This second volume of the series of books on the Refractive Indices of technical substances deals with Oils, Fats, and Waxes.

The importance of this determination has been brought to the notice of a wide circle of readers by the books of Lewkowitsch, Fryer & Weston, Pickering, Gill, Ingle, Newton, Friend, Leach, and others, and the examination of oils by the Refractometer has been a matter of routine in chemical laboratories for many years, particularly since the Abbe Refractometer and the Butter Refractometer have been available.

In consequence of the commercial importance of the Oil and Fat Industries there are very many sources of reference, and Mr Kanthack's task has been no light one. We have not aimed at placing on record every published determination of the Refractive Index of a given oil, but have endeavoured to give a sufficient body of evidence to establish the limits met with in the case of the commoner oils, and have sought to tabulate at least one value for the rarer oils.

More than 2500 references to the original sources are given, and it is hoped that the Bibliography will also be of service to those who wish to consult the original literature for other properties of the oils in the Tables.

Owing to the way in which the information required is scattered through the chemical literature of the world, it is evident that some publications on the subject, and possibly one or two important ones, may have escaped notice. Readers of this book, who may know of data which should be quoted, are invited to communicate with the publishers, so that the next edition may be rendered as complete as possible.

The limitations of the inferences to be drawn from Refractive Index measurements will be familiar to readers of this book, who are aware of the necessity of confirming the indications of the instrument by other tests.

The increasing interest taken in analysis with the Refractometer was well shown at a largely attended meeting of the Society of Chemical Industry, held in London on 3rd February 1919. There is an increasing tendency to include optical methods in the physical examination of technical products, as these methods have been found to give information which, in many cases, is not given by the Specific Gravity; moreover the determination of the Refractive Index has the advantage of being much more rapidly performed, and also of being a more accurate measurement. For rapid sorting tests the Refracto-meter has no rival.

Official Publications, such as the British Pharmacopoeia and the German Customs Regulations, specify limits of refractive indices, while the British Engineering Standards Association and the American Society for Testing Materials include this measurement in some of their specifications.

The following new applications, not referred to in the Refractive Index Tables, deserve special mention.

An authoritative statement on "The Change of Refractive Indices of Fixed Oils with Temperature" is given by A. F. Joseph in the Journal of the Society of Chemical Industry, Vol. 39, 15th March 1920.

An important paper on the "Dispersion of Oils" has been published by Fryer & Weston, and this property has been found to have discriminative value for Tung Oil and Coconut Oil. (The authors' values are given in this volume.) Dispersion measurements obtained with the Abbe Refractometer are sufficiently accurate for use in this connection, according to the experience of the present writer.

The instrument is also used in the analysis of ordinary soaps, soaps containing phenols, or derived from sulphonated oils. For the soaps last mentioned, Pickering (J. Soc. Chem. Ind., 1920, T. 305) found that "the refractive index and fat content are proportional. A graph plotted from these is very useful."

The measurements may gain in value when they are plotted against another physical property, such as the Melting Point, as shown by F. H. Trim in his paper on the analysis of mixtures of refined Arachis, Coconut, and Palm Kernel Oils (J. Soc. Chem. Ind. 1920, T. 307).

J. N. Goldsmith.