32 FOODS AND FOOD ADULTERANTS. 



ization. The arms of this cross arc parallel and perpendicular to the 

 primitive plane of polarization. Between the arms are generally to be 

 found rings which present the successive tints of the fringes of inter- 

 ference. In the extraordinary image the order of the phenomenon is 

 entirely reversed. 



Having now briefly described the more important optical phenome- 

 iion which forms the basis of the examination of butters with polarized 

 light, I will next say something of the nature of the substances to be 

 examined. 



The expressions "fats " and "oils " designate those natural products 

 of animals and vegetables known as glycerides. Chemically consid- 

 ered they are the normal propenyl ethers of the fatty acids, or, in other 

 words, compounds of the triad alcohol, glycerine, with the fatty acids. 

 The term "fat" is applied to such bodies when they are solid at ordi- 

 nary temperatures, and " oil" when they are semi-solid or liquid. Those 

 which are most important are: 



Tri-steariu, CsI^CiaHssOs^, occurs in natural fats. It may be ob- 

 tained in a considerable degree of purity by repeated crystallizations 

 from ether. It crystallizes in plates of a pearly luster. Its melting 

 point is 55 C. 



Tri-palmitiu, C 3 H5(Ci6H3,O 2 )3, is found in animal fats and palm oil. 

 It crystallizes with a pearly luster from ether. The crystals have a 

 melting point of from 50 to 66 0. 



Tri-butyrin, C 3 H5(C4H 7 O 2 )3, occurs chiefly in butter. At ordinary 

 temperature it is liquid, and has a distinct and peculiar odor and taste. 



Tri-olein, ^B. 5 (C ls n^O 2 ) 3) occurs in animal fats and in almond and 

 olive oil. At ordinary temperatures it is liquid, is neutral to test pa- 

 pers, and has neither taste nor smell. 



Minute quantities of tri-myristin, capriu, capryliu, and caproin are 

 also found in butter. 



Pure butter fat is supposed to contain 



Per cent. 



Tri-oleiu ? about 42. 5 



Tri-steariu, about - 51.0 



Tri-butyriu, about 0. 3 



Other glycerides, about 2 



100. 00 



Olive oil is composed chiefly of tri-palmitin and olein. 



Tri-stearin is the chief constituent of mutton fat, it having only small 

 quantities of oleiu and palmitin. 



Beef fat has somewhat more palmitin and stearin than mutton tallow. 

 Lard has more olein. 



It is thus seen that in dealing with butter fats and their substitutes we 

 have to consider chiefly tri olein and stearin, and, in smaller quantities, 

 tri-palmitin, butyrin, &c. It follows, therefore, that the chief differ- 

 ences in the several substances will be due to the different proportions 



