44 FOODS AND FOOD ADULTERANTS. 



The value of the melting point iii the examination of fats is at once 

 apparent, provided it is possible to be assured that it represents a 

 definite temperature which can be easily and accurately determined. 



At a temperature of 40 0. pure butter fat has a specific gravity of .912, 

 while the substitutes therefor, viz, lard, tallow, oleo oil, neutral lard, 

 &c., have specific gravities varying from .900 to .905. Yet even these 

 small differences are extremely valuable in distinguishing the fats from 

 each other. 



The differences in melting points, when they can be accurately deter- 

 mined, will also prove helpful to the analyst. The usual methods em- 

 ployed to determine melting points have been based on the assumption 

 that a fat becomes transparent at the moment it assumes the liquid 

 state. Usually the fat is melted and placed in glass capillary tubes, and, 

 after cooling, put into water near the bulb of a thermometer. The water 

 is slowly warmed, and the moment the fat in the tube becomes transpar- 

 ent the reading of the thermometer is taken. A careful observer is able 

 in this way to make multiple determinations which agree well together, 

 but the readings of different persons are apt to vary greatly. Moreover, 

 it is not the melting but the transparent point that is determined. 



In 1883, at the Minneapolis meeting of the Association for the Ad- 

 vancement of Science, I described a method of determining the llowiug 

 point of a fat. The melted fat having been put into a small bent me- 

 tallic tube, was, after cooling, placed in a bath of mercury. One arm 

 of this U-tub'3 was slightly longer than the other. The bent tube was 

 immersed in the mercury until the longer arm was just below the sur. 

 face. The fat in the tube was, therefore, subjected to a certain definite 

 pressure from the mercury, due to the difference in length of the two 

 arms. When the melted fat first appeared on the surface of the mer- 

 cury, the thcrmometric reading was made. It is scarcely necessary to 

 add that tbe bulb of the thermometer was wholly immersed in the mer- 

 cury. Fairly good results were obtained by this method. 



Another method, which gave rather good results, I tried at the same 

 time. A thin film of fat was spread over the surface of the mercury 

 and the temperature noted at which a platinum wire drawn through it 

 left no trace. The solidifying point was determined in the same op- 

 eration by observing where the wire left a mark. Various methods for 

 determining the melting point of fats are given by Ueichert. 1 The 

 method preferred by him is a modification of Guichard's process, 2 in 

 which the fat is forced out of the tube by a water pressure of a constant 

 magnitude. 



Dr. H. Kriiss 3 describes an apparatus for estimating the melting- 

 point by the completion of an electric circuit dependent on the melting 

 of the fat used as an insulating material. A platinum wire, bent into the 



1 Zeit. Anal. Chem., 1835, pp. 11 etseq. 

 *I1)id., 1883, p. 70. 

 3 Zeit. f. Instrumentenkunde, vol. 4, pp. 32, 33. 



