136 



such that when the Sp. T. R. is divided by this factor the quotient would 

 express the iodine value, this seems possible with fresh oils'or fats, but when 

 fats are exposed to air. partial oxidation takes place and this increases the 

 rise of temperature. The values in Table II, were determined from butter 

 fat of different degree of freshness, which had not been exposed to the air. 

 In considering other constants both physical and chemical no fixed rela- 

 tion exists. While it is true that the per cent, of olein influences the refrac- 

 tive index, no cpiantitative relation exists between the refractive index and 

 the per cent, of olein or oleic acid. This shows that oleic acid is not the 

 only varj'ing factor in butter fat influencing the refractive index. Since each 

 acid entering into the glycerids of butter fat has its own specific refractive 

 index, and from what has been said with reference to the mean molecular 

 weights of the volatile acids, we must expect the physical properties to vary 

 as the proportion of acids vary, which enter into the formation of a molecule 

 of the triglyceride'. The same reasoning applies to the insoUil)!e acids. This 

 variation of the proportion of the dififerent fatty acids entering into the gly- 

 cerides of butter fat, nnist also exert a varying influence on the ])hysical prop- 

 erties, such as the refractive index, nu'lting point, congealing point, specific 

 gravity, etc. 



DESCRIPTION OK CHEMICAL .\ND PHYSIC.VL CON.ST.VNTS. 



Soliflifyiny Point. — The solidifying ])oint indicates the temperatiu-e at 

 which butter fat solidifies. When butter fat is heated to 40° C. or 50° C, 

 then allowed to cool slowly, a point is reached when the temp(>rature remains 

 stationary. This depends on the property, that when substances solidify 

 on cooling, the latent heat of fusion, is liberated and the rise of temperature 

 due to the latent heat ecpials the lowering of the temi)erature of the fat. 

 When the temperature reaches this stationary point, the reading of the ther- 

 mometer is taken and is called the solidifijing point. 



Meltino Point. — The melting point indicatts tlie tenijierature at which 

 butter fat melts. Various methods arc used in determining t e melting 

 l)oint. To obtain comparable results the same method nuist be used. It 

 must also be borne in mind that fats do not show tlieii' norinnl melting point 

 shortlj' after being melted and then cooled. After butter fat has been melted 

 it should be allowed to cool at least twelve hours before the melting point 

 is determined. 



Refractive Index. — The refractive index expresses the ratio of the velocity 

 of light in vacuum to that of the velocity of light in the medium under inves- 



