TRANSACTIONS OF SECTION B. 589 



closely condensed or combined together than those of the hydroaromatic series. 

 This condensed condition of the carbon atoms is evident from the higher molecular 

 refractive energy of aromatic compounds and of the specific refractive energy of 

 the carbon in such combinations. 



Side chains such as do not exert selective absorption have no influence on the 

 character of the spectra, but they slightly increase the general absorption. 



Heterocyclic compounds possess greater absorptive power, both as regards the 

 general and selective absorption, than those which are homocyclic. 



The point which I particularly desire to draw attention to here is, that for the 

 first time Kekuld's remarkable benzene theory was supported by definite physical 

 measurements, and the closed-ring formula represented a veritable actuality. 



Of Molecular and Intra-molecular Vibrations. 



Johnstone Stoney was the first to show that the cause of the interrupted spectra 

 of gases is to be referred to the motions within the individual molecules, and not 

 to the irregular journeys or encounters of the molecules with each other ; and 

 this applies to the absorption as well as to emission spectra. He further advised 

 the use of oscillation frequencies instead of wave-lengths in describing the measure- 

 ments of spectra. Johnstone Stoney and Emerson Reynolds subsequently examined 

 the extraordinary absorption exhibited by chlorochromic anhydride, the bands in 

 which are evidently harmonically related. 



It has already been shown that the hydrocarbons of the aromatic series exert 

 two kinds of absorption, a general and a selective absorption. All the evidence 

 we possess warrants the belief that the general absorption is caused by the motion 

 of the molecules, while the selective absorption is due to the motion within the 

 molecules. 



When the molecule of a substance is capable of vibrating synchronously with 

 a radiation, the ray received on this substance is absorbed. The absorption is 

 complete if the direction of the vibration of the molecule and of the ray is the 

 same but the phase opposite, and if the number of molecules in the path of the 

 rays is sufficient to damp all the vibrations. 



When the quantity of substance in the path of the rays is reduced, the number 

 of molecules present is not sufficient to damp all the vibrations and some of the 

 rays are transmitted. If, however, certain carbon atoms within the molecule are 

 \'ibrating synchronously with certain rays, we shall have selective absorption of 

 these rays after the general absorption has been so weakened by dilution or other- 

 wise as to allow them to pass. 



It is evident, then, that general selective absorption exerted by carbon compounds 

 is due to the vibration of the molecules because the absorption increases with the 

 number of carbon atoms in the molecule ; or, in other words, in any homologous 

 series the greater the molecular mass the lower the rate of vibration of the molecule. 



It has not been found possible to associate any of the absorption bands of the 

 substances examined with any particular carbon atoms ; but the bands in benzene 

 are six in number, or the same in number as the carbon atoms. It has, however, 

 been shown that the rapidity of the intra-molecular vibrations was dependent 

 upon the rate of vibration of the molecules. From numbers representing approxi- 

 mately the mean wave-lengths of the four chief bands of rays absorbed by benzene, 

 naphthalene, and anthracene, and from the velocity of light, the mean rate of the 

 vibrations within the molecules was calculated (1881), the numbers being as 



follows : — 



Molecular 

 Vibrations, 



Benzene 1248'" 



Naphthalene 1177'° 



Anthracene ....;... 910'° 



The mean rate of vibration of the rays absorbed by naphthalene is less than 

 that absorbed by benzene, and those of anthracene less than those of naphthalene. 



