But when the reverse is the case, and the valuable constituents through their softness get reduced to a fine pulp long before the other parts, the ordinary operations of the ore-dresser become much more difficult to carry out. Most elaborate ore-reduction plants are constructed with the view to causing the crushing surfaces, whether of rolls or of jaws, to merely tap each piece of stone so as to break it in bits without creating much dust. This operation is repeated over and over again; but the stuff which is fine enough to go to the concentrator is removed by sieving after each operation of the kind; and the successive rolls or other crushers are set to a finer and finer gauge, so that there is a progressive approach to the conditions of coarse sand, which is that specially desired by the ore-dresser.
Much of this elaboration will be seen to be needless, and, moreover, better commercial results will be obtained when it is more clearly perceived that the recovery of a valuable ore in the form of a fine slime may be economically effected by the action of grinders specially constructed for the purpose of permitting the hard constituents of the ore to remain in comparatively large grains, while the other and softer minerals are reduced to fine slimes or dust. In other words, a grinding plant, purposely designed to carry out its work in exactly the opposite way to that which has been described as the system aimed at in ordinary crushing machinery, has its place in the future of metallurgy. Light mullers are employed to pound, or to press together, the crushed grains for a given length of time, and then sieving machinery completes the operation by taking out the dust from the more palpable grains.
In some cases it will be found that an improvement can be effected by bringing about the separation of a finer grade of dust than could be taken out by any kind of sieve which is commercially practicable on the large scale. This is more particularly the case in regard to sulphide ores containing very friable constituents carrying silver. A fine dry dust-separator may then be employed constructed on the principle of a vibrating sloping shelf which moves rhythmically, either in a horizontal circle or with a reciprocal motion, and which at the same time alters its degree of inclination to the horizontal. When the shelf is nearly level its vibration drives the coarser particles off; but the very finest dust does not leave it until it assumes nearly a vertical position. A large nest of similar shelves, set close to, and parallel with, one another, can separate out a great quantity of well-dried slimes in a very short space of time.
CHAPTER IX.
DOMESTIC.
The enormous waste involved in the common methods of heating is one of the principal defects of household economy which will be corrected during the twentieth century. Different authorities have made varying estimates of the proportion between the heat which goes up the chimney of an ordinary grate, and that which actually passes out into the room fulfilling its purpose of maintaining an equable temperature; but it cannot be denied that, at the very least, something like three-fourths of the heat generated by the domestic fires of even the most advanced and civilised nations goes absolutely to waste—or rather to worse than waste—because the extra smoke produced in creating it only serves to pollute the atmosphere. In the cities some degree of progress has been made in the introduction of heating appliances which really give warmth to a room without losing at least seventy-five per cent. of their heat; but in the country districts, where open fireplaces are the rule, it is not unusual to find that more than ninety per cent. of the heat produced behind the domestic hearth goes up the chimney.
Sentiment has had a great deal to do with retarding progress in the direction of improved house-heating appliances. For countless ages "the hearth" has been, so to speak, the domestic altar, around which some of the most sacred associations of mankind have gathered, and popular sentiment has declared that it is not for the iconoclastic inventor or architect to improve it out of existence, or even to interfere seriously with either its shape or the position in the living room from which it sheds its genial warmth and cheerfulness around the family circle. A recognition of this ineradicable popular feeling was involved in the adoption of the grate, filled with glowing balls of asbestos composition, by the makers of gas-heating apparatus. The imitation of the coal-filled grate is in some cases almost perfect; and yet it is in this close approximation to the real article that some lovers of the domestic fuel-fire find their chief objection, just as the tricks of anthropoid animals—so strongly reminiscent of human beings and yet distinct—have the effect of repelling some people far more than the ways of creatures utterly unlike man in form and feature.
Taking count of the domestic attachment to a real fuel-filled fireplace or grate as one of the principal factors in the problem of domestic heating, it is plain that one way of obviating the waste of heat which is at present incurred, without doing violence to that sentiment, is by making better use of the chimney. The hot-air pipes and coils which are already so largely used for indoor heating offer in themselves a hint in this direction. Long pipes or coils inserted in the course taken by the heated air in ascending a chimney become warm, and it is possible, by taking such a pipe from one part of the room up the passage and back again, to cause, by means of a small rotating fan or other ventilating apparatus, the whole of the air in the chamber to circulate up the chimney and back again every few minutes, gathering warmth as it goes. In this way, and by exposing as much heating surface to the warm air in the chimney as possible, the warmth derived by an ordinary room from a fuel fire can be more than doubled.