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