(g) Separation of a heavy and insoluble yellow powder, chrome yellow (see p. [130]).
Microscopic Examination.—The microscopic examination of butter has lately received considerable attention as a means for the detection of the presence of foreign fats. Genuine butter generally exhibits under the microscope a crowded mass of globules of fat, fatty crystals being commonly absent. In oleomargarine a more crystalline structure is observed, with pear-shaped masses of fat and but few globules. While the presence of crystals in a sample may justly be regarded as suspicious, it is by no means a positive evidence of adulteration, since, under certain circumstances, pure butter may present the same indications. In applying the microscopic test, a small portion of the fat is made into a thin layer on the slide, and then protected with a glass cover, applied with rather gentle pressure.
Plate VI.[40] represents the microscopic appearance of genuine butter and oleomargarine. It will be observed that in butter (Fig. 1) numerous globules but no crystals of fat are presented, the crystals present being those of salt. In oleomargarine (Fig. 2) the distinctive pear-shaped masses of fat, accompanied by only a small number of fatty globules, are to be seen. Dr. Thomas Taylor (of the U.S. Department of Agriculture), has made an elaborate investigation of the microscopic appearance of various fats when viewed by polarised light. He regards the presence of peculiar globular crystals and the black cross commonly termed St. Andrew’s cross as characteristic of genuine butter.[41] Lard, beef, and other fats are said to exhibit different and, to some extent, distinctive crystalline forms. Prof. Weber,[42] however, affirms that mixtures of lard and tallow fat, under certain conditions, cannot be distinguished from butter by means of this method of examination. More recently, Dr. Taylor states that the distinguishing difference between butter and other fats under the microscope is that the former, when observed by polarised light through a selenite, exhibits a uniform tint, whereas the latter shows prismatic colours. Although the results of these investigations cannot as yet be considered as perfectly satisfactory or conclusive, they certainly are entitled to rank as a highly valuable and important step in advance of the optical processes hitherto employed.
PLATE VI.
Fig. 1, Butter × 400.
Fig. 2, Oleomargarine × 400.
ARTOTYPE. E. BIERSTADT, N. Y.
PLATE VII.