CHAPTER XI. 



THE DETERMINATION OF THE MELTING AND SUBLIMING 

 POINTS OF MINUTE PARTICLES OF MATERIAL. 



The determination of the melting point of a compound is 

 usually one of the simplest and most reliable tests at our dis- 

 posal for ascertaining the purity of a known compound or for 

 obtaining an idea as to the probable nature of a substance of 

 unknown composition. In the case of organic compounds the 

 melting point is one of the first constants to be ascertained and 

 even with certain inorganic substances a melting point deter- 

 mination may often prove of great value. 



It not infrequently happens that such a small quantity of 

 material is available that the usual laboratory methods are im- 

 practicable and recourse must be had to some microscopic 

 method of procedure. Often, the chemist deals with material 

 containing a large proportion of amorphous matter mixed with a 

 crystalline substance and a satisfactory separation cannot be 

 effected; or again, a preparation is obtained in which there 

 appears to be two or more different crystalline substances but 

 no means for separating them can be found. In all these cases 

 a melting point would give the needed information were it possible 

 to effect a separation. 



By spreading out the material in a thin layer upon an object 

 slide and examining the preparation with the microscope, we can 

 almost always find crystals or fragments of material here and 

 there not in direct contact with others, but appearing in the 

 image isolated and free. We have thus in reality affected a 

 separation and if we apply heat, we should be able to make 

 reliable observations upon the behavior of each isolated particle. 

 If in addition we have some means of controlling and measuring 

 the heat applied, it is obvious that a melting point can be ascer- 

 tained. Inasmuch as a variety of methods for temperature 



