381 



the therrao couple indicated a temperature close to that at which there 

 was prospect of an oily deposit, the paper rolls were put in place, the 

 paper marked and the test started. When the tar commenced to show up 

 the temperature of the furnace was recorded and the pipe (15) changed 

 every 10 minutes. As the temperature of the furnace rose to 300° C. the 

 current was increased i of an ampere, at 400° C i of an ampere more, and 

 at 500° G. J of an ampere. This was done to take care of the increased 

 radiation, and to keep the rise in temperature at a constant rate. When 

 the paper ceased to show any signs of a tar deposit it was again marked 

 and timed and the current shut off. The strip of paper was then cut up 

 into lengths corresponding to 10 minute periods, and carefully weighed. 

 As the weight of the paper per inch was very constant, the excess in weight 

 over that of clean paper was in each case due to the tar. From this two 

 curves could be drawn, with time plotted horizontally, while one had 

 temperature centigrade and the other grams of tar plotted vertically. 

 These curves when placed one right over the other, as here given in Fig 3, 

 indicate the amount of tar coming off at each temperature. The points 

 where the tar starts and stops can not be indicated by this curve, as 

 the ends of the deposit are too thin to have appreciable weight. They are 

 consequently separately noted elsewhere. 



The first condensible gas to be driven off from the coal and appear 

 on the paper record is water vapor. After the last of the water has 

 disappeared there is quite a temperature range through which there is 

 no deposit at all. Then the paper will begin to show a slight trace of 

 oil. This will gradually increase in amount and give the paper the ap- 

 pearance of having been parafined. The deposit will then gradually as- 

 sume a brownish color, as though engine oil were appearing. Later a 

 temperature will be reached at which the deposit will increase very 

 rapidly in amount, and will assume a distinctly tarlike appearance. The 

 first tar to be deposited is usually very soft and sticky at room tempera- 

 ture. As the temperature rises the tar becomes steadily stiffer, until it 

 is finally hard and brittle when cooled. The temperature range through 

 which the maximum deposit occurs will vary from about 100° C, for some 

 western coals, to 175° for some of the samples from the east. At the 

 higher limit of this range the deposit becomes rapidly smaller in amount 

 until it is too small to weigh, but the paper is still distinctly browned. 

 This discoloration becomes less and less plain, until it finally disappears 

 entirely. There is no definite temperature at which the first and the last 



