The pressurized retardant tanks on the 747 Supertanker. The retardant is forced out by compressed air, much like on the MAFF C-130 air tankers.
And here's the exterior
Sam White of Evergreen points out the four nozzles that dispense retardant. Photos by Eric Engman
The Patent on the tanks is number 7413145 described as:
Content from external source
A method and apparatus for aerial fire suppression utilizing a potable fire retardant chemical dispensing system, readily adaptable, without extensive aircraft modification, to various makes of aircraft, for dispensing current types of forest and range fire fighting chemicals. The aerial delivery system is self contained and reusable. It enables cargo/utility aircraft to carry and dump a load, under control. The aerial delivery system is capable of attachment at the wing box, pressurized delivery from the nozzles, and nozzles directed straight downward.
Here's another couple of shots of the interior, taken in Hahn, Germany, July 7, 2009. This is probably when the above shots were taken, as Sam White appears in both, wearing the same suit, tie, and ID badge. But it's less useful to the chemtrail promoters, as it's more obviously from an air show - and people are not going to be showing a secret chemtrail plane at an air show.
Here's the supertanker doing its thing. Very unlike a contrail:
The barrels also clearly show up in old promotional films of the testing of the very first 747 in 1969: http://www.youtube.com/watch?v=xrDv4jX_MUs
I've seen an even older promo film of the testing of the 707 in the 1950s using the barrels. Will have a search for it later and post a link