184 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



become vastly extended by the presence of coal dust, and it 

 11 takes a very small proportion of fire damp or carbon-mon- 

 oxide explosive that would otherwise be harmless. Speaking 1 

 generally, a gas explosion develops centres of greatest vio- 

 lence in those localities where gas issues from the strata or 

 tends to accumulate, and the centres of violence are more pro- 

 nounced than in a dust explosion. A dust explosion feeds on 

 material scattered in its path, and follows those passages 

 that promise the largest supply of food in the form of 

 dust and oxygen. One of the characteristics of a dust ex- 

 plosion is the persistence with which it seeks the intake, and 

 advances against the current in its search for air. The flame 

 never enters far in the direction of the return air, because 

 it is extinguished by the products of its own combustion: gas, 

 on the other hand, is most likely to be found in the return 

 i\ir ways. At the time of the Bulli disaster, the return air was 

 made to ventilate other headings instead of allowing it to 

 pass over the main tunnel direct to the return air way. Sec- 

 ondary or back explosions are generally attributed to com- 

 bustible gases developed from highly heated coal dust, which 

 remains uiiburnt, also to gas sucked out of the faces 

 of the coal by the partial vacuum resulting from the ex- 

 plosion, or liberated by falls of roof. As soon as fresh air 

 rushes in, it forms an explosive mixture with these gases, 

 and a second explosion is liable to occur. It is known that 

 dust explosions have taken place in certain flour mills, where, 

 .so far as one is aware, there are not explosive gases, therefore" it 

 is quite possible that a pure dust explosion may occur in cer- 

 tain coal mines: but, under existing conditions, it is doubtful 

 whether a pure dust or pure gas explosion ever takes place 

 in a colliery, except on a minor scale, for one tends to create 

 the other. 



The days of this colliery are nearly numbered, most of 

 the coal that is won being taken from pillars. The coal in 

 this colliery is much disturbed by volcanic intrusions, which 

 occur as dykes and laccolites. The hot rock lias driven off 

 much of the volatile hydrocarbons, leaving a so-called natural 

 coke, which finds a limited market. 



The main surface buildings are located at Old Bulli, and 

 from here the seam is reached by two tunnels. About a mile 

 further north is another tunnel, known as No. 8. There is no 

 rope haulage to the surface from this tunnel, the skips being 

 drawn out by horses. At Old Bulli the rails of each tunnel 

 have a different gauge, so where the lines have a common 

 course at the surface the track is laid with three rails. The 

 skips are run into end kick-ups, which empty their contents 

 on to stationary screens. So that two skips shall not turn off 

 towards the same kick-up at one time a foot point is pushed 



