chemical equivalent of 981 2, and the others a chemical equivalent of
about half that number. Then the metals of one group have an extreme
specific gravity platinum being, in fact, the lightest of the three, or
as light as the lightest. Osmium has a specific gravity of 21.4, and is
the heaviest body in nature; platinum is 21.15, and iridium the same; the
specific gravity of the other three being only about half that, namely,
11.3, 12.1, and 11.8. Then there is this curious relation, that palladium
and iridium are very much alike, so that you would scarcely know one from
the other, though one has only half the weight of the other, and only half
the equivalent power. So with iridium and rhodium, and osmium and
ruthenium, which are so closely allied that they make pairs, being
separated each from its own group. Then these metals are the most
infusible that we possess. Osmium is the most difficult to fuse: indeed, I
believe it never has been fused, while every other metal has. Ruthenium
comes next, iridium next, rhodium next, platinum next (so that it ranks
here as a pretty fusible metal, and yet we have been long accustomed to
speak of the infusibility of platinum), and next comes palladium, which is
the most fusible metal of the whole. It is a curious thing to see this
fine association of physical properties coming out in metals which are
grouped together somehow or other in nature, but, no doubt, by causes
which are related to analogous properties in their situation on the
surface of the earth, for it is in alluvial soils that these things are
found.
Now, with regard to this substance, let me tell you briefly how we get it.
The process used to be this. The ore which I shewed you just now was
taken, and digested in nitro-muriatic acid of a certain strength, and
partly converted into a solution, with the leaving behind of certain
bodies that I have upon the table. The platinum being dissolved with care
in acids, to the solution the muriate of ammonia was added, as I am about
to add it here. A yellow precipitate was then thrown down, as you perceive
is the case now; and this, carefully washed and cleansed, gave us that
body [pointing to a specimen of the chloride of platinum and ammonium],
the other elements, or nearly all, being ejected. This substance being
heated, gave us what we call platinum sponge, or platinum in the metallic
state, so finely divided as to form a kind of heavy mass or sponge, which,
at the time that Dr. Wollaston first sent it forth, was not fusible for
the market or in the manufacturers workshops, inasmuch as the temperature
required was so high, and there were no furnaces that could bring the mass
into a globule, and cause the parts to adhere together. Most of our metals
75