408 



near them, or to send forth their gases upon any change in 

 the atmospheric pressure around them. 



To understand the danger of goafs it is necessary to know 

 that fire-damp is a compound of hydrogen and carbon, or 

 one volume of vapour of carbon and two volumes of hydrogen 

 condensed into one volume. Thus in numbers : — 



Specific Gravity. 



1 volume of gaseous carbon = 0.4 1 66 0.75= I prime. 



2 „ hydrogen = 0. 1 388 (0. 1 25 X 2) = 0.25 = 2 „ 



0.5554 1.00 

 This explosive mixture sometimes also contains a small 

 quantity of olefiant gas and of sulphuretted hydrogen gas, 

 and the mixture when pure is about half the weight of that 

 of air ; and, as Sir H. Davy has stated, any mixture of air 

 containing from J to tV of the gas will explode. These 

 mixtures are from 6 to 17 times greater in volume than the 

 fire-damp in them, and evidently not much lighter than air, 

 (0.91 and 0.96.) Hence to take away one cubic foot of fire- 

 damp, is to prevent the formation of from 6 to 15 feet of 

 explosive mixture. When sub-carburetted hydrogen is mixed 

 with twice its volume of oxygen and exploded, we obtain 

 exactly its own bulk of carbonic acid, while water is precipi- 

 tated, so that a perfect choke-damp is formed by the explosion. 

 Of the two* volumes of oxygen, one remains gaseous in the 

 carbonic acid, and another is condensed with two volumes of 

 hydrogen into water. 



The explosion at Haswell was from the firing of a goaf 

 13 acres in extent, the thickness of the seam of coal being 

 51 feet ; and as the country had not generally sunk in, there 

 was supposed to have been left a vault 5i feet high, and 13 

 acres in extent. The coal seam was not quite horizontal, 

 but rose 1 in 24. Messrs. Lyell and Faraday suppose that 

 at the lower edge of this goaf nothing but air might be 

 present, and likewise for a considerable distance up into the 



