Dissolved under pressure into the acetone soaked into the cylinder media. As I understand it, your standard acetylene cylinder only has a couple teaspoons worth of "free" acetylene at high pressure in it at any given moment - an amount insufficient to sustain a deflagration reaction to the point where it could be dangerous. When you draw that amount, however much it is, out of the tank, the pressure in the tank drops, allowing more acetylene to "boil" out of the acetone, rebuilding the pressure, lather, rinse, repeat until there's no more acetylene (or at least not enough to be useful) left dissolved in the acetone. Even if that small amount of "free" acetylene was enough to get a small deflagration reaction going, the resulting pressure increase makes it self-damping, since the rising pressure forces more more acetylene into solution with the acetone soaked into the cylinder media, keeping it from being able to participate in and help perpetuate the deflagration reaction. The "boiling" process isn't the result of heat, but a drop below the pressure needed to keep the acetylene dissolved in the acetone
- Kind of like taking water to very low pressure. At the right pressure for the temperature, you can get a "full rolling boil" going in a container of water at any temperature above freezing. You won't be able to *COOK* anything with it, but it'll look exactly the same as the pan of water boiling away merrily on the stove at 212 degrees F. Acetylene "boiling" out of solution to replace what you've tapped out of the cylinder is exactly the same mechanism in operation, just working on different materials.
Additionally, acetylene cylinder are *NOT* hollow cylinders, the way oxygen and other cylinders are - If you were to break one open somehow (HIGHLY not recommended outside of properly equipped facilities - unless you *WANT* to die) you'd find that it's almost entirely filled with a highly porous block of something that looks much like concrete. This "rock sponge" is what soaks up the acetone that the acetylene dissolves in, preventing you from getting spits and spurts of raw liquid acetone coming out the end of your torch. Gaps and voids in it are a certifiable Very Bad Thing(TM), which is one of the (if not the main) reasons that dropping an acetylene tank any distance is a Bad Thing(TM) and grounds for having that tank pulled from service and stored someplace "bomb-proof" until it's been inspected and tested to be sure the "stuffing" is still intact and working as intended. If you ever get handed a cylinder that "rattles when rocked" (something *INSIDE* the cylinder rattling, not "stuff attached to the cylinder") immediately hand it back - *GENTLY* - and demand another one. The "rattler" almost certainly has damaged media in it, and is unsafe. It's unlikely, due to the pre-fill inspection, that you'll ever get one, but "unlikely" and "impossible" aren't equal.
Unlike oxygen or other tanked gasses, filling an acetylene cylinder is a fairly time-intensive task - They don't just hook it up to a supply and open both valves then wait for the pressure to equalize - They have to let the fresh acetylene into the cylinder at a relatively slow rate so that it gets a chance to dissolve into the acetone. Depending on the size, filling one properly is a process that can take anywhere from a couple hours, to several days, or even longer for really monster-sized ones. Failure to fill properly can easily result in a bomb that's ready to go off at any moment, in response to any (or no apparent) provocation. Which is why, when you take an acetylene cylinder in to be refilled, the most common practice is to simply hand you another one, and put the one you brought in over with the rest of the "needs to be refilled" ones, rather than filling your cylinder "while-u-wait". If you want *THE SAME CYLINDER* back, you're going to have to wait for a while. But since the places that fill them have to inspect, certify, etc. etc. etc. every cylinder that goes through, "This other randomly selected cylinder of the same capacity" is effectively identical to "The cylinder I own and brought in to be filled", even if it isn't actually the same cylinder. Which means, in effect, that when Joe Sixpack "buys an acetylene cylinder", he's not really buying the cylinder itself - he's buying the right to use any single member of a "pool" of who-knows-how-many of that size cylinder, any of which is effectively identical to the one he "bought".