"The Gas Chamber at Seaford," Frederick Varley EvanHarper:I can’t find a source confirming it, but from its color the gas is probably chlorine, which was actually used in training. By late in the war it was understood that the terror effect of gas was potentially much greater than its effectiveness really justified, so the risk of mishap was considered worthwhile.

"The Gas Chamber at Seaford," Frederick Varley

EvanHarper:

I can’t find a source confirming it, but from its color the gas is probably chlorine, which was actually used in training. By late in the war it was understood that the terror effect of gas was potentially much greater than its effectiveness really justified, so the risk of mishap was considered worthwhile.