DAIRY PRODUCTS. 29 



from that for each of the butters. Tlie numbers remaining- are directly proportional 

 to the colors of the butters. lu the butters thus far examined a fair colored Jersey 

 butter was taken for a standard and called 100. The others were calculated to this 

 standard from the scale reading. 



The use of a small amount of vegetable coloring matters mentioned 

 above does not seem to be prejudicial to health. 



EXAMINATION OF BUTTERS. 



The examination of butters to detect adulterations may be divided 

 into two parts : (1) Determination of physical properties ; (2) deter- 

 mination of chemical properties. 



Physical properties. The physical properties of fats which are useful 

 in butter analysis are their crystalline state, specific gravity, and melt- 

 ing point. 



Pure fresh butter prepared in the ordinary manner is not crystalline. 

 The microscope shows the absence of all forms of crystalline structure, 

 and thin films of the butter fat have no influence whatever on polarized 

 light. 



On the contrary, old butters, or butters which have been melted and 

 allowed to crystallize, and oils and fats which have been once in a fluid 

 state, show, as solids, quite a distinct crystalline structure readily re- 

 vealed by the microscope and affecting, in a marked manner, the po- 

 larized ray. 



Recently much attention has been excited by a discussion of the ap- 

 plication of polarized light to the qualitative examination of suspected 

 butters, and since many analysts have not the time to fully investigate 

 this matter I have thought it useful to enter upon the discussion of it 

 in considerable detail. 



Polarization is a term applied to a phenomenon of light, in which the 

 vibrations of the ether are supposed to be restricted to a particular 

 form of an ellipse whose axes remain fixed in direction. If the ellipse 

 becomes a straight line it is called " plane polarization." This well- 

 known phenomenon is most easily produced by a Nicol prism, consist- 

 ing of a crystal of carbonate of calcium (Iceland spar). This rhoinbo- 

 hedral crystal, the natural ends of which form angles of 71 and 109, 

 respectively, with the opposite edges of its principal section, is prepared 

 as follows : 



The ends of the crystal are ground until the angles just mentioned 

 become 68 and 112. The crystal is then divided diagonally at right 

 angles with the planes of the ends and with the principal section, and 

 after the new surfaces are polished they are joined again by Canada 

 balsam. The principal section of this prism passes through the shorter 

 diagonal of the two rhombic ends. If now a ray of light fall on one 

 of the ends of this prism, parallel with the edge of its longer side, it 

 suffers double refraction, and each ray is plane polarized, the one at 

 right angles with the other. That part of the entering ray of light 



