72 DRESSING OF ORES 



better or worse conductor of heat. These tubes, into which the crayons intended for 

 baking are to bo put, traverse horizontally the laboratory, B, of the furnace, and ;iro 

 supported by two plates c, pierced with six square holes for covering the axles of the 

 tubes A. These two plates are hung upon a common axis D ; one of them, with a 

 ledge, shuts the cylindrical part of the furnace, as is shown in the figure. At the 

 extremity of the bottom the axis D is supported by an iron fork fixed in the brick- 

 work ; at the front it crosses the plate c, and lets through an end about 4 inches square 

 to receive a key, by means of which the axis D may be turned round at pleasure, and 

 thereby the two plates c, and the six tubes A, are thus exposed in succession to the 

 action of the fire in an equal manner upon each of their sides. At the two extremities 

 of the furnaces are two chimneys K, for the purpose of diffusing the heat more equally 

 over the body of the crayons. v,fig. 640, is the door of the fire-place, by which the 

 fuel is introduced; o, fig. 641, the ash-pit ; H, the fire-place ; i, holes of the grain 

 which separate the fire-place from the ash-pit; K, brick-work exterior to the fanner. 



General Lomet proposes the following composition for red crayons. He takes the 

 softest hematite, grinds it upon a porphyry slab, and then carefully elutriates it. 

 He makes it into a plastic paste with gum arabic and a little white soap, which ho 

 forms by moulding, as above, through a syringe, and drying into crayons. The pro- 

 portions of the ingredients require to be carefully studied. 



CRAYONS or CHALKS, lithographic. Various formulae have been given for the 

 formation of these crayons. One of these prescribes white wax, 4 parts ; hard tallow- 

 soap, and shellac, of each 2 parts ; lamp-black, 1 part. Another is, dried tallow-soap 

 and white wax, each 6 parts ; lamp-black, 1 part. This mixture being fused with 

 a gentle heat, is to be cast into moulds for forming crayons of a proper size. Sec 

 LITHOGRAPHY. 



DRESSING OP ORES. In metalliferous veins the deposits of ore are extremely 

 irregular, and usually intermixed with gangue or vein-stone. In excavating the 

 lode, it is usual for the miner to effect a partial separation of the valuable from the 

 worthless portion ; the former ho temporarily stows away in some open place under- 

 ground, whilst the latter is either employed to fill up useless excavations, or in duo 

 course sent to surface to be lodged on the waste heaps. From time to time the valu- 

 able part of the lode is drawn to the top of the shaft, and from thence conveyed to the 

 dressing floors, where it has to be prepared for metallurgic treatment. 



This process is known as Dressing, and in the majority of instances includes a 

 scries of operations. Hand labour, picking, washing, sizing, and reducing machinery, 

 together with water-concentrating apparatus, comprise the usual resources of the 

 dresser, but sometimes he may find it useful to have recourse to the furnace, since it 

 may happen that, by slightly changing the chemical state of the substances that 

 compose the ore, the earthy parts may become more easily separable, as also the 

 other foreign matters. With this view, the ores of tin arc often calcined, which, by 

 separating the arsenic and oxidising the iron and copper, furnishes the means of 

 obtaining, by subsequent washing, an oxide of tin much purer than could be other- 

 wise procured. In general, however, these are rare cases ; so that the washing 

 almost always immediately succeeds the picking, crushing, or stamping processes. 



Before entering upon the description of machinery employed in the concentration 

 of ores, it is important to notice the principles upon which the various mechanical 

 operations are based. 



If bodies of various sizes, forms, and densities be allowed to fall into a liquid, in a 

 state of rest, the amount of resistance which they experience will be very unequal, 

 and consequently they will not arrive at the bottom at the same time. This neces- 

 sarily produces a sort cf classification of the fragments, which becomes apparent on 

 examining the order in which they have been deposited. 



If it be supposed that the substances have similar forms and dimensions, and differ 

 from each other in density only, and it is known that the resistance which a body will 

 experience in moving through a liquid medium depends solely on its form and extent 

 of surfaces, and not on its specific gravity, it follows that all substances will lose 

 under similar circumstances an equal amount of moving force. 



This loss is proportionally greater on light brxlirs than in those having more con 



siderable density. The former for this reason fall through the liquid which less rapidity 

 than the denser fragments, and must therefore arrive later at the bottom, so that the 

 deposit will be constituted of different strata, arranged in direct relation to their 

 various densities, the heaviest being at the bottom, and the lightest at the top of 

 the series, 



Supposing, on the contrary, that all the bodies which fall through the water possess 

 similar forms and equal specific gravities, and that they only differ from each other in 

 point of volume, it is evident that the rapidity of motion will bo in proportion to their 

 sizes, and the larger fragments will be deposited at the bottom of tho vessel. 



