Chamber 15

Walk over to the red plunger button and press it to drop down the bridge above, which will catch the falling Experimental Cube.

Walk back to the switch on the ground, stand on it and use your portals to direct the transportation beam so it picks up the Cube.

Fire a portal under the Cube so it catches it as it falls and brings it right over to you.

Take the Cube over to the corner with the hole in the glass and fire a portal through the hole and on the wall to your right.

Take the Cube in with you and set it down on the red switch in here, doing this will cause the orange propulsion dispenser to start.

Walk back outside and position yourself back on the round switch to reactivate the transportation beam device.

Get your first portal on the ceiling by the beam and the second portal on the single wall panel halfway up the back wall. The Propulsion Gel will now get carried across the room.

Turn to your right and wait for the gel to have just passed the ramp, now disengage the transportation beam and the gel will fall down, coating the ramp and the floor leading up to it.

Go back to the Experimental Cube, pick it up, then drop it on the red switch outside.

Run to the left side of the ramp and look across to the shoot-able over the chasm on the right-hand side (behind the force field).

Fire a portal all the way over here before placing the second one on the ceiling for the transportation beam.

Now walk to the start of the orange gel on the floor, line yourself up with the ramp, run forwards and you’ll catapult yourself into the beam, which will now carry you all the way over to the exit.