379 
the body of coal by some good conductor, as the coal itself is a very poor 
conductor of heat and all particles in the body of coal must always be at 
a uniform temperature during the heating. 
(3) The temperature of the coal must be accurately known at all 
times. 
(4) The gases driven off from the coal must be swept out as soon as 
formed. 
(5) The gases must be cooled down and continuously tested for tar. 
(6) <Any tar deposited in the pipes at a low temperature must not be 
allowed to re-distill at a higher temperature and then appear in the gas. 
1. The Furnace. After considering the various possible means of heat- 
ing the coal it was decided to use an electric resistance furnace. By this 
means the coal could be heated at any rate desired and the rate of heating 
cou:d be controlled at all times, or the coal held at any desired tempera- 
ture for any length of time. The furnace used is shown in cross section in 
Fig. 1 of the accompanying drawing, and was made as follows: <A cylin- 
der, (1), 33” inside diameter and 20” long was made out of 16 B. & S. gage 
sheet iron, riveted at the seam. Around this was wound four thicknesses 
of wet asbestos paper (2). About 75 feet of 12 B. & S. gage nichrome re- 
sistance wire (3), was wound around this, four turns to the inch. The 
asbestos paper iDsulated the wire from the cylinder. Around this was 
wound more wet asbestos paper, to hold the wire in place, and the whole 
Was covered by a layer of asbestos pipe covering (7), about 2” thick. The 
ends of the resistance wire were fastened to electric terminals on the fur- 
nace. The current passed through this furnace was taken from a 110 volt 
alternating current circuit, and was varied by means of a smal! water 
rheostat. The amount of currelit was measured by an ammeter in the cir- 
cuit, as is shown in the sketch of the apparatus. 
Six or seven amperes were required to bring the temperature up 
from 20° C. to 600° C. in four or five hours. Direct current would have 
been somewhat preferable had it been available, but the alternating cur- 
rent used did not jar the coil sufficiently to do any damage. 
2. The Coal Cartridge. The cartridge in which the coal was placed 
” 
to be heated was about 24 ” inside diameter by 6” long. It was made up 
as shown in Fig. 1. (4) is a 23%” short nipple, having 24” couplings 
(5) screwed on to both ends. Into these couplings pipe plugs (6) were 
screwed, thus forming a closed cartridge. One plug was drilled and 
