PROPERTIES OF FOREIGN SUBSTANCES. 21> 



clinker. For domestic purposes, where the temperature is 

 low, the quantity of an ash is of more importance than its 

 fusibility. 



The percentage of ash may vary from two to thirty-six, 

 but the best fuel should not contain more than 7 per cent., the 

 medium more than 14 per cent., while that which goes over 

 14 per cent, is looked upon as poor. The proportion of ash is 

 somewhat more in slack than in lump coal from the same 

 seam. 



When iron pyrites occurs in a coal, since 3 atoms of oxy- 

 gen replace 4 atoms of sulphur, the weight of the ash is less 

 than the weight of the mineral matter in the coal by 5-8ths r 

 the weight of the sulphur of the pyrites. Corrections for this r 

 however, are not made in proximate analysis. 



Nitrogen in a state of chemical combination is always 

 found in coals, its percentage generally being between one and 

 two per cent. 



Sulphur. This element generally occurs in coals as iron 

 pyrites, but may also be present as gypsum, or even baryte 

 It is the sulphur of the pyrites that is hurtful for most econo- 

 mic purposes, for it. is readily given off as sulphur dioxide, or 

 sulphuretted hydrogen, while the sulphur of gypsum and 

 baryte is fixed in the ash. When present in gas or household 

 coals, the sulphur tarnishes silver and silver prints; when used 

 for boilers it corrodes iron and copper, and it is bad for forge 

 work. Pyrites is also disliked by coal miners, for the nodules 

 are harder to work out than coal, and pyrites is often credited 

 with being the cause of spontaneous combustion in coal. 



Phosphorous. This has been found in the ashes of coals 

 combined with one or more of the bases present: as this tends 

 to pass into pig-iron when smelting iron ores, the absence 

 of phosphorous is an important matter when the iron is re- 

 quired for the Bessemer process of steel-making. 



Coke is the residue left after driving off moisture and vola- 

 tile matter: it is therefore richer in ash than the coal from 

 which it is made. All the sulphur is not driven off by coking, 

 so it is often advisable to get rid of as much of this and also 

 of the ash present in the original coal as possible by washing, 

 since the presence of these substances may have an important 

 bearing on the production of a coke for blast furnace or foundry 

 purposes. Coke has, weight for weight, more carbon than the 

 fuel from which it was obtained ; and also a higher calorific 

 value. Coal loses on an average about l-3rd of its weight in 

 coking, and increases about l-10th in bulk. 



The specific gravity of a coal is important, and may be due 

 either to combustible matter or impurities that form ash. The 

 specific gravity of anthracite is greater than that of bitumin- 

 ous coal, and in consequence it takes up 10 to 15 per cent, less- 



