ced 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