472 W. P. White — Melting Point Determination. 



Hiittner arid Tammann* point out the connection between 

 impurity and the obliquity of the melting curve, here devel- 

 oped in equations (1) to (4), and suggest that the form of the 

 curve may be a convenient means of estimating non-isomor- 

 phous impurities. They give a numerical computation for the 

 last half of the melting interval (or the first half of the freez- 

 ing interval), but appear to have overlooked the result which 

 would be obtained by carrying their calculation farther down 

 the curve, for they have no other explanation of the large 

 obliquity usually found there than conduction of heat along 

 the (bare) thermoelement, which they assign as the principal 

 source of obliquity. Their view as to the practical cause of 

 such obliquity is thus decidedly opposed to that reached in the 

 present investigation. 



Summary. 



1. Actual melting and freezing point curves are nearly 

 always oblique— that is, they show, not the constant tempera- 

 ture called for by elementary theory, but instead, an interval 

 within which the temperature continuously rises or falls. 



2. The prime cause of obliquity in melting curves is the 

 obliquity of the melting itself, due to impurity. The true 

 melting point is the high end of the oblique melting interval. 



3. The melting hysteresis of some very viscous substances 

 (mostly compounds of boron and silicon) is also an occasional 

 (and then serious) cause of obliquity. 



A number of causes of obliquity lie in the experimental 

 determination of the behavior of the melting and freezing 

 substance. 



4. The determination of a melting curve necessarily involves 

 two factors : temperature rise, and heat supply ; the latter 

 depends on the temperature difference of furnace and melting 

 charge ; if this varies, the curve is distorted in a way striking 

 but easy to correct. The most common and conspicuous 

 example is where the furnace temperature is allowed to rise or 

 fall continuously, while the substance, melting or freezing, 

 remains nearly stationary. 



5. The freezing point, coming at the beginning (in time) of 

 the interval, where temperature distribution in the charge is 

 relatively uniform, is easier to observe than the melting point, 

 but is inadmissible in substances where undercooling is 

 marked. 



6. The melting point, coming at the end (in time) of the 

 interval, is liable, where stirring is not practiced, to obliquities 

 resulting from uneven temperature distribution : First, due to 

 the inevitable temperature difference between inside and out- 



* K. Hiittner and G. Tammann, Uber die Schmelzpunkte und Umwand- 

 lnngspunkte einiger Salze. Zeitschr. f. Inorgan. Cliem., xliii, 218, 1905. 



