- Does the radium salt (or metal for instance) glow only because the emitted alpha particles bombard the nitrogen atoms in the air? Still, how does that lead to a glow anyway?
- What's the color of the glow?
- In that case, it wouldn't glow in vacuum, right?
Radium that was used for luminous indicators was combined with a phosphor. The radiation excited the phosphor and that was what glowed (usually green).
If you have enough radium in the dark, it can ionize the nitrogen in the air. As the nitrogen recombines with electrons, various emissions are possible, but in the visible band, blue is strong. You'll see a similar glow from an electric spark, because that too is ionizing nitrogen in the air.
You're correct, it would not glow in vacuum unless it had some contamination or attachment that was phosphorescent.
From The Becquerel Rays and the Properties of Radium (R. J. Strutt, 1904)
Lenard found, as we have already seen, that the cathode rays produced a blue luminosity in the gas through which they pass. This luminosity is visible both inside the vacuum tube and outside it, though it is much more conspicuous inside in the neighborhood of the cathode, especially where the pressure is not too low. This luminosity is called the negative glow, and possesses a spectrum characteristic of the gas in question.
Sir William and Lady Huggins have recently obtained evidence that the β-rays of radium also produce the same characteristic glow in gases. It was noticed in the early days of the discovery of radium that radium preparations were feebly self-luminous in the dark. It was generally thought that this was due to a slight fluorescence of the salt under the influence of its own rays -- at least that was the view with the author took of it, and probably others thought the same. Sir William and Lady Huggins, however, making use of their unrivalled experience in the photography of feeble spectra gained in working on the nebulæ, were able to photograph the spectrum of the radium glow, and they found it to give the characteristic bands of nitrogen. Thus it seems likely that the glow is due to the air, and it is produced by the action of the β-rays upon it, just as the cathode rays produce luminosity in gases.