DETERMINATION OF MELTING POINTS 



221 



butyrorefractometer may be employed. By regulating the 

 heating flame and the rate of flow of hot water, very gradual or 

 very rapid rises of temperature may be obtained or the temper- 

 ature may be maintained almost constant. Jacketing the cell 

 with asbestos simplifies the regulation of temperature. Heaters 

 functioning on the principle of the thermo-siphon, Fig. 133, may 

 also be employed for temperatures up 

 to 85 to 90 C.; but above 90 de- 

 grees the regulation of the height of 

 the heating flame becomes rather diffi- 

 cult and the sudden formation of 

 steam usually results in a blow-off 

 through the safety tube, in which the 

 thermometer is only very closely in- 

 serted. 



Substituting brine or oil for water, 

 the temperatures can be raised to 

 125-150 degrees if the heating coil be 

 used, but the author has never found 

 hot oil to give satisfactory results in 

 any thermo-siphon system, since the 

 viscosity of the oil in the glass cell 

 is too great to permit an even and sufficiently rapid rate of flow 

 unless large conducting pipes be employed, necessitating a cell 

 far too thick for use. 



The temperatures may be conveniently measured by means 

 of a set of Anschiitz thermometers. Thermometers of this 

 type are sufficiently small, so as not to project too far, and their 

 graduations are such as to permit readings to be taken to o.i 

 degree. 



A convenient arrangement for reading the thermometer and 

 observing the melting point of the substance under observation 

 is given below. 



With hot stages of the sort just described it is always a wise 

 precaution to place the cell in a glass tray or shallow crystallizing 

 dish to guard .against damage to the microscope should the hot 

 stage break. 



FIG. 133. Heater for Melting 

 Point Apparatus. 



