6 INFRA-RED ABSORPTION SPECTRA. 



between the one and the other, since such an agreement could not be 

 fortuitous." 



From 1889 to 1890 Angstrom,^ using a bolometer and rock-salt prism 

 and lenses, found the absorption of CO and COo. He also found the 

 absorption spectra of methane, ethylene, ether, benzene, and carbon 

 disulphide, several of the latter in the vapor and also the liquid state. 

 H is work extended to about 8 [x, and showed that the maxima of liquids 

 and their vapors are coincident. He found the absorption band of CO 

 at about 4.6 [x, and that of COo at about 4.3 fx, which would indicate that 

 the location of the maximum does not depend upon molecular weight. 



About this time Julius- investigated the absorption spectra of some 

 20 organic compounds. He used a bolometer, and a prism and lenses 

 of rock salt. His work extends to 10 /j., which by his straight-line 

 extrapolation from 5 /j, was supposed to be 16 /a. He used a cell hol- 

 lowed out of rock salt, about 0.2 mm. in thickness, and found that about 

 one-third of the substances, mostly alcohols, became opaque at 7 /x. He 

 found that all compounds containing the methyl, CH3, group had an 

 absorption band at 3.45 /x. The conclusion reached from this investiga- 

 tion, which had been the most extensive thus far, was that the absorp- 

 tion of heat waves is due to intramolecular movements ; in other words, 

 the internal structure, i. e., the grouping of the atoms in the chemical 

 molecule, determines the character of the absorption. He found that 

 the chemical atom lost its identity in a compound ; i. e., the effect is not 

 " additive," so that one can not foretell the absorption spectrum of the 

 compound from a knowledge of the spectra of the constituent elements. 

 This conclusion that the constituent atom loses its absorbing power 

 in a compound is further substantiated by the fact that of the six com- 

 pounds containing chlorine investigated by Julius not one showed the 

 CI band found by Angstrom^ at 4.28 /x. 



Donath,* in 1896, using a quartz prism and a bolometer, investigated 

 half a dozen compounds (mostly the essential oils), and concluded that 

 the absorption is intermolecular and not intramolecular, as found by 

 Julius. His work extended to 2.7 [x, and covered the region of " prac- 

 tical significance," as he expressed it. For the aromatic compounds 

 and the fatty oils he found maxima common at 1.69 /x and 2.2 /x. To my 

 mind there is not sufficient evidence to draw conclusions like the above. 

 However, after subsequent investigations by other observers on pure 



^Angstrom : Ofversigt Af Kongl. Vetenskaps-Akadem. Forhandl., Stockholm, S. 

 549, 1889; S. 549, 1889; S. 331, 1890. 



^Julius : Verhandl. Konikl. Akad. Amsterdam, Deel I, No. i, 1892. 

 ^Angstrom & Palmer: Ofversigt Kongl. Vet. Akad., No. 6, p. 389, 1893. 

 *Donath : Ann. der Physik (3), 58, p. 669, 1896. 



