CHEMICAL MICROSCOPY 



271 



similar to those in Fig. 11.4, are derived. In this graph, the tem- 

 peratures read off the thermometer are shown as ordinates and the 

 potential differences as abscissae. Two curves are plotted: the first 

 corresponds to a temperature rise of 2°C per minute, the other 4°C 



300 



5> 200 



a> 

 a. 



Fig. 11.4. Calibrating the hot stage. 



per minute. If a temperature rise of 2°C per minute is required at 

 200' C approx., the graph shows that the rheostat adjustment is to 

 provide a 73-volt potential difference. 



4. DETERMINING THE MELTING POINT 



Before attempting measurements of undetermined substances, it is 

 indispensable to find the melting points of several compounds the 

 melting point of which is known. After putting several crystals of the 

 substance on the object-slide, and covering the latter with the cover- 

 slip, the cover-slip is rotated by means of, for example, an india-rubber 

 eraser, thus spreading out the sample since the crystals adhering to 

 the cover-slip underface melt at approximately 1° above the melting 

 point of the crystals on the object-slide, the best procedure is to remove 

 the latter, clean it and re-set it on the specimen. In this way, the crystals 

 are on the object-shde and not on the cover-slip under-face. 



All the accessories being in order, focusing is made, selecting an 

 area of the specimen where discrete crystals are discernible. The 

 rheostat is adjusted so that temperature rises promptly to about ten 

 degrees below the melting point. Heating is stopped until temperature 

 begins to drop. Using calibration curves similar to those shown in 

 Fig. 11.4, heating is resumed so that temperature rises about 2° per 

 minute. When 2° below the melting point, the rate of temperature 



