The model of a spherically expanding envelope is employed to find equivalent widths of C iii lines in the rocket ultra-violet. The calculations include electron-ion collisions, and absorptions and emissions associated with the stellar and difftise radiation fields. Effects of optical depth are taken into account by calculating photon escape probabilities. We find that the envelope of 72 Vel, WC8 +08, has an electron temperature of o 000 to 5 coo0 K and electron density of about 4. io11 cm-3. For tbe ratio of carbon density to electron density we find Nc/Ne> . io-8. Our calculations allow for laser action, but in no case does that process occur.