36 ANALYSIS AND ADULTERATIONS OP BUTTBE. 



solidification and subsequent melting point of fats. It may 

 be noticed when a butter bas been beated in the air-batb for 

 a long time, in order to drive off water, tbat the fat will in 

 some instances remain fluid long after it is cooled down to 

 the temperature of the surrounding atmosphere. In conse- 

 quence of these and other collateral influences, we must 

 advise that a very limited amount of reliance be placed upon 

 the point of fusion in fats, as a proof of their identity or 

 purity. 



If a poi-tion of butter-fat which has been cooled down 

 quickly from a long-continued temperature of 100°, be 

 examined under the micro-polatiscope, it will be found to 

 consist of a mass of minute and imperfect crystals ; if, how- 

 ever, the same fat be cooled down slowly, the usual stellate 

 crystals wiU be formed, a change from the first to the second 

 may produce in the capillary tubes the so-called first allo- 

 tropic condition, and bring about an apparent fusion. We 

 are inclined to the behef that these changes in fats are not 

 due to true allotropy. This view is strengthened by the 

 fact that the fusing point, when taken by our method, with 

 a larger quantity of fat, is not effected, as will be seen by the 

 following experiment : — 



A test-tube full of butter-fat was kept at 100° for two 

 hours and then cooled down to 16° in running water ; the 

 normal sinking point of 35 '5 was then found not to have 

 been changed ; the gas was then turned out and the whole 

 arrangement left to cool down slowly. The sinking point 

 was again taken, and found not to have changed from the 

 original figures of 35 '5, 



