What makes an X-ray machine's white photographic film go black?
X-ray film is very similar to standard black and white negative film. It's basically a transparent plastic base coated with an emulsion of silver halide particles (a salt of $\ce{Ag+}$ with a halide, usually $\ce{Cl-}$ or $\ce{Br-}$). As shown below, when light hits one of these particles, it reduces a small amount of the halide to elemental silver (1). When this is exposed to a photographic developer (essentially a mild reducing agent), more elemental silver is produced around the initial sites (2).$$\ce{Ag^+ +e- -&gt; Ag}$$

Finally, a fixing agent is added which solubilizes the unexposed silver halide so it can be removed from the emulsion, preventing further light exposure from fogging the film.

Thus, the regions that are exposed to light appear dark when developed as the tiny silver particles ($\ce{Ag}$) block light passing through the film and the unexposed regions are transparent, taking on the colour of the plastic base (often blue or grey). X-ray films are typically viewed on an illuminated white background, making the unexposed areas appear white (or off-white, coloured by the plastic base).

Normal B&amp;W film is actually slightly sensitive to x-rays but, the emulsion of dedicated x-ray film contains other components to facilitate the silver reduction from x-ray exposure. 

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