. Substitution in the or</io-position in the phenol from which the 

 indophenol is derived causes a considerable displacement of the minimum 

 of transparency (maximum of absorption), whatever the solvent may be. 

 This displacement may even exceed the foregoing in degree. The im- 

 portance of substitution is thus evident ; the auxochromic groups are not 

 the only ones to modify the nature of the dye. 



c. A substitution in the meto-position in the phenol from which the 

 indophenol is derived causes a -yery slight displacement of the minimum 

 of transparency towards the red or towards the blue ; the shifting is 

 often so slight as not to exceed experimental errors in measurement. 



The preceding two laws, the authors remark, enable the formula of a 

 phenol to be determined ; it is converted into the indophenol with a 

 primary or a tertiary nitrogen, and the alcoholic solution is then examined. 

 Only an extremely small quantity of the substance is required. 



Note. — Hartley has shown ^ the relationship of the absorption spectra 

 of benzene and triphenylmethane to the colouring matters derived there- 

 from by means of curves of molecular vibrations. 



No matter what their colour may be, a band of red rays is transmitted 

 with greater persistency than the rays in any other part of the spectrum, 

 and that these red rays are materially modified by the introduction of 

 alkyl radicals into the NH.2 groups of the rosaniline molecule, as in 

 methyl- violet^ and they are more modified by the presence of iodine, as in 

 iodine green. 



To illustrate this the following measurements of the transmitted red 

 rays in solutions at different dilutions and of different thicknesses are 

 stated both in wave-lengths and oscillation frequencies. The fiducial 

 lines in the solar spectrum are also given as useful for reference. 



' Chevi. Soc, Trans., vol. li. 1887, p. 152 ; see also the report of this Committee, 

 1899, p. 31. 



