Why couldn't it be solid CO2??
It's white
It sublimates

just asking

Note: I'm a radiologist.

The device the guy is holding looks exactly like an x-ray tube from that era (in fact one of the organizations I belong to uses a representation of that on their logo.)

Now can that be used as a "death ray"? Certainly, but with the usual output, it would take some long exposure to kill a human.

This strikes me as more of the fantastical, breathless hype that surrounded physics in general, and x-rays in particular at that time. It was touted as a cure for virtually everything. Indeed, x-ray therapy was used for benign conditions like ringworm of the scalp (a fungal infection) as late as the mid 50's.