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it into the large air tank, where it was stored for use. By this means 

 the supply of gas for a complete test could be stored up before the test 

 itself was started. One more precaution had to be taken in using this gas, 

 as it could not be allowed to affect the temperature of the coal as it 

 passed through. To prevent this, a coil of pipe was placed over a gas 

 flame and the gas passed through and heated up to the temperature of 

 the coal, before it was allowed to enter. The temperature of the gas was 

 measured by a thermo couple which extended into it through a tee in the 

 pipe line. To make assurance doubly sure the end of the cartridge itself 

 was filled with steel chips, as is shown at 22 in the sketch in Fig. I. The 

 entering gas was thus forced to pass through a considerable volume of 

 these chips before coming into contact with the coal. It was found difficult 

 to heat the gas up to the highest temperature of the producer. This 

 might tend to affect the seeming temperature at which the last traces of 

 tar appear. The tendency of the gas would always be to be lower than 

 that of the coal. For this reason the end of the thermocouple was placed 

 in the coal at the end where the gas enters it, and therefore at its cooler 

 end, in case there should be any difference at all. Thus the temperature 

 reported as the one at which the last trace of tar appears is as accurate 

 as it is possible to make it. 



5. To test the gases for tar. The next problem was to find a means 

 of subjecting the gases from the coal to a continuous test for tar. The 

 most searching and satisfactory test known to the author, and the one 

 used by the gas companies over the country, is to allow a small stream 

 of the gas to strike a piece of white paper at a high velocity. If there 

 is any trace of the tar at all in the gas, it soon leaves a spot on the paper. 

 This test was adopted. To use it, the gas must be cooled down before it 

 strikes the paper. This was accomplished by keeping a cloth filled with 

 cold water constantly lying on pipes (12) and (18). It was desired to 

 have the test continuous. The device used to accomplish this is shown 

 in Figs. 1 and 2. The rollers (18) are about 10" in diameter and are sup- 

 ported by steel rods through their centers which turn freely in iron sup- 

 ports at either side. On one end of each of these steel rods was placed a 

 small wooden spool, around which was wound a cord, supporting weights Wi 

 or W 2 . A long strip of cloth was wound around one of these rollers and 

 its end started around the other. A piece of paper ribbon was wound on 

 with the cloth. Wi and W 2 tend to turn the rollers in opposite directions, 

 thus keeping the cloth and paper strip tight. Wi is enough heavier than 



