The melting point of otto of rose is usually taken in a similar tube to the setting point, and is considered to be the point at which the last crystal disappears. _Iodine Absorption._--In the authors' opinion, this is of some value in conjunction with other data in judging of the purity of otto of rose. It is determined by Huebl's process as described under Fats and Oils, except that only 0.1 to 0.2 gramme is taken, and instead of 10 c.c. of chloroform, 10 c.c. of pure alcohol are added. The rest of the process is identical. SOAP. In the analysis of soap, it is a matter of considerable importance that all the determinations should be made on a uniform and average sample of the soap, otherwise very misleading and unreliable figures are obtained. Soap very rapidly loses its moisture on the surface, while the interior of the bar or cake may be comparatively moist, and the best way is to carefully remove the outer edges and take the portions for analysis from the centre. In the case of a household or unmilled toilet soap, it is imperative that the quantities for analysis should all be weighed out as quickly after each other as possible. _Fatty Acids._--Five grammes of the soap are rapidly weighed into a small beaker, distilled water added, and the beaker heated on the water bath until the soap is dissolved. A slight excess of mineral acid is now added, and the whole heated until the separated fatty acids are perfectly clear, when they are collected on a tared filter paper, well washed with hot water and dried until constant in weight. The result multiplied by 20 gives the percentage of fatty acids in the sample. A quicker method, and one which gives accurate results when care is bestowed upon it, is to proceed in the manner described above as far as the decomposition with mineral acid, and to then add 5 or 10 grammes of stearic acid or beeswax to the contents of the beaker and heat until a clear layer of fatty matter collects upon the acid liquor. Cool the beaker, and when the cake is sufficiently hard, remove it carefully by means of a spatula and dry on a filtering paper, add the portions adhering to the sides of the beaker to the cake, and weigh. The weight, less the amount of stearic acid or beeswax added, multiplied by 20 gives the percentage of fatty acids. Care must be taken that the cake does not contain enclosed water. The results of these methods are returned as fatty acids, but are in reality insoluble fatty acids, the soluble fatty acids being generally disregarded. However in soaps made from cocoa-nut and palm-kernel oils (which contain an appreciable quantity of soluble fatty acids) the acid liquor is shaken with ether, and, after evaporation of the ethereal extract, the amount of fatty matter left is added to the result already obtained as above, or the ether method described below may be advantageously employed.