HISTORICAL. 13 



violet is approximately proportional to the square root of the atomic 

 weight; and that this wave-length is independent of the state of aggre- 

 gation of the element, and is unchanged in solution. 



The most recent investigation in this region is that of Magini,^ who 

 examined some of the compounds already mentioned under the work 

 of Pauer/ as well as 12 isomers, such as resorcin, hydroquinon, the 

 oxybenzoic acids, and the pthalic acids. Like Pauer, he observed a 

 shift toward the longer wave-lengths when an amido or carboxyl group 

 is substituted for a hydroxyl group. For total absorption the arrange- 

 ment of isomers is mcta, ortho, para. Para-compounds have greater 

 effect in forming entirely different absorption spectra. He concludes 

 that the double bond causes the absorption in the ultra-violet, w4iile it 

 makes no dift'erence, in the infra-red, whether the bands are double or 

 single. The double bond can be thought of as changing the elasticity 

 and cohesion in the molecule in such a manner that the molecule will be 

 resonant with ultra-violet light. He finds that the effect of absorption 

 is greatest when the replaceable groups are joined to opposite carbon 

 atoms of the benzene ring, and also when the molecule is symmetrical. 

 Isomers having a double bond have dissimilar absorption spectra. As 

 a whole, he thinks the absorption is caused chiefly by the molecular 

 configuration on the one hand, and by the nature of the compound on 

 the other. 



TOTAL ABSORPTION TEMPERATURE. ETC. 



The change in diathermancy of liquids with temperature was investi- 

 gated by Dechant,^ who found that the transparency of mica did not 

 change for a rise of 120°, while the diathermancy of water decreased. 

 Konigsberger* found that, in solid selective absorbing media, a rise of 

 temperature shifts the absorption curve toward the long wave-lengths, 

 while for the metals the absorption is constant. The work of Hagen 

 and Rubens^ on the metals, in which a film so thin that it transmits light 

 but is uniformly opaque in the infra-red, can not be discussed here. 

 Neither the work of Aschkinass and Schafer,® v/ho determined the 

 dielectric constants of several compounds for electrical waves, and 

 found that, with increase in molecular weight, the maximum of reso- 

 nance shifted toward the shorter resonators, which, if properly inter- 



'Magini : Phys. Zeit., 5, p. 69, and p. 145, 1904. 



^Pauer, loc. cit. 



^Dechant : Wien Ber., in, p. 264, 1902 ; Beiblatter, 27, i, 1903. 



*Konigsberger : Ann. der Physik (4), 4, p. 796, 1901. 



^Hagen & Rubens : Ann. der Physik (4), 8, p. 432, 1902. 



^Aschkinass & Schafer : Ann. der Physik (4), 5, p. 489, 1901. 



