62.  The operation of pulverizing solid substances and of separating the powders of various degrees of fineness, is common in the arts:  and as the best graduated sifting fails in effecting this separation with sufficient delicacy, recourse is had to suspension in a fluid medium.  The substance when reduced by grinding to the finest powder is agitated in water which is then drawn off:  the coarsest portion of the suspended matter first subsides, and that which requires the longest time to fall down is the finest.  In this manner even emery powder, a substance of great density, is separated into the various degrees of fineness which are required.  Flints, after being burned and ground, are suspended in water, in order to mix them intimately with clay, which is also suspended in the same fluid for the formation of porcelain.  The water is then in part evaporated by heat, and the plastic compound, out of which our most beautiful porcelain is formed, remains.  It is a curious fact, and one which requires further examination than it has yet received, that, if this mixture be suffered to remain long at rest before it is worked up, it becomes useless; for it is then found that the silex, which at first was uniformly mixed, becomes aggregated together in small lumps.  This parallel to the formation of flints in the chalk strata deserves attention.(2*)

63.  The slowness with which powders subside, depends partly on the specific gravity of the substance, and partly on the magnitude of the particles themselves.  Bodies, in falling through a resisting medium, after a certain time acquire a uniform velocity, which is called their terminal velocity, with which they continue to descend:  when the particles are very small, and the medium dense, as water, this terminal velocity is soon arrived at.  Some of the finer powders even of emery require several hours to subside through a few feet of water, and the mud pumped up into our cisterns by some of the water companies is suspended during a still longer time.  These facts furnish us with some idea of the great extent over which deposits of river mud may be spread; for if the mud of any river whose waters enter the Gulf Stream, sink through one foot in an hour, it might be carried by that stream 1,500 miles before it had sunk to the depth of 600 or 700 feet.

64.  A number of small filaments of cotton project from even the best spun thread, and when this thread is woven into muslin they injure its appearance.  To cut these off separately is quite impossible, but they are easily removed by passing the muslin rapidly over a cylinder of iron kept at a dull red heat:  the time during which each portion of the muslin is in contact with the red-hot iron is too short to heat it to the burning point; but the filaments being much finer, and being pressed close to the hot metal, are burnt.