GAS-MAKING AND OTHER COALS 371
pipe, in which the greater water velocity creates a zone of maximum lifting effect, below which only the lieavier particles can descend, and in the cone itself the decreasing velocity produces a minimum lifting effect, being just sufficient to float the lighter particles over the sili into the shoot.
Coal after washing is usually allowed to drain in bunkers and to dry in air, and although many attempts have been made to effect artificial drying, by means of heat and the centrifuge, the method has not developed to any extent in this country. As regards the proportion of water in washed coal, this depends largely
Fie. 259.— The Dkawb Coal Washbb.
upon the size of the coal, but it may be as much as 10 per cent, above that in the normal sample.
THE STORAGE, SPONTANEOUS COMBUSTION, AND DETERIORATION OF COAL
The problem of the spontaneous combustion of coal is one which the gas engineer is continually called upon to face, owing to the faet that the bituminous coals used on gasworks suffer most in. this respect. Camiel coals are less affeeted, and steam coals and anthracite are least liable of any. Originally, there was a general opinion among gas engineers that .spontaneous ignition was due to the oxidation of the ‘brasses” or pyrites in the* coal, and that the action was greatly accelerated by