Alternating Current in a Chemical Laboratory. 115 
Besides this two pieces of ordinary insulated copper wire No- 
18, 19, 20 or 21 and 70 cm. long will be required. All the 
wires can be supplied and fixed by any dealer in electrical sup¬ 
plies. 
The rheostat is the most expensive part of the apparatus. 
Any commercial rheostat capable of carrying 12 amperes will 
answer all ordinary purposes. It is necessary to have a resist¬ 
ance in the circuit and the current must be started with a high 
resistance. It may be lessened afterwards until the proper 
strength is obtained. 
The assorted carbons may be J to in. in diameter. I use 
the cored carbons almost entirely. They are much better than 
the uncored. 
Carbons differ much in quality; some contain considerable 
quantities of metallic carbides and carbonates. I have found 
that those manufactured specially for optical projection gener¬ 
ally contain less of these substances, but these also show vari¬ 
able quantities of metals and earths. 
The arc lamp may be used for the following purposes: 
1. To show the effect of high temperatures upon difficultly 
fusible and non-volatile substances. 
2. For reduction of ores. 
3. As a partial substitute for the blow pipe in qualitative 
analyses. 
4. For synthetic preparation of compounds of carbon from the 
elements. 
There is a wide range of experiments showing the effect of 
the electric arc upon substances almost infusible by other means. 
One may proceed as follows: 
Fasten or suspend a cored carbon in a vertical position so 
the lower end is about 10 cm. from top of table. Connect by 
wrapping around the carbon a piece of insulated copper wire 
stripped of insulation where contact is made with carbon. 
Take a piece of cored carbon about 4 cm. long and bore a 
conical shaped cavity in one end about 5 mm. deep. Connect 
this carbon with insulated wire like the first and join both 
wires with opposite poles of circuit. Fix the lower carbon in 
a holder, a clamp fastener, a wire test tube holder or a pair of 
