1 74 //u/elt, Mack and Smyth — 



Art. XII. — The Moisture Content of some Typical 

 Coals; by G. A. Hulett, E. Mack and C. P. Smyth. 



In a previous article "On the Moisture Content of 

 Coal and some Ideas on the Genesis of Coal." 1 we 

 obtained results which were readily interpreted by 

 assuming that coal is largely in the colloidal state with 

 water as the external phase and absorbed on such a large 

 surface that a measurable portion of it is not removed 

 in the usual analytical method of heating to 110° in a 

 drying oven nor even by any process of vacuum desicca- 

 tion at ordinary temperatures. Indeed it was found 

 necessary to heat coal to near 300° C, with a continual 

 removal of the water vapor, in order to secure results 

 which represented the moisture content of the coal. It 

 is probable that all water, which is present as such in 

 coal, is not removed until a temperature of some 365° C, 

 the critical temperature of water, is reached; but the 

 amount of water retained by a coal after heating for an 

 hour or so at 250° to 300° is, we believe, quite negligible. 

 The constitutents of coal decompose or "crack" at all 

 temperatures but at ordinary temperatures the reactions 

 are so slow as to require geologic periods to produce 

 measurable results, and even for temperatures well over 

 200° the rate of these reactions is still so slow 2 that it 

 seemed quite possible to heat coal for the time and tem- 

 perature needed for our purpose without causing a 

 noticeable decomposition or the formation of a measur- 

 able amount of water. In view of this situation it has 

 been of interest to make a more detailed study of some 

 typical coals. 



Coal in the uncovered seam always appears to be 

 "dry" but immediately it is broken down and exposed 

 to the mine air, there begins a loss or gain of moisture, 

 depending on the kind of coal, temperature, humidity of 

 the mine air, &c. Furthermore, it has been shown 3 that 

 coal, like other colloidal substances, does not possess a 

 definite vapor pressure. When water was removed from 

 a coal by lowering the vapor pressure, the coal showed a 

 continually decreasing vapor pressure as it lost water. 



1 This Journal, 43, 89, 1917. 



2 L. c, p. 103. 



3 Ralston and Porter, Tech. Paper 113, U. S. Bureau of Mines. 



