Fusion and Vaporisation of Carbon

Carbon vaporises in the electric arc under atmospheric pressure at about 3600° C., and condenses as graphite. Moissan has shown that when carbon is heated in an electric furnace in a carbon tube at 2000 amperes and 80 volts it vaporises, and the tube is filled with a light deposit, which by chemical tests is proved to be graphite. Moissan was of opinion that carbon vapour always yields graphite by condensation, but according to Berthelot the film deposited on the glass of carbon filament electric lamps is amorphous carbon. Moissan concluded, moreover, that carbon might be liquefied under great pressure; and showed that diamonds having the appearance of congealed drops, besides occurring naturally, might be prepared artificially under great pressure. That carbon powder cannot be welded by great pressure alone was shown by Spring.

In a study of the thermal effects of the musical arc, La Rosa has obtained evidence of the fusion of carbon and of its crystallisation in particles which sink in bromoform and methylene iodide and scratch ruby.

The physical constants of carbon have been calculated by Crookes in the following manner:

The critical temperature of a substance is about 1.5 times its boiling or vaporising temperature at atmospheric pressure. The vaporising temperature of carbon is about 3870° abs. ( =3600° C.); therefore its critical temperature is about 5800° abs. Further, the critical temperature of a substance is numerically equal to about 2.5 times its critical pressure; so that the critical pressure of carbon is about 2320 atmospheres.

Lastly, the melting-points of substances that sublime (e.g. arsenic) are about 1.1-1.2 times as high as their vaporising temperatures at atmospheric pressure; consequently the lowest melting-point of carbon is about 4400°. By the application of the Rankine-van der Waals equation: log P = 10.11 – 39,210/T, the pressure corresponding to this melting-point is found to be about 16 atmospheres. Thus the following physical constants of carbon have been arrived at:

Temp, (abs.)	Press, (atm.)
3870°	1	vaporisation-point
4400°	16	melting-point
5800°	2320	critical point