W. P. White — Melting Point Determination. 459 



Instead of a constant temperature, the observer has to deal 

 with one varying continually from beginning to end.* The 

 course of the phenomenon, so far from being independent of 

 the furnace temperature, follows it closely and registers its 



Fig. 3. 



Heat ( = Time approximately) 



Fig. 3. Typical melting curve. Points taken from an observed curve 

 for Na 2 S0 4 containing \ per cent NaCl. X-O, melting point depression be- 

 low pure substance. Smooth curve calculated by formula 3 for the depres- 

 sion, X-O. Data page 484. 



every fluctuation. — Some consequences of this point of view 

 deserve discussion. 



1. The accuracy of the thermal method. — The obliquities of 

 thermal curves have been cited by several observers as an 

 evidence of the inaccuracy and untrustworthiness of the ther- 

 mal method. Rather are they the very sign of its fidelity to 

 the phenomena. If a method could be devised which gave 

 perfectly sharp points in ordinary substances, it would by that 

 very fact be convicted of misrepresenting the facts and of con- 

 sequent untrustworthiness. And while the method, for in- 

 stance, of locating the melting temperatures by watching for 

 changes in the appearance of the charge shows in a way the 

 gradual progress of the melting, its results are much further 

 removed from the quantitative. There is no definite connec- 

 tion between the amount of material melted and the appearance 

 of the charge to the eye. The thermal method alone at high 



*The fact that a melting interval and not a melting point is really in 

 question has been recognized by some observers, who regularly determine 

 the upper and lower limits of this interval. It is clear, however, from the 

 discussion above that the assignment of any lower limit is entirely arbitrary. 



