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the distance of target to specimen could be reduced to perhaps one third that required by the tube used here. This would permit a ninefold gain in intensity.The geometry of the simple arrangement shown in Figure 1 can readily be altered to accommodate several specimens simultaneously, if desired. A four-window x-ray tube can obviously serve four samples simultaneously. After the spectrum is once

Infrared absorption spectra from 2 to 15 microns are given for a number of pure long-chain saturated and monounsaturated fatty acids, methyl esters, triglycerides, and alcohols. The four classes of compounds studied are readily distinguished by spectral features common to the members of each class. Within each class, trans compounds are readily distinguished from cis and/or saturated compounds, but the two last-named types can be differentiated only by careful examination of their AS A preliminary step in the application of infrared spectros-_i\_ copy to a variety of problems being conducted in the authors' laboratories, and in particular to studies of the reaction of fatty materials with oxygen, it was necessary to obtain reference spectra from 2 to 15 microns on a wide variety of pure longchain compounds. With few exceptions (1, 21) published spectra on long-chain aliphatic compounds cover a limited range {10, 18, 20) or the purity of the starting materials is unknown {3).

SummaryThe scope and limitations of the lead salt‐alcohol method (11, 19) have been defined as a result of observations in the authors' laboratories and in others. Trans octadecenoic acids and esters are not determined specifically by this method, and when large amounts of cis acids or esters are present, they are also isolated with the solid acid fraction.The infrared spectrophotometric method, previously described by the authors (14), and the lead salt‐alcohol method have been applied to a variety of synthetic mixtures of known composition and to other materials. Comparison of the data indicates that the infrared method is more rapid, specific, and accurate than the lead salt‐method; it is directly applicable to the determination of trans isomers in acid or ester mixtures; only small samples are required; and they can be recovered if necessary.The infrared method is suggested as a necessary tool to investigators conducting research on the oxidation, isomerization, polymerization, composition, and hydrogenation of fats and their components and derivatives, and on the preparation of pure unsaturated acids and esters.