a small quantity of a
glass-like residue is left, which, I suppose, is melted alumina. When
compressed strongly they conduct very well, but not as well as
ordinary carbon. The powder, which is obtained from the crystals in
some way, is practically non-conducting. It affords a magnificent
polishing material for stones.

The time has been too short to make a satisfactory study of the
properties of this product, but enough experience has been gained in a
few weeks I have experimented upon it to say that it does possess some
remarkable properties in many respects. It withstands excessively high
degrees of heat, it is little deteriorated by molecular bombardment,
and it does not blacken the globe as ordinary carbon does. The only
difficulty which I have found in its use in connection with these
experiments was to find some binding material which would resist the
heat and the effect of the bombardment as successfully as carborundum
itself does.

I have here a number of bulbs which I have provided with buttons of
carborundum. To make such a button of carborundum crystals I proceed
in the following manner: I take an ordinary lamp filament and dip its
point in tar, or some other thick substance or paint which may be
readily carbonized. I next pass the point of the filament through the
crystals, and then hold it vertically over a hot plate. The tar
softens and forms a drop on the point of the filament, the crystals
adhering to the surface of the drop. By regulating the distance from
the plate the tar is slowly dried out and the button becomes solid. I
then once more dip the button in tar and hold it again over a plate
until the tar is evaporated, leaving only a hard mass which firmly
binds the crystals. When a larger button is required I repeat the
process several times, and I generally also cover the filament a
certain distance below the button with crystals. The button being
mounted in a bulb, when a good vacuum has been reached, first a weak
and then a strong discharge is passed through the bulb to carbonize
the tar and expel all gases, and later it is brought to a very intense
incandescence.

When the powder is used I have found it best to proceed as follows: I
make a thick paint of carborundum and tar, and pass a lamp filament
through the paint. Taking then most of the paint off by rubbing the
filament against a piece of chamois leather, I hold it over a hot
plate until the tar evaporates and the coating becomes firm. I