Absorption and Fluorescence. 



421 



region o£ the spectrum of pyridine is somewhat complex 

 owing to the bands at 2*95 /uu and 3*25^, due to the nitrogen 

 atom and the hydrocarbon chain respectively. The first and 

 second harmonics o£ the fundamental band for a thin film 

 of liquid pyridine have, however, clearly been observed 

 by Coblentz at 6*25 (j, and 9*35^ and by Spence at 6*35 fi 

 and 9*50 ya. 



We have also examined the less refrangible band of 

 salicylaldehyde, and the values obtained are shown in 

 Table III., the solvent in this case being alcohol. 



Table III. 



V. 



1/X. 



Factors. 



Infra-red Band. 



0-1042 



3084 



- ■ 

 19x162-35 



6-16 /x 



0-208 



3074 



19x161-8 



6-18 



1 



3059 



19x161 



6-21 



10 



3045 



19x160-27 



6-24 



16 



3040 



19x160 



6-25 



20 



3039 



19x159-9 



6-254 



320 



3049 



19x160-4 



6-23 



1280 



3066 



19x161-4 



6-20 



50000 



3101 



19x163-2 



6-13 



It was not found possible to examine solutions of less con- 

 centration than V = 50,000 owing to the limitations set by 

 the spectrophotometer, but it is obvious that the limiting 

 value of l/\ is greater than 3101. Now Purvis* has 

 measured the wave-lengths of the component lines of the 

 more refrangible band of salicylaldehyde vapour, and there 

 is no doubt that the central line in this case is at A, = 2514 

 or l/\ = 3978. On the other hand, the infra-red spectrum 

 of salicylaldehyde has not been observed, and therefore we 

 can gain no direct information as regards the value of the 

 fundamental wave-number in this region. 



It is, however, clear from analogy with other compounds, 

 since 1/A, = 3978 is the central line of one absorption band 

 ■of salicylaldehyde vapour, that, if v x be the fundamental 

 .infra-red wave-number, xv x must equal 3978 and yv x must 

 equal some number which is rather larger than 3101, x and 3/ 

 being two integers. We have observed both absorption 

 bands of the aldehyde in alcoholic solution when V = 1280, 

 * Trans. Chem. Soc. cv. p. 2482 (1914). 



