Carborundum can be obtained in two forms—­in the form of “crystals” and of powder.  The former appear to the naked eye dark colored, but are very brilliant; the latter is of nearly the same color as ordinary diamond powder, but very much finer.  When viewed under a microscope the samples of crystals given to me did not appear to have any definite form, but rather resembled pieces of broken up egg coal of fine quality.  The majority were opaque, but there were some which were transparent and colored.  The crystals are a kind of carbon containing some impurities; they are extremely hard, and withstand for a long time even an oxygen blast.  When the blast is directed against them they at first form a cake of some compactness, probably in consequence of the fusion of impurities they contain.  The mass withstands for a very long time the blast without further fusion; but a slow carrying off, or burning, occurs, and, finally, 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.