W. P. ^Yhite — Melting Point Determination, 469 



element. These are in part antagonistic to those required- for 

 diminishing effects V and VI, but with apparatus of the 

 dimensions given above, page 467, all these errors are reduced 

 to one or two-tenths of a degree or less. 



Minor Causes of Obliquity. 



VIII. Contamination of the thermoelement. — If the furnace 

 is heated continuously and more or less regularly throughout 

 the melting interval, the temperature of the furnace end 

 relatively to the charge steadily rises, and if the thermoelement 

 is not homogeneous between these two, its reading will rise 

 also.*\ In the case of a badly contaminated element at 1400°, 

 this apparent increase of obliquity was found seldom to exceed 

 1°. It practically vanishes if a constant temperature differ- 

 ence is maintained between furnace and charge, as also of 

 course if the element is in good condition ; hence, while not to 

 be overlooked, it should never constitute a serious difficulty. 



IX. Differentiation in the charge. — The portions of an 

 impure charge which crystallize last may have more impurity 

 in them, hence there may be a tendency for that part of the 

 charge immediately around the thermoelement to melt at a 

 lower temperature than the outside. This would tend to 

 increase the obliquity of the observed curve. Thus far, we 

 have not been able witli certainty to distinguish this effect 

 from the other accompanying causes of obliquity. 



X. Radiation through the charge. — If the charge which is 

 beino; melted is diathermanous, direct radiation from the 

 crucible wall to the thermoelement will heat the latter, causing 

 a temperature rise which is greatest at the beginning of the 

 melting and which depends upon the fluctuations of the furnace 

 temperature. This effect, also, has not yet been distinguished 

 with certainty. 



Present Standing of the Problem. 



Fig. 4A shows the typical melting curve of a fairly limpid 

 silicate (diopside). That the obliquity in the lower part of this 

 and similar curves is due to the substance itself (and therefore 

 to impurity) can be easily shown as follows : The obliquity 

 always precedes the melting, wherever that may occur, hence 

 it is due to the melting in some way or other, and not to the 

 furnace; that is, it indicates a true heat absorption accompany- 

 ing the melting. When the heat absorption begins, the 

 thermoelement in the charge reads about 30° lower, and a bare 

 element exposed directly to the furnace outside the charge 

 reads (usually) 25° lower than the final melting point. Hence 

 *Pliys. Eev., xxvi, 535, 1908. 



