70 INFRA-RED ABSORPTION SPECTRA. 



parent. The enormous drop of 97 per cent in transparency in going 

 from 2 to 3.5 ju, has been observed in no other substance except resin. 

 The transparent region at 5 /x is to be found in many compounds. The 

 5.88 fi band is very sharp and occurs in nearly all of this group of com- 

 pounds. The bands at 7.2 [i and 14 /a are to be noticed frequently. The 

 increased transparency beyond 12 /x reminds one of eucalyptol. As a 

 whole, this compound is extremely opaque, and the film of o.oi mm. 

 made it barely possible to explore the regions from 7 /a to 8/x and 



10 /J. to 12 fi. 



Le Page's Glue. (Fig. 55.) 

 Having used this glue (fish glue) in making some of the thicker cells, 

 an examination of a dry film, on rock salt, was imperative. The curve 

 is given with that of acetic acid, since that substance is used as a solvent 

 of the glue. Two dried films, 0.17 mm and 0.06 mm. in thickness, were 

 explored. Although six times thicker than acetic acid, they have the 

 same transparency up to 3.5 [x, beyond which the glue becomes more 

 opaque and is entirely so at 6 ju,. The 0.06 mm. film was likewise found 

 to be entirely opaque beyond 6 fi. It will thus be seen that in the cases 

 where glue was used, if any of it had dissolved, it would have mani- 

 fested itself. This occurred but once, for a solution of iodine in acetic 

 acid, and the result was opacity beyond 3 /x. None of the other sub- 

 stances was found to dissolve glue. 



Valeric Acid (Normal). C5Hio02=CH8(.CH2)3CO— OH. (Fig. 57.) 

 Both Kahlbaum's and the museum specimens were examined, but no 

 difference was found in the spectra. The curve is marked for the 

 depth and size of its absorption bands, especially the ones at 3.45 ju. and 

 13.5 fx, which are to be found in all the fatty acids. If the presence of 

 oxygen sharpens absorption bands, as noticed by Abney and Festing, 

 the effect is to be found in these compounds. The height of the general 

 transmission is about 50 per cent beyond 3/*. The 9.15/* band is 

 deepest in this compound and is smaller in the following compounds. 

 An examination with the large spectrometer showed no new bands 

 except at 3 fi, which appears complex. 



CaproicAcid. CoHi^O^. CH3(CH2)iCOOH. (Fig. 57.) 



This compound is a little more opaque than valeric acid. The trans- 

 mission seems to be in two steps, just as in acetic acid, with an opaque 

 region at 8 ju, and 10 /t, due to large absorption bands. The band at 8 /x 

 is double in valeric acid, the maxima being at 7.9 [i and 8.3 fx. The 

 10.8 fi band is also more opaque than in valeric acid. 



