SPONTANEOUS COMBUSTION. 13- 



He proposes the above structural formula, not as ex- 

 pressing' the exact constitution of the compound, but as an 

 expression of the iron in the molecule, and as embodying in a. 

 quantitative way the results of his investigation into its con- 

 stitution. It will be noticed that the sulphur of the Fe u S^ is 

 made to link entirel}* with iron. He considers the ferric iron 

 is most likely Fe lii ; anyhow, he thinks that the simplest way 

 to regard it. 



Fires may occur in coal seams from natural causes, even 

 in cases where the seam has never been worked. In 1838, Sur- 

 veyor-General Mitchell, referring to the so-called burning 

 mountain, Mount WingenJ, stated that there was evidence of 

 the fire having existed for a considerable time previous to his 

 visit. This fire is still burning, its progress being indicated 

 by cracks taking place in the overburden on ahead, and a 

 subsidence of the ground behind the fire. This coal seam be- 

 longs to the Greta series of the Lower Coal Measures, and from 

 the fused condition of the rocks about it at Cessnock, it has 

 evidently been alight there also. This seam appears to be 

 very liable to spontaneous combustion, for the Homeville Col- 

 liery, in the Maitland District, was sealed down for fourteen 

 years on account of a fire, and when re-opened, it fired again. 

 The Greta and East Greta Collieries have also had cases of 

 fire. J. E. Carne !i writes: "On the edge of the Coal 

 Measures, south-east of portion thirty-eight, Parish Coco, 

 County Roxburgh, the second lowest coal-seam about one 

 hundred and ten feet above the base of the Marangaroo Con- 

 glomerate is on fire over a small area, probably having been 

 ignited by a burning root during a bush fire, or, possibly, by 

 decomposition of secondary pyrites by infiltration of water." 



Coal, strictly speaking, is not a mineral, for the definition 

 of a mineral is a "natural homogeneous, inorganic substance, *' 

 and coal is of organic origin. Nevertheless, as it occurs in 

 nature like a mineral, it is convenient to look upon it as 

 such. A satisfactory definition of coal has not yet been 

 made, for coal is not uniform in composition, or physical fea- 

 tures, and there is no sharp line between the different types 

 of coals, which run one into the other. For the sake of con- 

 venience, we may look upon coal as being a solid fuel, which 

 occurs in seams, formed by the fossilised remains of organic 

 matter. This would exclude oil and natural gas, since these 



t "Three Expeditions into the Interior of Australia. 

 I.p. 23 (London, 1838). 



!l"The Kerosene Shale Deposits of New South Wales," 

 p. 136 Sydney (1903). 



