382 
trace of the tar appears. in the same sense that water has a boiling tempera- 
ture. The first deposit is so indistinct that it is almost impossible to tell 
whether there is a deposit or not. The increase is also so very gradual 
that it is difficult to choose the temperature at which to report the firsi 
appearance of a deposit. This gradual increase will extend over a tempera- 
ture range of from 50° to 150° C. before there will be a sufficient deposit 
of tar to feel sticky to the finger. The determination of the temperatures 
between which the maximum deposit occurs is likewise an arbitrary 
matter, and also the temperature at which the last trace of tar appears. 
Therefore the results as here reported must not be too literally interpreted. 
However they are a very careful estimate of the facts as they are and the 
highest temperature reported is one at which one may feel assured that 
the very last trace of tar has disappeared from the coal. 
