You fill the bottle with enough of the cauldron's poisonous concoction for a suitable sample to be studied.
Welcome back to the relative safety of Chillwind Camp, <name>. What progress on the cauldrons do you have to report?
Well done <name> - it should take Alchemist Arbington no time at all to analyze this and devise a counter agent to introduce into the cauldron. If we are able to pump out enough of it into the air, we might be able to weaken all the Scourge present here.
This will no doubt be an ongoing ordeal; we will need to continually hit these cauldrons and introduce our reagents into their mix. Still, if we can keep it up, we will have a huge advantage over Scourge when the time comes for battle!