224 ELEMENTARY CHEMICAL MICROSCOPY 



to be determined. A lens attached to the body- tube or held in 

 a separate stand serves to magnify the thermometer scale. It 

 is thus possible to look into the tube of the instrument and to 

 watch both the material and the thermometer. This arrange- 

 ment and its applications will be readily understood by refer- 

 ence to the illustration. 



More serviceable and reliable than a small thermometer is 

 a thermocouple, with cold terminals in melting ice, and sensitive 

 millivoltmeter. A couple consisting of copper and copper- 

 nickel wire will be found satisfactory for a range of temperatures 

 from 20° C. to 400 C. or a little higher. Twisting the ends 

 of the wires together and fusing the tip in the flame of a blast 

 lamp with borax as a flux gives a good hot terminal. The cold 

 terminals should be placed in a receptacle and surrounded with 

 crushed ice; conveniently in a Dewar beaker or in a beaker 

 covered with cotton-wool or with felt. 



The thermocouple must be calibrated by means of compounds 

 of known melting points. Select a series of substances which 

 will cover the range through which the hot stage will be operated. 

 Determine their melting points in small melting-point-tubes in 

 the usual manner. Then take each of the substances in turn 

 and read the voltmeter as they are observed to melt in the hot 

 stage; observations being made under the microscope. On 

 cross-section paper plot the millivoltmeter readings against the 

 corresponding temperatures. The curve obtained will serve for 

 the determination of the melting points of substances under 

 future investigation. 



With platinum wire coils a temperature somewhat higher than 

 700 C. may be obtained in the apparatus. 



The material to be tested may be either crystallized upon or 

 supported on a small thin cover glass held by the wire fingers C 

 or may be placed in a short piece, 5 millimeters long, of tiny thin- 

 walled capillary tube fastened to the thermometer by a wire 

 band. For ordinary materials these tubes are best held horizon- 

 tally but for fats, waxes, etc., better results are obtained by 

 slightly inclining the capillary and taking as the melting point the 

 thermometer reading at the instant the fat slides out of focus. 



