IT seems a happy necessity that has finally extended the number of Dr. Ernest Guenther's volumes to six. In this respect, as in its total of well over a million words, the work now matches Gibbon's "Decline and Fall"; and in well-ordered presentment of the fruits of enormous study, it is not unqualified to sustain the comparison. Fittingly, therefore, recalling Gibbon's memorable introspection on the completion of his task, one may congratulate Dr. Guenther on the recovery of his freedom, sympathize with his sober melancholy in taking leave of an old and agreeable companion, and assure him as to the establishment of his fame. Admittedly, the scientist's achievement is less spectacularly single-handed than the historian's. As noted in the reviews of the earlier volumes1 and exemplified in Dr. Leo Goldblatt's monograph on the American turpentine industry in Vol. 6, Dr. Guenther has made way for able specialists in particular sections, constituting perhaps one-tenth of the whole. Nor can the scientific world be unmindful of a great debt to the firm of Fritzsche Brothers, without whose unstinted support the publication, could evidently not have been a practical possibility. Yet Gibbon's monomachy was a triumph exclusively of the library, while Dr. Guenther has refought his battles on the very ground, in the pleasant fields of Grasse and in regions much more remote: a valuable instance is his first-hand study, in Vol. 5, of the complex frac-tionation and grading of ylang-ylang oil as practised in the Madagascar islet of Nossi-Be. Before relinquishing the comparison, let it be said that Dr. Guenther's prose, while rightly refraining from the Roman majesty, is noticeably free from those infelicities to which the merely industrious compiler is so commonly prone.The two present volumes, continuing from Vols. 3 and 4 their account of individual essential oils, deal chiefly with the products of those plant families which, though low in the scale of quantitative output, gain importance from their commercial history and from their special characters as perfumes and flavours. Here, therefore, are most of the examples of the preparation of concretes and resinoids by non-distillation methods: processes as old as Homer, such as the extraction of rose oil into fatty pomades, or as recent as the extraction with such newer solvents as butane, by which the hitherto unattainable concrete of lily of the valley is now being prepared. Though the pure chemistry of the constituents of essential oils was formally dealt with in Vol. 2, Dr. Guenther has profited by his opportunity of recording subsequent advances in this field. To quote only a few examples from the present volumes, the monograph on Thuja plicata discusses the naturally occurring ct/cfoheptanone derivatives (tropolones) down to the synthesis of the three isomeric thujaplicins in 1951. The account of oil of hops includes a survey of recent investigations on the structure of oc-earyophyllene and its identity with humulene. Among other compounds for which these volumes record structural formulae proposed, revised or confirmed since the publication of Vol. 2 are cedrene, lanceol, (3-santalol, zingiberene, kessyl alcohol, luparol and the poly-ffcoetylenic compounds of the "matricaria-ester" type. Noteworthy also are the first records of the natural ooccurrence of a sulphoxide (sulphoraphene, in oil of Radish), the tricyclic terpene tricyclene in hemlock-spruce oil, and piperettine, a homologue of piperine, in oleoresin of pepper.Analytical procedures described in Vol. 1 have also been amended where necessary in view of later work: for example, the thiosulphate factor in ascaridole odetermination. One might wish, perhaps, for fuller?documentation of the statement that the halogen ^test (for synthetic benzaldehyde as opposed to natural oil of bitter almonds) has lost much of its importance owing to the production of the synthetic material in a higher state of purity. It is not clear whether this opinion takes account of very delicate chlorine determinations by the lamp method2, to which the book does not explicitly refer. Consideration of the six volumes as a whole may well prompt the reflexion that the essential oil industry has been singularly blessed in the production of such a book at such a juncture as the present. Even a few years ago, it would have been impossible to include much of the material descriptive of new and rationalized production techniques, which may be considered as prefiguring the industry's future; while in even a few years more, oblivion perhaps may begin to overtake much of its historic and romantic past-the buccaneers and galleons, and such strange contrasts as that between the sandalwood of Mysore, owned and accounted for to the last stick by a solicitous government, and the copaiba of Brazil, produced obscurely out of Amazon jungles by native woodsmen calling no man master. To students of either past or future, the book must be equally invaluable.In view of the progressive additions of new work, to which reference has already been made, a comprehensive index is a necessity for making the fullest use of the book. This is provided in the sixth volume, and extensive trials of its adequacy indicate it as an ending which may be said to crown even a maximum opus.