Run along the ledge until you see a solid floor beyond the poison. Shoot a portal onto the wall over there, then another near you that you can go through. There is a tiny little grate in the ceiling in this area. Shoot a portal through it and use another portal to get up there.
This highly unpleasant area is filled with a nasty (but at least not poisonous) sludge that makes you slip and slide as you try to move around. Shoot a portal onto a wall above the ledge, but be warned: there are turret doors that will open as soon as you get up there. Quickly look around to see which turret door is about to open. Shoot a portal on any wall that you can walk into, and then make a portal of the other color inside of the turret's little room, so you can go in and pick up the portal and drop it out, disabling it.
Very soon, another turret room will open, so make a portal in there and disable the turret the same way. Keep doing this until all of the turrets are disabled. Eventually you'll end up in a room near the ceiling, and from there you can see an angled piece of wall on the opposite wall down below. Put a portal on it, then make a portal on the floor and fall through it. As you fly up, shoot a portal on the angled piece of wall that you can see in front of you, and fall through another floor portal. You should end up on a metal ledge.
A couple of turrets will be placed at the end of the hallway facing you. Quickly make a portal on the floor behind them before scurrying off to the side to avoid getting hit. Then make a floor portal here and drop through. You'll end up behind the turrets. Disable them.
Once those turrets are dealt with, look around at the ceiling of the big room that you jumped from before, and you'll see some walkways up there. Shoot a portal onto the floor of the ledge up here, in a position where you will be next to a walkway if you jump from the portal. Then when you have positioned it to your liking, shoot a portal onto the floor of the big room and drop into it. You will be launched up next to the walkway, so move so that you land on the walkway.
At the end of the walkway is a door into more back offices. Look around if you wish. There is a door at the top of a short flight of stairs, and opening it leads to a corridor with windows on the sides. This leads to the final chamber