HISTORICAL. 7 



compounds, the work is valuable in showing that the spectra of the 

 pure compounds were not seriously disturbed when they existed in the 

 impure state in the essential oils. 



Aschkinass/ using a flint-glass and a fluorite prism, and a mirror 

 spectrometer, investigated the absorption spectrum of water to 8 fi. He 

 found characteristic maxima located at o.yy fi, i.o fx, 1.25 /a, 1.50^1, 

 1.94 ju, 2.05 fi, 3.06 /x, 4.7 fi, and 6.1 fi. Paschen^ found the 3 fi band at 

 2.916 fi, 2.975 /*» and 3.024 ix, depending upon the thickness of the film, 

 while the second large maximum was found at 6.06 /t. This was fol- 

 lowed by the study of six alcohols, by Ransohoflf,^ his object being to 

 learn the effect of the OH-group. He found marked bands at 1.71 /x, 

 3.0 fi, and 3.43 fx, and tacitly concludes that since this 3.0 fi band agrees 

 with the one found by Aschkinass, it is a characteristic of the hydroxyl- 

 group. Although the alcohols were " chemically pure," that is a differ- 

 ent question from the one of having them " water free," which he does 

 not consider. This question will be considered later on. It is of 

 interest to note that the 1.71 /x band is harmonic with the one at 3.43 ^, 

 just as is true of the water bands at 3 /* and 6 fi. 



The deepest exploration ever made into the infra-red is that of Rubens 

 and Aschkinass,^ who, using a sylvite prism, found the absorption of 

 CO2 and water vapor to 20 /a. They found CO2 transparent except at 

 14.7 fi, while water vapor has a series of maxima throughout the whole 

 region. 



In 1899 and 1900 Puccianti^ explored a number of benzene deriva- 

 tives by means of a quartz prism, mirror spectrometer, and radiometer. 

 He found that all compounds, in the molecules of which carbon is com- 

 bined directly with hydrogen, presented a maximum at 1.71 fi, while all 

 the benzene derivatives have two other maxima in common at 2.18 ^u, 

 and 2.49 fi. He found his results in agreement with the " hypothesis 

 that the absorption depends upon the groups of atoms which exist in 

 the molecule." The isomeric xylenes exhibit absorption spectra almost 

 but not completely identical. 



A series of double refracting crystals have been investigated by 

 Konigsberger.^ He used a fluorite prism and bolometer. The observa- 

 tions of most interest here are that impurities changed the absorption 

 curve, but not the maxima, and that wafer of crystallisation, in corn- 



^Aschkinass ; Ann. der Physik (3), 55, p. 406, 1895. 

 ^Paschen : Ann. der Physik (3), 53, p. 336, 1894. 

 ^Ransohoff : Inaug. Diss. Berlin, 1896. 

 *Rubens & Aschkinass : Astrophys. Jour., 8, p. 181, i8g8. 

 ^Puccianti : Nuovo Cimento, 11, p. 241, 1900. 

 *K6nigsberger : Ann. der Physik (3), Gi, p. 687, 1897. 



