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. 
