1869.] Cod Washing. 495 



to have at least three such tanks, each of which should be in 

 duplicate, so that one set of tanks may be kept at work whilst the 

 coal which has collected in the other set is being removed. Some- 

 times a fine mesh wire gauze, or finely perforated plate, is placed at 

 the outlet of the first setthng tank for the purpose of keeping back 

 all pieces of coal above a certain size, but it is very doubtful whether 

 such a precaution is necessary, as the larger pieces are sure to be 

 deposited in the first tank, whilst the finest of all will be found at the 

 bottom of the last settling tank. Even after all these precautions 

 have been taken to save as much of the coal as possible, the author 

 has witnessed instances where samples, collected promiscuously from 

 the residue which escapes with the water after it has passed through 

 the last setthng tank, have been found to contain seventy-five per 

 cent, of combustible matter, but it is certain that a small portion only 

 of this consisted of pure carbon. Under a judicious arrangement 

 of setthng tanks it will be almost invariably found that the de}io- 

 sited coal becomes more pure in each successive tank up to the 

 third, and that what is subsequently found to be held in suspension 

 by the water contains too large a proportion of impurities to render 

 it worth the trouble and expense of collecting. 



Too much attention can hardly be given to the construction of 

 settling tanks, whatever form of washing machine is employed, 

 since, in the first place, the fine coal passing away with the water 

 is the purest and best adapted for the manufacture of coke, and 

 secondly, unless this be carefully conserved the loss consequent upon 

 the operation of washing may be such as to make it very question- 

 able whether the cost is not out of all jDroportion to the benefits 

 otherwise obtained. Even in a well-arranged coal-washing esta- 

 bhshment the loss in weight by washing will often be found equal 

 to from twelve to fifteen per cent., consisting of the impurities 

 extracted, as well as a certain amount of small coal, which, as has 

 been before explained, will always escape with the waste water. 

 So far as the author's experience goes, there appears generally to 

 exist at coUieries a strong objection to devote a sufiicient extent of 

 ground to the proper construction of settling tanks. In setting 

 them out care should be taken that the tanks are made only of 

 such a width that they can be readily cleared out without the use 

 of wheel-barrows ; for this purpose they should not be more than 

 6 feet wide, and about 3 feet deep at the outside, and on either side 

 of each tank trams should be laid, at a level somewhat below the 

 surface, so that men may shovel the deposited coal directly into 

 waggons. The tanks also should not consist of one long narrow 

 trench each, as is most generally the case. For a large coal- 

 washing establishment such an arrangement would be very incon- 

 venient, and besides, it would not be found to work so well nor to 

 deposit so much coal as if each tank were made to consist of three 



