514 Prof. E. C. C, Baly on 



it is obvious that the band will become very narrow at very 

 low temperatures. 



Although nothing is known about the ultra-violet absorp- 

 tion band of water, there are, on the other hand, several 

 substances of which the complete systems of absorption lines 

 in the characteristic ultra-violet band groups have been 

 measured with considerable accuracy. It may be pointed 

 out that the conception may in such cases be put to a more 

 rigid test than is possible in the case of the infra-red bands 

 calculated by Fraulein v. Bahr and by Eucken, since the 

 measurements of the fine-line absorption composing a single 

 band group in the ultra-violet are capable of far greater 

 accuracy than those comprising a band group in the 

 infra-red. 



We are indebted to Coblentz for a series of very accurate 

 measurements of the infra-red bands of a large number of 

 organic compounds between the limits of 1 /jl and 13 /x, and 

 one outstanding conclusion can be drawn from this work*. 

 The characteristic infra-red bands of a substance are also 

 found to be in the spectra of its derivatives. Thus benzene 

 possesses a very pronounced band at 3*26 /z, and this band is 

 also found in a considerable number of benzene compounds. 

 Again, the characteristic bands of water are shown by salts 

 with water of crystallization, and moreover at least one of 

 these is shown by substances containing the hydroxyl group. 

 These facts at once suggest that in the spectrum of a com- 

 pound there is an additive function of the spectra of its 

 constituents. For example, in the case of phenol the cha- 

 racteristic basis constants of both water and benzene are 

 functionally active and their values are not altered in the 

 compound. It is only in this way that it is possible to 

 explain the occurrence of the principal bands of the 

 constituents in the spectrum of a given compound. 



Before this question can be discussed it is necessary to 

 deal with the infra-red spectra of simple substances, and find 

 out whether the absorption bands can be expressed in terms 

 of multiples of one or more basis constants. 



Benzene may be taken as the first example, and in the 

 case of this substance there are twenty absorption bands 

 recorded by Coblentz between the limits 1 fj, and 13 /i. These 

 vary very much between themselves in intensity, and the 

 most pronounced bands are at 978, 6*75, 3*25, 2*48, 2*18, 

 and 1*68 jjl. Now if the wave numbers of all the infra-red 

 benzene bands be considered, it will at once be seen that 

 they are nearly all multiples of 4, and indeed it appeared at 



* Publications of the Carnegie Institution, Washington, No. 35 (1905). 



