September 17, 1903] 



NATURE 



475 



spectra : First, those of substances the rays of which are 

 freely transmitted, the absorption being at the more re- 

 frangible end of the spectrum, and the spectrum of which 

 is readily increased in length by dilution ; secondly, those 

 in which the spectra are of the same kind, but the absorptive 

 power is greater, so that they withstand dilution to a much 

 greater extent ; thirdly, those spectra which exhibit selective 

 absorption, and which at the same time exert great absorp- 

 tive power, or, in other words, can undergo great dilution 

 before the absorption bands are rendered visible, and still 

 further dilution before they are extinguished or obliterated. 



Spectra of the First Variety belong to substances which 

 are constructed on an open chain of carbon atoms, thus : 

 CCCCC or C=C.CC.C and C = CC.C.C. 



The introduction in place of one or more atoms of 

 hvdrogen — of hvdroxyl, OH, carboxyl, COOH, methoxyl, 

 dCHj, CO, COH, or NH,, or of side chains such as CH,, 

 CjHj, &c. — does not affect the character of the spectra, but 

 merely the absorptive power, which is increased when 

 oxygen or an oxygenated radical is introduced. 



Spectra of the Second Variety are spectra of substances 

 so constituted that the carbon atoms form a closed chain. 

 It is immaterial whether the closed chains are homocyclic 

 or heterocyclic ; thus : — 



They possess greater absorptive power than open-chain 

 compounds, but do not exhibit absorption bands. It is 

 manifestly the chain or ring structure of the compounds 

 that gives them greater absorptive power, and not the 

 number of carbon atoms in the molecules. 



Spectra of the Third Variety. — These show absorption 

 bands, and the substances yielding them are generally con- 

 stituted on the type of benzene, naphthalene, anthracene, 

 phenanthrene, &c. ; but the rings may be either homocyclic 

 or heterocyclic without the character of the spectra being 

 altered ; thus : — 



C C 



/\ /|\ 

 C C C\. j/C 



II i I ^ '^ I 



II • I I _^ \^ I 



C C C^ ^C 

 C C 



Beniene Benzene Naphthalene Pyridin* Quinoline Pyrazine 



If we say that the compounds which are homocyclic are 

 constituted of at least three pairs of carbon atoms doubly 

 linked, which are themselves singly linked together, we 

 may make use of the formula of Kekuld for benzene as the 

 simplest e.xpression of their constitution ; if we assume that 

 each of the six atoms is linked to at least other two atoms 

 we adopt what is practically the prism formula of Laden- 

 burg, or the same idea expressed in space of two 

 dimensions. It is difficult to express the physical condition 

 by the Armstrong-Baeyer formula or centric arrangement 

 because this does not clearly suggest to one's mind what 

 is manifestly the fact, namely, that the carbon atoms in 

 the nucleus of benzene are much more 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 hav* 

 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 direct attention 

 to here is, that for the first time Kekul6's remarkable 

 benzene theory was supported by definite physical measure- 

 ments, 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 

 measurements of spectra. Johnstone Stoney and Emerson 

 Reynolds subsequently examined the extraordinary absorp- 

 tion 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 vibr.ating 

 synchronously with a radiation, the ray received on this 

 siabstance 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 trans- 

 mitted. If, however, certain carbon atoms within the 

 molecule are vibrating synchronously with certain rays, we 

 shall have selective absorption of these rays after the general 

 absorption has been so weakened by dilution or otherwise 

 as to allow them to pass. 



It is evident, then, that general 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 showrt that the rapidity of the intra- 

 molecular vibrations was dependent upon the rate of vibra- 

 tion 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 vibra- 

 tions within the rnolecules was calculated (iS8i), the 

 numbers being as follows : — 



• Molecular 

 Vibr.itions 

 1248" 



Benzene 



Naphthalene 



Anthracene 



"77 

 .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. It 

 follows from this that the vibrations within the molecules 

 are not independent, but are a consequertce, of the fuxjda- 

 mental molecular vibrations, like the harmonics 'of a 

 stretched string or of, a bell. 



The term absorptive power has generally been used with 

 respect to the extent of rays of the spectrum absorbed; hut 

 there is intensity of absorption to be considered. In the 

 case of a vibrating string or tuning-fork greater amplitude 



NO. 1768, VOL. 68] 



