Physical Ghemlsfri/ of Kssentiol OiU 



'J'hk Densities. 



(.2) Tlie Refractive Index. 



Of recent papers on the relation between refractive index and 

 temperature the most interesting are those of Falk^, who found a 

 linear relation for those substances he examined, none of which, 

 however, were terpenes. 



In the present research in every case there are found divergences 

 from this simple proportionality, the temperature coefficient in- 

 creasing with the rise of temperature except in the cases of cineol 

 and of l-pinene, where it decreases, and the divergences bein^" 

 most marked in the results for l-phellandrene. 



Like the coefficients of expansion, these temperature coefficients 

 varv only within narrow- limits from .000402 for d-pinene to 

 .000480 for l-phellandrene. Since the coefficients for those organic 

 substancs, whose refractive indices are tabulated in Landolt- 

 Bornstein, vary from .0003 to .0008, this magnitude appears to 

 be a constitutional one. The measurements of refractive index 

 between lOo and 60^ are tabulated in the table. They were ob- 

 tained in a Zeiss refractometer, fitted wuth a constant temperature 

 device, which ensured constancy to at least .1°. 



Three formulae have been suggested for calculating the refractive 

 power — 



(1) n-\ld (2) n'-\:d and (3) ir - \ ; {n^ + '2)d 



and Falk found that ir-ijd gave values that decreased as the 

 temperature I'ose, tr — \j{>r -\-'2d) values that increased, while, when 

 n — \jd\yA>i used for the calcidation. they increased in some cases 

 and decreased in others. 



1 J. Amer. Cheni. Soc, 1909, 31. 86, 806. 

 Zeit. Physik. Cheiu., 1913, 82, 504. 



