30 INFRA-RED ABSORPTION SPECTRA. 



present. As an illustration of the aforesaid, a series of '' petroleum 

 distillates " may be mentioned, of which 25 samples were obtained from 

 crude petroleum, while one, " hexane," was made synthetically. The 

 samples obtained by distillation have the same number of large absorp- 

 tion bands, while the synthetic product showed one new band, in addi- 

 tion to those common to the distillates. 



For the earlier part of the work, the samples of the compounds were 

 obtained from the m^useum of the department of chemistry. Many of 

 these had been obtained from Merck, while some were prepared by 

 students. Many of them were redistilled just before using. Out of 

 a total number of 70 compounds, 43 were imported directly from Kahl- 

 baum. Of this number, 23 were duplicates of the aforesaid compounds. 

 They came in sealed glass-stoppered bottles, in quantities of 5 to 10 

 grams, and were insured to be the purest obtainable. He omitted to 

 state the boiling points, however, as requested. With these small quan- 

 tities it was difficult to determine the boiling points with precision, but 

 of the samples tested every one had a boiling point so close to the theo- 

 retical value that its purity could not be questioned. 



In addition to the above compounds, a series of 25 distillates was 

 obtained from Prof. C. F. Mabery, of Case School of Applied Science 

 (see p. 73). The gases were manufactured in the laboratory, as indi- 

 cated elsewhere. 



It wall be noticed presently that water is the most opaque substance 

 investigated; hence it is of the greatest importance to have the com- 

 pounds free from water. The fact that ordinary alcohol blurred and 

 glycerin etched the rock-salt plates of the cell, while all the other com- 

 pounds did not, would indicate that the quantity of water in the latter 

 must have been exceedingly small. Thus, for the petroleum distillates, 

 of which a great number were examined in one cell, after completing 

 the work the interior of the cell was found as clear and highly polished 

 as when it was new. 



PROBLEMS ATTACKED. 



The problem before me was to determine the effect of molecular 

 weight upon absorption spectra ; also the effect of chemical structure, 

 i. e., the arrangement of the atoms in the molecule, and the effect pro- 

 duced by the substitution of a CH3 or OH group of atoms. 



As a criterion for the effect of the substitution of a CHg-group, the 

 conspicuous band occurring between the wave-lengths 3.0 fi and 3.5 fj. 

 Avas critically examined. Julius^ found this band at 3.45 /x for com- 



^Julius, loc. cit. 



