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IN VENTIONS OF NIKOLA TESLA.

against them they at first form a cake of some compactness, prob-ably in consequence of the fusion of impurities they contain. Themass withstands for a very long time the blast without furtherfusion; 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 verywell, but not as well as ordinary carbon. The powder, which isobtained from the crystals in some way, is practically non-con-ducting. It affords a magnificent polishing material for stones.

The time has been too short to make a satisfactory study ofthe properties of this product, hut enough experience has beengained in a few weeks I have experimented upon it to say thatit does possess some remarkable properties in many respects. Itwithstands excessively high degrees of heat, it is little deterioratedby molecular bombardment, and it does not blacken the globe asordinary carbon does. The only difficulty which I have experiencedin its use in connection with these experiments was to find somebinding material which would resist the heat and the effect of thebombardment as successfully as carborundum itself does.

I have here a number of bulbs which I have provided withbuttons of carborundum. To make such a button of carborun-dum crystals I proceed in the following manner: I take an or-dinary lamp filament and dip its point in tar, or some otherthick substance or paint which may be readily carbonized. Inext pass the point of the filament through the crystals, and thenhold it vertically over a hot plate. The tar softens and forms adrop on the point of the filament, the crystals adhering to thesurface of the drop. By regulating the distance from the platethe tar is slowly dried out and the button becomes solid. I thenonce more dip the button in tar and hold it again over a plateuntil the tar is evaporated, leaving only a hard mass which firmlybinds the crystals. When a larger button is required I repeatthe process several times, and I generally also cover the filamenta certain distance below the button with crystals. The buttonbeing mounted in a bulb, when a good vacuum has been reached,first a weak and then a strong discharge is passed through thebulb to carbonize the tar and expel all gases, and later it is broughtto a very intense incandescence.

When the powder is used I have found it best to proceed asfollows : I make a thick paint of carborundum and tar, and passa lamp filament through the paint. Taking then most of the