29 



holes are forced through the covering on the second or third 

 day, particularly on the lee side. 



A bursting forth of gases of a blueish hue indicates active 

 burning, and under such circumstances the access given to 

 the air must be closed, while new perforations are made in 

 any yet inactive portion of the meiler. 



Over-great activity of fire is suppressed by water applied 

 to the covering, or by adding to the thickness of the latter. 

 Strong sinking of the cover during the earlier burning proves 

 more or less complete combustion of the coal, and it may then 

 become necessary to refill hurriedly the holes with wood or 

 coal, underclosure of all openings, and careful restoration of 

 the cover thus temporarily removed on one spot. This refill- 

 ing in large meilers may be required for five days in succes- 

 sion ; but the more carefully the mound has been built, and 

 the more watchfully the early glowing process has been con- 

 ducted, the less necessity will arise for the troublesome and 

 wasteful process of refilling. A final additional covering 

 becomes frequently needful. The operation closes by the 

 sinking of the cover, or by its being partially forced down- 

 ward, and the ready coals are removable one day afterwards. 

 Partial withdrawals of coal can be effected from the lee side 

 while the meiler is still active. 



[The lecturer here described the process of charcoal-burn- 

 ing-] 



The specific gravity of charcoal stands generally in a precise 

 proportion to the specific weight of the wood employed. 

 Dryer wood realises a heavier, moister wood a lighter coal. 

 {Slow combustion also renders the coal heavier than a more 

 rapid burning process, because in the latter case more carbon 

 is consumed for various volatile products formed from the 

 wood. As a rule, the quantity of coal obtained is about a 

 quarter of the weight of wood employed. Good coal has a 

 slight metallic lustre, is firm, not friable, causing a clear 

 sound when thrown on the ground. It must burn without 

 flame and smoke. For trade purposes coal must be kept dry, 

 as its absorption of humidity is considerable.* The heating 

 power of coal as compared to wood is ascertained to be as 100 

 to /55 or 60. An equal volumen of wood produces less heat- 

 ing effect than the same space of coal. For technic operations 

 the equable and more lasting heat, and the great power of 



* For extensive details consult Von Berg's Anleitung zum Ver- 

 kohlen; also Muspratt's Chemistry* 



