— 160 — 



Harvey and Wilkie have determined the indices of refraction of numerous 

 essential oils and they set forth in table form the limits of values observed 

 by them for the various oils. Oil of rue had the lowest index of refraction 

 (n D20 o 1,4331), cassia oil the highest (n D20 o 1,6052). Another table shows the 

 changes which the indices of refraction of the essential oils undergo per 

 degree of temperature. These vary somewhat with the different oils, ranging 

 from 0,00039 for sandalwood oil to 0,00052 for anise oil, the average being 

 0,00046. The values agree with those ascertained by Harvey 1 ) in his earlier 

 investigations and published by him elsewhere, but on the present occasion 

 the material available for examination was much greater. 



Investigations which were made in our laboratory some time ago 

 as to the effect of temperature upon the indices of refraction of essential 

 oils do not quite agree with the results obtained by Harvey and Wilkie. 

 We obtained on the whole somewhat lower figures, with an average of 

 0,00035 per degree of temperature. But a determination, to be accurate, 

 must be made at the required temperature; it is decidedly inadvisable to 

 proceed by calculation in a matter of this kind. 



Parry 2 ), whose view regarding the value of the refractive index we 

 recorded in our last Report (p. 158) has since then again broken a lance 

 in favour of this constant. He is of opinion that, in conjunction with the 

 sp. gr. and the rotation, the refractive index may very well be used to judge 

 essential oils, notwithstanding the fact that its chief value is only revealed 

 in the testing of the various fractions of an oil. 



Parry has examined several oils by this method, by splitting them up 

 into 10 equal fractions and determining the refractive indices of the separate 

 fractions, and in a further publication 3 ) he makes known the results 

 obtained by him with spike oil, peppermint oil, and sandalwood oil 4 ). He 

 also quotes the refractive indices of numerous essential oils as well as of 

 separate constituents of essential oils. 



In an article entitled "Oil of anise and otto of rose" 5 ) Umney doubts 

 the reliability of the solidifying-point, and proposes, in the case of oils 

 mentioned above, and of similar oils, to substitute for it the melting-point 

 as a standard of value. He attempts to justify his proposals by declaring 

 that the solidification of the oils apparently depends largely upon chance, 

 and takes place sooner in one case than in another, while the results are 

 moreover influenced by the temperature of the cooling liquid and of the 



x ) Journ. Soc. chem. Industry 24 (1905), 717; Report October 1905, 87. 



2 ) Perfum. and Essent. Oil Record 1 (1910), 147. 



3 ) Chemist and Druggist 77 (1910), 314. 



*) Parry, in conjunction with Bennett, had already previously carried out similar 

 investigations with oils of rosemary, peppermint, and sandalwood. [Chemist and Druggist 64 

 (1904), 854; 68 (1906), 671 ; 71 (1907), 19.] For spike oil also compare the present Report, p. 131. 



5 ) Perfum. and Essent. Oil Record 1 (1910), 85. 



