278 REPORT OF THE COMMISSIONER OF AGRICULTURE. 



under examination would produce. These differences, however, hap- 

 pily appear greater when subjected to the analysis of polarized light 

 than the foregoing resumd of their chemical properties would indi- 

 cate. In other words, the physical differences in the various natural 

 fats are as important as the chemical. Advantage has been taken of 

 these dift"erences of physical structure to discriminate between fats 

 and oils of different origins. The specific gravity and the melting 

 point furnish two valuable points of discrimination, but both of these 

 are perhaps inferior in veJue to the evidence afforded by the crystal- 

 line structure of the fats. The observation with tlie microscope of 

 the crystals obtained in various ways furnishes valuable data for dis- 

 crimination, and if polarized light and a selenite x^late be used, these 

 data become still more valuable. 



The first account of the use of the selenite plate in such examina- 

 tion was given by Dr. J. Campbell Brown in the Chemical JSfeivs, 

 Vol. 28, page 1. He gives the following directions for the polaro- 

 microscopic work : 



Exahiiine several portions of the original samples by means of a good microscop&, 

 usi)ig a one-quarter or one-fifth inch object glass. In butter made from milk or 

 cream nothing is seen except the characteristic globules and the granular masses of 

 curd and the cubical crystals of salt. The hard fats of butter are present in tho 

 globulus in a state of solution, and are not recognizable in a separate form. 



If stearic acid, stearin, or palmitin be present in separate form, they will be recog- 

 nizable by single fusiform crystals, or star-like aggregations of acicular crystals. 

 They indicate the presence of melted fats. 



Other substances, such as starch, flour, palm-oil, corpuscles, Irish moss, coloring 

 matter, &c., may also be distinguished by the microscope, as distinct from butter 

 or fata. 



Examine the same portions with the sa,me object glass, together with a polar i- 

 Bcope, consisting of two Nicol's prisms and a selenite plate. The crystals referred 

 to polarize light, and when viewed by the polariscope are distinctly defined. Par- 

 ticles of suet and other fats which have not been melted may also be distinguished 

 by their action on polarized light, by their amorphous form, and by their membranes. 



The value of this deportment of fresh butter fat with elliptically 

 polarized light did not meet with the appreciation its merits deserved 

 until attention was again called to it by Dr. Thomas Taylor, of the 

 Department of Agriculture. 



Any fat or oil which is homogeneous and non-crystalline will 

 present the same phenomena when viewed with polarized light and 

 selenite plate; in other words, will have no effect on the appearance 

 of the field of vision. It is only, therefore, fats which are in a crys- 

 talline or semi-crystalline state that can thus be distinguished from 

 fresh, amorphous butter. Naturally it follows that a butter which 

 has been melted and cooled, or butter which has stood a long time, 

 would impart a mottled appearance to the field of vision. For a sim- 

 ple preliminary test, however, the procedure is worthy of more atten- 

 tion than its discoverer, Dr. Campbell Brown, accorded to it. 



FORMS OF FAT CRYSTALS. 



The forms of fat crystals differ greatly with the kinds of fat and 

 the proportions in which they are mixed. It would be idle to attempt 

 a description of all these modifications. 



Husson (Ann. _d. Chem, et d. Pharm., 5, 12, 469) has published an 

 illustrated description of some of the more important fat crystals. 

 Suet crystals, according to Husson, are very characteristic of stearin. 

 They are small rounded or elliptical masses, formed by stiff, needle- 

 like crystals, and resemble a sea-urchin or hedge-hog. 



