375 



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 

 could 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), 3i" 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 insulated 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 small water 

 rheostat. The amount of current 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 2\ " inside diameter by 6" long. It was made up 

 as shown in Fig. 1. (4) is a 2i" short nipple, having 2i" couplings 

 (5) screwed on to both ends. Into these couplings pipe plugs (G) were 

 screwed, thus forming a closed cartridge. One plug was drilled and 



