probes comparisons of results obtained on the same sample by various methods could in many cases be used for the estimation of their accuracy various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention therefore it should be understood that this invention is not to be limited to the following examples in which parts are given by weight in this example the apparatus was like that of fig1 having ______________________________________length of furnace 11 30 cminside diameter of furnace tube 12 2 5 cminside diameter of inlet tubing 16 0 4 cm______________________________________ hydrogen 190 cc min was bubbled through boiling ch 3 2 sicl 2 to produce a gaseous mixture that was passed through a 20 u00b0 c reflux condenser and then through conduit 28 of fig1 called 34 flux 1 34 in table i argon 2100 cc min was passed through conduit 30 called 34 flux 2 34 in table i the apparatus was used to coat a 2000 denlet 760 filament tow mullite with a 2 b 2 o 3 content available as nextel u00ae 480 that had a 0 2 carbon coating this tow was pulled through the furnace tube kept at 1050 u00b0 c at 37 cm min to provide a cvd coating about 100 nm in thickness as measured by electron microscopy and verified by calculations based on elemental analysis of carbon the resulting filaments of the tow had infinite electrical resistance their barrier coatings of sic afforded a golden color to the filaments uniform orange colored lines observed under cross polarized light indicated a uniform coating thickness of about 130 nm the coating thickness distribution as shown by.