Chap. 5-1 Ignes Fatul and Fire .T)amp* 393 



inverted bottle, fo Js to permit the bubbles of air 

 which rife to be received in the inverted bottle ; and 

 this air will be found Co be inflammable. 



The ignes fatui are fuppofed to proceed from the 

 inflammable air which abounds in marfhy grounds, 

 and to be fet on fire by eledbric fparks. 



This gas, as well as fixed air, was long known to 

 miners before it was noticed by philofophers ; and 

 among the colliers and other workmen of that clafs, it 

 obtained the name of the fire damp. It is, however, 

 feldom found pure in mines or coal works, but is gene- 

 rally combined with fulphureous matter, or what is 

 called hepatic gas, or with carbonic acid air j and this 

 admixture varies its fpecific gravity, and in general 

 renders it fomething heavier than pure inflammable 

 air. The fire damp generally forms a whitifh cloud 

 in the upper part of the mine, and appears in fome- 

 thing of a globular form ; from its levity it will not 

 mix with the atmofpheric air, unlefs fome agitation 

 takes place ; and it is difpofed to lodge in any little 

 cavity in the fuperior part or roof of the mine. 

 When it appears in this form, the miners generally 

 fet fire to it with a candk, lying at the fame time 

 flat on their faces to efcape the violence of the 

 fhock. It will not, however, take fire unlefs in 

 contact with atmofpheric air, for the obvious rea- 

 fon, that a mixture of oxygen ga$ is necefiary to 

 its inflammation. The danger arifes entirely from 

 its inflammability on the approach of any ignited body, 

 for when the fire damp confifts of pure inflamma- 

 ble air, the explofion is like that of gunpowder; 

 but when it is mixed with carbonic acid, it burns 

 with a lambent flame. The eaficft and fafeft me- 

 thod, therefore, of clearing the mine from this for- 

 midable fluid is by leading a long pipe through the 



fhaft 



