and changes with the nature of the fatty substance. The groups of fatty acids are distinguished by a characteristic deportment toward halogens; while members of the first series are indifferent to haloids, those of the second and third class combine readily, without suffering substitution, with two respectively four atoms of a haloid. In view of this behavior the first series is termed saturated, the second and third that of unsaturated acids. Addition of halogen to one of the unsaturated acids yields on subsequent examination an invariable quantity of the former, representing two or four atoms, according to one or the other of unsaturated groups; and as the molecular weights of fatty acids are unequal, the percentage quantity of halogen will be found varying with regard to members belonging to the same series. The amount of iodine absorbed by some of the fatty acids is illustrated by the following items: Hypogallic acid, C_{16}H_{30}O_{2}, combines with 100.00 grammes. iodine. Oleic acid, C_{18}H_{34}O_{2} " " 90.07 " " Erucic acid, C_{22}H_{42}O_{2} " " 75.15 " " Ricinoleic acid, C_{18}H_{34}O_{3} " " 85.24 " " Linoleic acid, C_{16}H_{28}O_{2} " " 201.59 " " Of the halogens employed in the examination, iodine is preferable to either chlorine or bromine; it acts but slowly at ordinary, but energetically at elevated temperatures. The reagents are solution of mercury iodo-chloride prepared by dissolving of 25 grms. iodine, 500 c.c. alcohol of 95 per cent., and of 30 grms. mercury chloride in an equal measure of the same solvent; both liquids are filtered and united; a standard solution of sodium hyposulphite produced by digestion of 24 grms. of the dry salt with 1 liter water and titration with iodine solution; solution of potassium iodide of 1:10; chloroform, and finally a solution of starch. The above solution of mercury iodo-chloride acts on both free unsaturated acids and glycerides, producing addition products. For testing a sample of 0.2 to 0.4 grm. of a liquid, and from 0.8 to 1.0 grm. of a solid fat being used, which is dissolved in 10 c.c. chloroform and treated with 20 c.c. mercury iodo-chloride solution run into it from a burette, if the liquid appear opalescent a further measure of chloroform is introduced, while the amount of mercury iodo-chloride must be such as to produce a brownish coloration of the chloroform for two subsequent hours. The excess of iodine is determined, on addition of from 10 to 15 c.c. potassium iodide solution and 150 c.c. distilled water, by means of caustic soda. From a burette divided into 0.1 c.c. a solution of caustic soda is poured with continual gyration of the flask into the tinged liquid, and the percentage of combined iodine ascertained by difference; for this purpose 20 c.c. of mercury iodo-chloride are tested, on introduction of a solution of potassium iodide and starch, previously to