rites us that the article
on "Hot Water in Dressing Ores" in the March number "is another good
illustration of how great men will stumble over little things. Permit me
to express a principle with regard to the same matter, by which without
Rittinger's profound calculations, without Ransom's laboratory
experiments, the entire question about the best medium (liquid or fluid)
for separating two equal sized particles of solids according to their
density (specific gravity) can be settled for every special case." His
"principle" is that the ideal fluid for this purpose is one that is more
dense than the lighter of the two particles and less dense than the
heavier. But this is no new revelation. The difficulty is that there is
but one fluid of the kind, and only one metal (disregarding the very
rare ones) to which it can be applied. The fluid is mercury and the
metal gold. The latter has a specific gravity of say 19, and therefore
sinks when it is carried upon a bath of fluid quicksilver, with a
specific gravity of say 13.6. The sand with which the metal is mixed has
a specific gravity of only 2.6 to 5, and floats over the mercury bath
and away into the waste, thus effecting the desired separation. This
operation, and the fact that there is such a thing as a theoretically
ideal fluid, was clearly pointed out by Rittinger, for whom Mr. Cazin
appears to have so little respect. The latter gentleman does bring
forward one new point, and it is an important one. He asserts that air
can be made to act as an "ideal" fluid, in the sense referred to here,
by imparting motion to it. This conclusion depends on the consideration
that "motion of the fluid in an opposite direction to the fall of the
solid particles is equivalent (by friction, adhesion, resistance) to an
increase of density of the fluid. Therefore air may by imparted motion
have the same separating effect, in a specified case, as water would
have without motion."

If Mr. Cazin would state his case differently, he would see more
clearly the place that air has as a separating medium. It cannot be
made an _ideal_ fluid, but it is comparable with water, which also is
never an ideal fluid, for there is no ore of common occurrence that is
lighter than water. The question in ore dressing really is whether air
can be made to work as well as water. Theoretically we can see no
objection, but in practice a great many obstacles arise. The cost is
greater both for machinery and operating