FUEL BY-PRODUCTS 57 



these gases may have time to pass away before the crop 

 begins to grow. 



Soot. Soot has long been known and highly esteemed 

 as a useful manure. It is obtained by the combustion of 

 either coal or wood but the major part of that obtained 

 to-day owes its origin to the combustion of coal. When 

 coal is burnt in a fireplace the greater part burns away, but 

 some portions are only incompletely burnt, and pass up the 

 chimney to form minute particles of smoke. Soot consists 

 of the deposit of these minute particles of carbon, tar, ash 

 and other products of imperfect combustion. Some of the 

 nitrogen in the coal is evolved as ammonia, and much of 

 the sulphur as sulphur dioxide. The two passing together 

 up the chimney, oxidize and condense as ammonium 

 sulphate. Ordinary soot, which is usually acid to litmus, 

 consists mainly of carbon, tar and mineral matter with small 

 quantities of nitrogen and sulphur compounds. Analyses 

 of soot show very great variations in composition, but this 

 is not to be wondered at, considering the great variation of 

 conditions under which coal is burnt. Soot is a product of 

 incomplete combustion, and its composition will therefore 

 depend upon the particular degree of incompleteness with 

 which the fuel has been burnt. The degree of draught up 

 a chimney will also have a powerful influence on the amount 

 of mineral dust carried forward. With the high tempera- 

 tures and strong draughts of a factory furnace, a large 

 proportion of the soot is produced by the mechanical removal 

 of small particles due to the strong draught ; but in the 

 domestic fireplace, where the draught is small, it is the 

 incomplete combustion of the fuel to which must be at- 

 tributed the larger part of the materials found in the soot. 

 Further, the actual distance from the fire to the place where 

 soot is deposited will cause a great variation in the composi- 

 tion of the soot, since the draught is a cause of the separation of 

 the constituents of the soot and the temperature of the place 

 of deposition will control the proportions of partly volatile 

 substances deposited thereat. Table 10 gives the result 

 of examining soots produced from the same coal under 



