Very interesting asemery, your bottle uses the same principle as the 'choggle' water carrier that we used in the forces many years ago (they might still do?). The choggle was a large (4 litre I think) water bottle shaped bag made from a very tightly woven cotton material (courser than a Millbank bag) and which packed (usefully) flat when empty. Once filled water would (very) slowly bleed through the material and evaporate on the outside causing that on the inside to cool down (quite considerably too if the weather was hot and you hung them off the sides of your vehicle). Of course you did loose some water to evaporation but not that much if you were keeping your fluids intake up. I picked one up at a militaria fair some years back for one of my sons to take to Afghan with him - just in case!
