Carbon and arsenic are the only two elements that have a melting-point above the boiling-point; and among compounds carbonic acid and fluoride of silicium are the only other bodies with similar properties. Now the melting-point of arsenic is about 1·2 times its absolute boiling-point. With carbonic acid and fluoride of silicium the melting-points are about 1·1 times their boiling-points. Applying these ratios to carbon, we find that its melting-point would be about 4400°.
Therefore, assuming the following data:
| Boiling-point | 3870° Ab. |
| Melting-point | 4400° |
| Critical temperature | 5800° |
| Critical pressure | 2320 Ats. |
the Rankine or Van der Waals formula, calculated from the boiling-point and critical data, would be as follows:
log. P = 10·11 - 39120/T,
and this gives for a temperature of 4400° Ab. a pressure of 16·6 Ats. as the melting-point pressure. The results of the formula are given in the form of a table:
| Temperature | Pressure | |
| Ab. | Ats. | |
| 3870° | 1·00 | Boiling-point. |
| 4000° | 2·14 | |
| 4200° | 6·25 | |
| 4400° | 16·6 | Melting-point. |
| 4600° | 40·4 | |
| 4800° | 91·2 | |
| 5000° | 193 | |
| 5200° | 386 | |
| 5400° | 735 | |
| 5600° | 1330 | |
| 5800° | 2320 | Critical point (15 tons per square inch). |