On Underground Temperatures. 



17 



minerals, which is of rare occurrence in coal mines, and the heat 

 arising from the men, horses, and lights, which can only be of im- 

 portance in shallow mines, for in deep mines it is rarely that that 

 source of heat brings up the temperature of the air to the normal 

 temperature of the rock, there are few causes likely to produce an 

 abnormal rise of temperature. There is one, however, which though 

 of exceptional occurrence, should be noticed in connexion with this 

 subject, and that is the heat produced by the crushing of the rocks 

 which sometimes takes place in coal mines. 



The Crushing of Rock. — It is certain that in coal mines where pillars 

 are crushed and the strata pressed up in creeps, more or less heat is 

 liberated. No experiments, however, have been made to determine 

 what, in these cases, are the exact effects produced, but some of tho 

 evidence given before the Coal Commission fully establishes the 

 general fact. Mr. Elliot,* in speaking of one of the pits in South 

 Wales, mentioned that when " a creep takes place, he has known the 

 temperature very much increased," and in one case where " the pres- 

 sure began to crush the pillars, the heat produced was so great that 

 he feared it would set fire to the coal." In some cases the pressure 

 has been such as "to grind the rock to powder, like the effect of a dozen 

 mill-hoppers, and this was accompanied by considerable heat." He had 

 often found the air very hot when a sort of temporary creep occurred. 



Escape of Gas. — On the other hand, a cause productive of a loss of 

 heat is a more constant disturbing cause. There are few coals which 

 do not give off gas when first exposed. In some seams it may be 

 observed exuding from the freshly broken surfaces with a hissing 

 sound; and, if in large quantity, as in the case of the so-called 

 "blowers," or sometimes near faults, it issues with a rushing noise 

 like the steam from a high-pressure boiler. Some of these blowers will 

 be exhausted in a few minutes, others will last for years — like that at 

 Wallsend, which gave off 120 feet of gas per minute. f The common 

 gas on these occasions is light carburetted hydrogen. It must exist 

 in the coal under the enormous pressure either highly condensed, if 

 not in a liquid state, otherwise it is hardly conceivable how the dis- 

 charge could be maintained so long. The pressure of this gas is said 

 to equal sometimes 300 to 400 lbs. to the square inch. J 



In any case the escape of this gas from the coal, in which it appa- 

 rently exists in an infinite number of small cavities, must be to 



* Coal Commission Report " On Possible Depths of Working," p. 112. 

 f " Coal and Coal Mining," p. 204. 



% [Sir Frederick Abel states that, if cavities are bored into tbe coal and plugged, 

 the gas speedily accumulates so as to exercise a pressure of several hundred 

 pounds upon the square inch, as indicated by pressure-gauges fixed into the cavities. 

 " Nature," December 3rd, 1885. Under this enormous pressure we do not know 

 what the critical point may be. — January, 1886.] 



VOL. XLI. C 



