NATURE 



241 



THURSDAY, DECEMBER 



1 90S 



ESSENTIAL OILS AXD ESSENCES. 

 The Chemistry of Esscnlinl Oils and Artificial Per- 

 fumes. By Ernest J. Parry. Second edition, revised 

 and enlarged. Pp. viii + 546. (London : Scott, 

 ('•reenwood and Son, igoS.) Price 12s. 6d. net. 

 'pHE chemistry of tiie essential oils is one of the 

 -L most interesting, and at the same time one of 

 the most complicated, sections of plant chemistry. 

 To begin with, the true function of an essential oil in 

 the economy and life-history of a plant is by no means 

 clear. It might at first sight be thought to be con- 

 nected with the reproductive agencies of the plant, 

 and possibly as regards the flower this may be the 

 case ; a fragrant smell in the flower may be the means 

 of attracting the insects which carry the fecundating 

 pollen. But that the attraction of insects is not the 

 sole function of an odoriferous oil must be obvious 

 from the circumstance that many essential oils, as in 

 the conifers, are to be met with in practically all parts 

 of ihe plant; some are found in the fruits, and a few- 

 even in the roots. At the saine time, there is much 

 evidence to show that the occurrence of an essential 

 oil in a plant is frequently directlv related to processes 

 which ensure the continuance of the species. The 

 amount increases with the growth of inflorescence and 

 decreases after the flowering period is past. But 

 certain oils appear to be the result of metabolic 

 changes which are not necessarily connected with 

 reproductive processes. Thus the essential oil of 

 .almonds is a product of the decomposition of 

 .imygd.ilin under the influence of the ferment emul- 

 sin. Oil of mustard is similarly produced from a 

 specific glucosidc by the agency of myrosin. 



The whole subject has hardly received that attention 

 from plant physiologists that its importance and in- 

 terest demand. E.\perimentally it is confessedly most 

 difficult, owing to the imperfection and restricted 

 character of the analytical methods at the in- 

 vestigator's disposal, especially quantitatively; and 

 it is, of course, accurate quantitative methods which 

 are most needed in correlating the life-historv of the 

 plant with the occurrence and distribution of (he 

 fragrant oil. 



The volume before us is the second edition of a 

 work which made its first appearance in 1899, and it 

 differs from its predecessor in several important par- 

 ticulars, partly in omissions, but more especially in 

 e.\tensions. Thus as regards the chemical nature of 

 the essential oils, a comparison of the present volume 

 with that of the first edition will serve to show the 

 striking developments that have taken place in the 

 interval. It is true that we have had no substantial 

 addition to the main groups to which the various 

 proximate principles occurring in essential oils may 

 be referred, but the number of such substances has 

 been greatly increased, and their physical character- 

 istics, lonslitution and mutual relations have been far 

 more accurately determined. The general survev of 

 tli( chemistry of the subject given bv Mr. Parry in 

 I lie second chapter is as full as the limitations of space 

 NO. 2044, VOL. 79] 



in a book not priinarily intended for theoretical 

 chemists would allow. The chapter on the prepara- 

 tion of the essential oils is, however, disappointing, 

 especially in a work which professes to deal with the 

 technology of the subject. The author practically con- 

 tents himself with a statement of the principles of the 

 main processes — expression, distillation and extraction 

 — in general use, and omits all detail on the ground 

 that details could not yield the practical man nearly 

 so much information as could be obtained during a 

 short stay in a factory. No doubt, in general, practice 

 is worth more than precept, but if every technologist 

 treated the practical aspects of his subject in this way 

 technology would cease to exist. 



By far the largest, and indeed the most valuable, 

 section of the work is concerned with the systematic 

 study of the more commercially important essential 

 oils. This section constitutes about half the entire 

 work. Great pains have evidently been taken in its 

 compilation, and, so far as we have been able to 

 discover, nothing of material importance relating to 

 the origin, mode of manufacture and properties of 

 anv particular oil would appear to have been omitted. 

 It need hardly be said that the well-known contribu- 

 tions which Messrs. Schimmel and Co. periodically 

 make to the literature of the subject have afforded 

 the .author much of his information. Indeed, it must 

 be admitted that the amount of actual original matter 

 other than analytical data which he has been able 

 to contribute to his work is not very extensive, and he 

 has to depend upon others for the greater part of what 

 he has to say relating to the origin and mode of 

 extraction of the products he describes. 



Since Tiemann's discovery of vanillin, which he first 

 obtained from the glucoside coniferin more than a 

 third of a century ago, a considerable number of 

 so-called synthetic perfumes have been prepared, 

 notably coumarin, heliotropin, ionone, artificial musk, 

 and neroli. Vanillin, which is methyl protocaiechuic 

 aldehyde, is now obtained on a considerable scale 

 from eugenol, the main constituent of oil of cloves, 

 and also from guaiacol, as well as by other methods, 

 and is largely used in confectionery. Costing as much 

 as 160L per lb. in 1876, it is now quoted at about 184'. 

 Coumarin, originally discovered in the tonka bean, is 

 found in a great number of plants, and was first 

 svnthetically obtained by Perkin. The synthetic pro- 

 duct is now employed to a considerable extent in place 

 of the tonka bean, as, for example, in the prepara- 

 tion of the well-known perfume foin-coupi. 



Heliotropin is the methylene ether of protocatechuic 

 aldehyde, and was first obtained from piperine, the 

 active principle of pepper, and is now prepared from 

 safrol. When first discovered its price, in 1880, was 

 701. per lb. ; it now costs about 85. per lb. 



Ionone, the artificial violet perfume, now largely 

 employed in perfumery, was first obtained by Tiemann 

 and Kruger in 1893. The preparation of these various 

 substances artificially constitutes one of the triumphs 

 of synthetic chemistry, and has given a great impetus 

 to the manufacture of artificial perfumes. Practically 

 all that has been up to the present made known on this 

 subject is set out in detail in the last chapter of Mr. 



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