DAIRY PRODUCTS. 33 



in which these glycerides are mixed and to such other physical differ- 

 cnces as the various sources of the substances under examination would 

 produce. These differences, however, prove greater when subjected to 

 physical and chemical analysis than the foregoing resume" of their 

 chemical constitution would indicate. Advantage has been taken of 

 these differences 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 value to the evidence afforded by the crystalline 

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

 crystals obtained in various ways furnishes valuable data for discrim- 

 ination, and if the light employed be plane polarized or elliptically 

 polarized by a seleuite plate, these data become still more valuable. 



The first account of the use of the selenite plate in such examinations 

 was given by Dr. J. Campbell Brown in the Chemical News, vol. 28, 

 pages 1, ct scq. He gives the following directions for the polaro-micro- 

 scopic work : 



Examine several portions of the original sample by means of a good microscope, 

 using a one-fourth or one-fifth inch object-glass. In butter made from milk or cream 

 notliing 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 the globules 

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



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

 nizable by simple fusiform crystals, or starlike aggregations of aeicular crystals. 

 They indicate the presence of melted fats. 



Other substances, such as starch. Hour, palm oil, corpuscles, Irish moss, coloring 

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

 fats. 



Examine the same portions with the same object-glass, together with a polariscopc, 

 consisting of two Nicol's prisms and a seleuite plate. The crystals referred to polarize 

 light, and when viewed by the polariscopo are distinctly deiiucd. Particles 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 po- 

 larized light did not meet with the appreciation its merits deserved un- 

 til attention was again called to it by Prof. Thomas Taylor, of the De- 

 partment 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 arc in a crystalline or semi- 

 crystalline state that can thus be distinguished from fresh, amorphous 

 butter. Naturally ifc 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 simple preliininaiy test, how- 

 ever, the procedure is worthy of more attention than its discoverer. Dr. 

 J. Campbell Brown, accorded to it. 

 19330 No. 13 3 



