136 



such that whni tlie Sj). 'I\ R. i.s divided by this factor the ((uoticiit would 

 express the iodine vahie, this seems possibh' with fresli oils oi' fats, l)ut 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 

 f;it of different degree of freshness, which had not Ijeen exjjosed 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 quantitative relation exists between the refractive index and 

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

 only varying 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 insoluble acids. This 

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

 c(>iides of butter fat, must also exert a vai'ying influence on the j)hysical prop- 

 erties, such as the refractive index, melting point, congealing point, s]M'cific 

 gravitj'^, etc. 



DESCRIPTION OK fHEMICAL .VND PHVSIC'.VL CON.STANTS. 



Solid if ijiny Point. — The solidifying point indicates the temiH-rature 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 tempcu-ature remains 

 stationary. This depends on the property, that when sul)stances solidify 

 on cooling, th(! latent heat of fusion, is liberated and the rise of temi)erature 

 due to the latent heat equals the lowering of the tenii)erature of the fat. 

 When the temperature reaches this stationary ])oin1, the reading of the ther- 

 mometer is taken and is called the soliiliji/ing poinl. 



Meltimj Poinl. — 'I'he melting point indicates the tenip(M-atur(> at which 

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

 point. To obtain conip.ar.able results the same met hod must be used. It 

 nnist also l)e borne in mind that fats do not show tlieir normal melting ])(>int 

 shortly after being melted and then cooled, .\fter butter fat lias l)een melted 

 it should !)(■ ;iihiwc(l to cool at least twelve hours before tlie melting i)()int 

 is determine(i. 



Rcfrnrlivc Indc.r. — 'I'lie refractive index expresses Ihi' ratio of t he \ clocity 

 of I i gill in wicuuin to Ih.at of t he \-elocil \ of light in t iie medium mider inxcs- 



