116 
Walker—Some Uses of the Low Potential 
tongs will do. To avoid shocks the handle should be insu¬ 
lated. 
Bring the shorter carbon so the lower end of the vertical 
carbon touches the edge of the cavity. Separate the carbon 
slightly and adjust the rheostat so an arc % to -J in. long 
passes between the carbons. 
Only a little practice is required to obtain very high tem¬ 
perature under good control. 
It is better to protect the eyes with a dark glass. 
Place in the conical cavity a small piece of a substance to be 
tested. Most minerals and the common metals fuse easily. A 
piece of quartz the size of a grain of wheat can be fused com¬ 
pletely. A piece the size of a pea will fuse on one side. 
Iron, copper and manganese are easily obtained from their 
oxides in nearly pure form. 
If the core is removed from the upper carbon and a stream of 
hydrogen or coal gas passed through the carbon tube thus 
formed, molybdenum and tungsten can be obtained from their 
oxides in more or less pure condition. The stream of reducing 
gas prevents reoxidation while cooling. 
The manipulation is so easy that even young students can 
obtain beads of iron, copper or manganese after a few minutes 
practice. 
In using the arc as an aid to or partial substitute for the 
blow pipe the carbons should be tested for the metals they gen¬ 
erally contain and allowance made for such. However, these 
give but little trouble in ordinary qualitative analyses. 
For preliminary tests the upper carbon should be 10 cm. in 
length and to make the test as delicate as possible it should be 
passed through a cork fitted into one end of a large glass tube 
8 cm. long. A lamp chimney is best. Through the cork a 
glass tube 5 mm. diameter and 6 cm. long should be passed 
near the carbon and bent above the cork so as to form an angle 
of 30° with the carbon. 
Adjust the large tube so the point of the carbon is about 3 
cm. above the opening. 
Put the substance to be tested in the cavity of shorter car¬ 
bon as in case of simple fusion and heat in the same way. 
