It should be mentioned that this table is by no means certain when the mixtures are in some other proportions. Thus, a mixture of 10 steario and 90 palmitio will show a melting-point of 60.1° F.), but its cooling-point will be about (130° F.), instead of 56.5° (1331° F.), so that even to get fair approximation, both the melting- and the resolidifying-points must be taken. Pure stearin acid melts at 69.2° (156° F.).
(6) Analysis of Fats containing Glycerides of both Soluble and Insoluble Fatty most gene rally interesting fat iu this series is butter, which has to be distinguished from the artificial butter manufactured from solid beef-suet, and known in commerce as " oleo-margarine " or "butterine " (see p. 1362). Remarks will therefore be specially directed to this subject, merely pointing out that the same process would be available for coconut and palm-nut-oils, or any of the few other fats oontaining soluble acids. Suet, and the artificial butter manufactured from it, will yield on melting, settling, and filtering, a fat containing about 95.5 per cent. of insoluble fatty acids ; but the fat of butter will he found to possess on an average very nearly the following composition :—Insoluble fatty acids '(chiefly oleic and margaric), 88 ; soluble fatty acid (calculated as butyric), 6 ; total, 94. From an estimate of both the soluble and insoluble acids, it follows that deficiency of the former and excess of the latter gives a good basis for detecting adulteration. This may be done in two ways, viz., by specific gravity, and by analysis.
(a) Examination of Butter by " actual density."—The term " actual density " was applied by Dr. Muter to this method as meaning the weight of any given volume of butter at (100° F.), divided by that of the same volume of distilled water at the same temperature, to distinguish it from ordinary sp. gr. compared with water at 15i- (60° F.). The actual density of pure butter ranges from 0.912 to 0.914, while that of butterine is Consequently there is a difference on the lowest estimate of 0.006, and an approximate judgment can be made as follows :— All butter, 0.912 or over ; -I butter, 0.910 ; butter, 0.909 ; butter, 0.908; all butterine, or under. It is, however, customary to pass as "good," butter having an actual density of anything over because, by keeping, the density increases ; but, to cause a fall below this point, it would require a degree of rancidity ao great as to produce complete unfitness for either alimentary or analytical purposes. The process is carried out as follows :—The butter is first kept melted till
all the impurities settle down, and the clear fat is filtered, as already directed in the estimation of "total fatty acids." A special ep. gr.-bottle is procured, of a pear shape, and having a thermometer fused through the stopper. The thermometer has a long narrow bulb, running right through the centre of the bottle, and its scale, which is from 15ii to 48.9° (60°-120° F.), is entirely above the stopper. This bottle is exactly counterpoised, and is then filled with recently boiled distilled water at 35° (95° F.). The stopper is inserted, and the whole is at once plunged up to the neck into a 12-oz. squat beaker, partially filled with distilled water at 39i° (103° F.), in which is placed a thermometer. As the temperature rises in the bottle, the water leaks out at the stopper, and in a feW minutes (if the quantity of water in the beaker be properly regulated), a time arrives at which the two thermometers equalize themselves at 37.8° (100° F.). The joint between the stopper and the bottle is instantly wiped by a small piece of blotting-paper to absorb loose water, and the bottle is lifted out, wiped thoroughly dry, and weighed. This process having been repeated three times, the average weight is scratched on the bottle with a diamond, and it is then ready for use. The pure butter-fat, prepared as already described, is melted in the water-oven, and cooled to 35° (95° F.). It is then poured into the bottle till full, the stopper is inserted, and the whole is plunged into the beaker of water at 39f° (103° F.). The same operations are gone through as just directed for the water, and the weight so obtained is divided by that marked on the bottle. The contrivance of having a rising fat heated by a falling water until the two equalize is the perfection of accuracy, and moreover gives an appreciable rest in the variation of temperature, sufficient to enable the excess of fat which has leaked out to be removed exactly at the required point.