The apparatus which I propose to call by the above name
([mu][epsilon][lambda][delta][omega], to melt) consists of an adjunct to
the mineralogical microscope, whereby the melting-points of minerals may
be compared or approximately determined and their behavior watched at
high temperatures either alone or in the presence of reagents.
As I now use it, it consists of a narrow ribbon of platinum (2 mm. wide)
arranged to traverse the field of the microscope. The ribbon, clamped in
two brass clamps so as to be readily renewable, passes bridgewise over a
little scooped-out hollow in a disk of ebony (4 cm. diam.). The clamps
also take wires from a battery (3 Groves cells); and an adjustable
resistance being placed in circuit, the strip can be thus raised in
temperature up to the melting-point of platinum.
The disk being placed on the stage of the microscope the platinum strip
is brought into the field of a 1" objective, protected by a glass slip
from the radiant heat. The observer is sheltered from the intense light
at high temperatures by a wedge of tinted glass, which further can be
used in photometrically estimating the temperature by using it to obtain
extinction of the field. Once for all approximate estimations of the
temperature of the field might be made in terms of the resistance of the
platinum strip, the variation of such resistance with rise of
temperature being known. Such observations being made on a suitably
protected strip might be compared with the wedge readings, the latter
being then used for ready determinations. Want of time has hindered me
from making such observations up to this.
The mineral to be experimented on is placed in small fragments near the
center of the platinum ribbon, and closely watched while the current is
increased, till the melting-point of the substance is apparent. Up to
the present I have only used it comparatively, laying fragments of
different fusibilities near the specimen. In this way I have melted
beryl, orthoclase, and quartz. I was much surprised to find the last
mineral melt below the melting-point of platinum. I have, however, by me
as I write, a fragment, formerly clear rock-crystal, so completely fused
that between crossed Nicols it behaves as if an amorphous body, save in
the very center where a speck of flashing color reveals the remains of
molecular symmetry. Bubbles have formed in the surrounding glass.
Orthoclase becomes a clear glass filled with bubbles: at a lower
temperature beryl behaves in the same way.
Topaz whitens to a milky glass--apparently decomposing, throwing out
filmy threads of clear glass and bubbles of glass which break,
liberating a gas (fluorine?) which, attacking the white-hot platinum,
causes rings of color to appear round the specimen. I have now been
using the apparatus for nearly a month, and in its earliest days it led
me right in the diagnosis of a microscopical mineral, iolite, not before
found in our Irish granite, I think. The unlooked-for characters of the
mineral, coupled with the extreme minuteness of the crystals, led me
previously astray, until my meldometer fixed its fusibility for me as
far above the suspected bodies.
Carbon slips were at first used, as I was unaware of the capabilities of
platinum.
A form of the apparatus adapted, at Prof. Fitzgerald's suggestion, to
fit into the lantern for projection on the screen has been made for me
by Yeates. In this form the heated conductor passes both below and above
the specimen, which is regarded from a horizontal direction.