Raytheon 2051 Thyratron vacuum test

The WikiPedia page about Thyratrons mentions that they contain some gas. Either neon, argon, mercury or xenon. This means that the tube can be tested in the same way I do with Nixies, using the high-voltage AC generator.

Now that I built a HV-AC tester out of a CCFL driver crcuit the test is fast. And the picture shows the result:

My Thyratron lights up, so it contains some gas. Which one, according to the color?!

Only the lower half of the glass lights up this way, which confirms something I've read: the gas tends to leave deposits on the glass. I am now holding the tube upside down to see if something changes.