When was the 'ray-gun' first mentioned?

I was asked this question via email, and it got me thinking. First port-of-call: the Clute-Nicholls-Langford SF Encyclopedia (of course), whose Langfordian entry for 'RAY GUN' reads in its entirety:

This generic energy weapon, usually hand-held, is one of the best-known sf Clichés established in the Pulp magazine era; hence the Retro-Pulp magazine title Ray Gun Revival. Ray guns may of course project any of the exotic Rays imagined by sf authors: popular varieties include the purely destructive energy of the Blaster or Disintegrator, and the theoretically non-lethal Stunner or paralyser. Star Trek's phasers are famously switchable between these two settings, as are earlier sf handguns like the Denton (an imaginary brand name) in A Tale of Two Clocks (1962; vt Legacy 1979) by James H Schmitz. Another notable ray gun is Isaac Asimov's agony-inducing neuronic whip.

Not much help with the specifics: 'pulp magazine era' covers a lot of ground.

Google returns plenty of searches for 'X-ray gun' from scientific publications, mostly from the 1920s (for instance, the following account of an X-ray experiment from the US Proceedings of the 9th Annual Educational Conference (1929): 'At the left there appears what may be referred to as the "X-ray gun," which shoots a few rays through a cloud-expansion chamber') although some from as early as the late 1890s. (As you know, Bob, these 'rays' were first discovered in 1895). That's no good for us, obviously. Indeed, I wonder if the first SF ray-guns weren't specifically "x-ray guns", from before people realised that such a weapon would be more likely to fix an image of your enemy's skeletal structure upon any photographic plates that happened to be about rather than kill him:

And then, as with the snap of a switch, he was galvanized into sudden furious energy. The old gentleman was ... aiming the x-ray gun at Charles! [Robert Myron Coates, The Eater of Darkness (1929), 216]

Here's a later paperback cover of Coates's 1929 novel. Cool, no?

I like to think the "!?" is a blurb provided by a reader rendered speechless by the book. But there are earlier examples. Here, for example, is a passage from from a story in Everybody's Magazine of 1917:

"All is not going well, Arnold: the ray-rods are emptying fast, and our attack upon the lower level of the wing has failed. Sanson has placed a ray-gun there. All depends on the air-scouts, and we must hold our positions until the battle-planes arrive."

That's the climactic battle of Victor Rousseau's The Messiah of the Cylinder, published, in revised form, as a book in 1917.

Here's the book version of that same quotation:

"All is not going well, Arnold: the ray-rods are emptying fast, and the storage batteries within the Airscouts’ Fortress have been destroyed, so that we cannot recharge them; the attack upon the lower level of the Wing has failed. Sanson has placed a Ray gun there. All hangs upon the battleplanes, and they have not returned." [294]

This whole battle has a rather pleasing Star Wars, or 'Tis Sixty Years Hence vibe to it. 1917, remember!

Streaks of light, pitiably thin, flashed from their Ray rods, and, with exultant shouts, the Guard sprang forward to meet them. They were dragging lighter Ray guns behind them. For an instant it seemed as if the revolutionists would scale the walls before the heavy Ray artillery could be reaimed at them. The foremost files of the opposing forces clashed and surged and swayed in a rain of meteor flashes. The blackened corpses heaped the bridges, hung, toppled over, and went to swell the heaps below. [294-5]

No SFE3 entry on Victor Rousseau, which seems to me a shame [I'm corrected in this point in the comments]. Maybe I should write one; just as soon as I've cantered through this facsimile first edition of his book. That's the earliest 'ray gun' reference I have found. I also had a look for 'ray pistol', and found this intriguing stage-direction from Glenn Hughes's 1932 play Green Fire: A Melodrama of 1990, in Three Acts -- what an excellent title!

S.D. (Picks up the death ray pistol, and points it at the door. The door r. opens. Enter Ferguson, MacRobert, Alan, Wills, Thompson, Brand and Vera. The Men hesitate and put up their hands as they enter and see the pistol.)

Golly, what happens next? I'm agog. (Glenn Hughes is another character without an SFE3 entry; I assume we're not talking about the Black Sabbath/Deep Purple bassist). His play was based on John Taine's 1928 novel of the same title, summarised here at Wikipedia. There's also a ray pistol in Burroughs' Pirates of Venus (also 1932) I think. Perhaps you know of one earlier? I'd love to know, if so.

[PS: Ian Sales draws my attention to this, where the earliest OED-sourced citation is 1931. My 1917 example is quite a bit earlier than that, I'm pleased to say.]