396 Prof. Johnston on certain Mineral Substances, ^c. 



bustible matter may be anticipated ; while the probability of 

 such escape affords a rational explanation of those sudden and 

 unexpected emissions of gaseous matter which have occasion- 

 ally been followed by consequences so disastrous*. 



An observation familiar to practical men in the English 

 coal fields leads to the same conclusion. In mines where can- 

 dles or open lamps are used, it is by the appearance of the 

 flame that the miner judges of the purity of the atmosphere, 

 and the presence of combustible matter. When little inflam- 

 mable gas is mixed with the air, the flame carries over it a very 

 short pale blue head^ which increases in length as the quantity 

 of the carbo-hydrogen increases, until the whole atmosphere 

 becomes one explosive mixture. But in different coal fields, 

 the length of head, as it is called, which indicates an approach 

 to the explosive state, is very different. In the Newcastle and 

 Leeds coalfields 1 finches indicate danger; in S. Wales 4 or 

 5 in. are not unusual. The colour of the head is also a criterion 

 by which the miner judges; when blue, combustible matter is 

 present, and an explosion is to be feared ; if brown and muddy, 

 carbonic acid is suspected, and the danger is less. 



Though no particular conclusions can be drawn from these 

 observations, yet the general result does force itself upon us, 

 that various compounds of carbon are at different times present 

 in the atmosphere of coal mines and in various quantities ; 

 and that sudden explosions may often be caused by the escape 

 from cavities in the coal strata of other compounds than that 

 usually called the fire damp, and to which all the mischief is 

 usually attributed. 



Durham, March 1838. 



Note. — I have just seen in the possession of Prof. Graham, of University 

 College, a candle formed of a substance said to be found in considerable 

 quantity in the coal mines near Linlithgow in Scotland. It resembles in 

 every respect the Ozocerite candles of Moldavia. The substance is dull 

 brown, and after fusion almost black, reflected and reddish brown by 

 transmitted light; mass opake but translucent at the edges and in thin 

 layers ; is greasy to the touch (like Hatchetine), easily scratched by the 

 nail, has a conchoidal fracture, and when cold has no perceptible smell. 



[ may here mention also that the Middletonite described in a former 

 paper, has since been met with in the mass of the coal in the Newcastle 

 coal-field. May not this substance be the resin of the trees of the car- 

 boniferous aera more or less changed ? — April 16. 



[* Another explanation had previously been given by Mr. Hutton,in fol- 

 lowing up an idea originally suggested we believe by Dr. Paris : see L. and 

 E. Phil. Mag., vol. ii. p. 303 ; and Paris's Life of Davy, p. 395. Both are 

 probably true. — Edit.] 



