44 INFRA-RED ABSORPTION SPECTRA. 



Acetylene. C2H:. 

 (Cell, 6.3 cm. ; barom., 74.5 cm. ; temp., 22°. 5 ; fig. 13.) 



The acetylene used was generated from " selected calcium carbide, 

 commercially pure, for bicycle lamps." 



In the preliminary examination the gas was washed in c. p. H2SO4 

 and dried in P2O5, curve a. For a subsequent examination the gas was 

 washed by passing it through a solution of 20 grams of HC1+ 10 grams 

 HgCla + So grams HgO to remove the phosphene (PH3) ; then through 

 a solution of KOH to remove HjS ; then through concentrated H2SO4 

 to dry it, and finally through a U-tube cooled in CO2 snow. In the 

 latter there was a slight deposit of vapor on the side where the gas 

 entered. This was found to be water vapor. After all this treatment, 

 as well as an additional one, to be mentioned presently, not a single 

 band was changed appreciably in transmission, nor in the position of its 

 transmission minima, curve h. That ordinary acetylene is quite pure 

 has been found by Rands,^ who found a purity of 99 per cent. 



Acetylene is distinguished for three deep, narrow regions of absorp- 

 tion at 3.08 II, 7.73 IX, and 13.63 jx, and two other bands, one at 3.7 fi, 

 which is harmonic with the one at 7.4 /x. In order to be able to locate 

 the band at 13.63 /u, the cell had to be exhausted to a pressure of 7 cm., 

 curve c. Elsewhere the gas is perfectly transparent. At first it seemed 

 that the band at 3.08 /x might be due to water, but washing it thoroughly 

 in new cone. H2SO4 and leaving it stand in contact with it for three 

 days, in a pipette, failed to alter the intensity or position of this trans- 

 mission minimum, curve d, which would indicate that this band belongs 

 to acetylene. 



As a further check, a thin film of water, curve h, between fluorite 

 plates, was examined. The first band occurs at 3.02 p., while the second 

 lies at 6.02 II. 



If this band at 3.08 /* in acetylene were due to water, one would expect 

 a similar one at 6.0 /x, but in this region the gas is perfectly transparent. 



It will be interesting to note the shifting of this band toward the infra- 

 red for an increase of two H atoms (keeping the number of C atoms 

 constant) in each of the following series of compounds. 



Acetylene brought to a red heat will polymerize, three molecules 

 uniting to form one molecule of benzene (CeHg), (Berthelot). This 

 is one of the most striking transitions from the aliphatic to the aromatic 

 series, and is at the same time a synthesis of the parent hydrocarbon 

 of the aromatic substances. Acetylene is also chemically allied to ethyl 

 alcohol, yet there is not the slightest trace of resemblance in the absorp- 

 tion curves in either case. 



^Rands : Phys. Rev., xiv. 



