302 



Messrs. Scott and Galloway on the Connexion [Apr. 18, 



which is filled down to the level d c with a mixture of air and gas. Gas 

 may flow into such a cavity from a fissure in the sides, or from any lower 

 part of the seam along the roof and under the edge at a. 



In fig. 2, d b c g is an unbratticed bord, receiving gas from the face b c, 

 filled with a similar mixture down to d c. The space occupied by the foul 

 air is shaded in each case ; and ef is a plane in that space which is chosen 

 arbitrarily. 



The plane d c, which bounds the lowest part of the accumulation, is 

 level, and here the mixture contains the least proportion of fire-damp ; the 

 impurity of the air increases with the height, and the foulest atmosphere is 

 found at b, the highest point of the cavity. 



If any gas flows into the cavity a corresponding volume of the contents 

 of the cavity is displaced and escapes into the ventilating-current. 



Diffusion is also constantly going on ; and as the result of this process is 

 to produce below the plane c d a mixture lighter than the ventilating- 

 current, this mixture rises along the roof and is carried away at C (fig. 1), 

 d(fig.2). 



Gas is therefore constantly being removed from the cavity, partly by 

 displacement, partly by diffusion, and its quantity is exactly equal to that 

 entering the cavity. 



If, now, the ventilating-current become fouled in any of the ways we 

 have described, the space below the plane c d will be filled with the same 

 mixture as that in the ventilating-current itself. 



If we suppose that the mixture in the cavity up to a certain level 

 ef be less foul, and therefore heavier than that which now forms the 

 ventilating-current, the volume of air &c. in the space defc will be 

 displaced, and no further escape of gas can take place from either ac- 

 cumulation until the whole of the contents above the line c d are more 

 foul than the mixture below cd. As soon as this condition is fulfilled, 

 diffusion and displacement will go on. 



In order, however, that the rate of diffusion may be the same as 

 before, the specific gravity of the whole mass must be reduced, for the 

 square roots of the specific gravities of the mixtures above and below 

 the plane cd must bear the same ratio to each other that they did 

 before the current became fouled. This reduction of the specific gravity 

 is, in other words, increased foulness of the air. 



If, then, the mixture in the space cbf was previously near the firing- 

 point, it is obvious that any impurity in the ventilating-current will 

 cause it to approach nearer to that point, and so eventually an explo- 

 sive mixture may be generated in a cavity while the ventilating-current 

 itself is non-explosive. 



It will be seen that by the above process a quantity of fire-damp may 

 be stored up in such a cavity, which can only escape very gradually, 

 after the ventilating-current has become purer. 



It follows, then, that if an explosive mixture has been formed in places 



