REPORT OF THE CKEMIST. 281 



of adulteration would be indicated by the number expressing tlie rel- 

 ative weight. In order that these numbers give reliable results it is 

 necessary that the manipulation whereby they are obtained be made 

 with the greatest care. 



Melting point. — The melting point of a fat is also a physical prop- 

 erty which is of value in determining its character. Unfortunately 

 the methods heretofore in use for determining the molting point have 

 not been reliable. 



The difficulty has been to fix upon a point that really represents 

 the passage of the fat from a solid to a liquid state. Since, however, 

 in this transit the fat passes through all the grades of solid, semi- 

 solid, and liquid, it has been found impracticable to fix the point with 

 any degree of definiteness, 



I have sought to overcome this difficulty by fixing on some definite 

 physical phenomenon which can be observed with a considecfable de- 

 gree of accuracy. 



Since fats i)ass gradually from a soft solid to a mobile liquid, it oc- 

 curred to me that the point at which the molecular attraction of the 

 particles overcome the molecular adhesion might be determined. 

 in order to do this, however, it was necessary to subject the particle 

 of fat to be observed to conditions in which it would be affected by 

 no other force except its own molecular stress. For this purpose the 

 fat was raised to a temperature a few degrees above the melting point, 

 and in this condition dropped onto the surface of ice or water cooled 

 to a degree which would enable it to fix the thin film of fat before it 

 could recoil into a spheroid state. 



The diameter of these fat disks should be about 1. 5 centimeters, and 

 each of them should weigh about 200 milligrams. 



To free these disks from all external force I submerge th-em in a 

 mixture of alcohol and distilled water, each recently boiled, to free it 

 of all air bubbles. 



The distilled water is first poured into a large test-tube until it is 

 one-third full. The alcohol is now added, pouring it in rather care- 

 fully until the tube is nearly full. After standing for an hour all 

 parts of the liquid are of the same temperature, but the heavy liquid 

 remams at the bottom of the tube. One of the disks of fat prepared 

 as above is now placed in the test-tube. It sinks until it comes to a 

 ponit where the liquid of the tube is of the same specific gravity as 

 itself, and there remains stationary. (The fat disks are kept on the 

 cold water until they are to be used. They are moved by lifting them 

 with a metallic spatula). 



The test-tube is now placed in a tall beaker of water, and the bulb 

 of a delicate thermometer is brought near the disk of fat. (These 

 thermometers should indicate tenths of a degree. They were made 

 especially for this work by Emil Greiner, of New York.) The water 

 in the beaker is now heated very slowly, and constantly stirred either 

 by blowing into the beaker with a rubber bulb, or by a paddle. The 

 thermometer is also gently moved from side to side in order to secure 

 a perfect uniformity of temperature in the liquid in contact with the 

 disk. A thermometer is also placed in the water in the beaker, the 

 temperature of which is not allowed to rise more than 2° above the 

 melting point of the fat. As the temperature approaches the critical 

 point the disk is observed to roll up into a rod, which gradually 

 shortens until the mass becomes a perfect sphere. The movement of 

 the thermometer imparts a rotatory motion to the globule, and thus 

 enables the observer to determine the exact poinl at which it becomes 



