The Rough Science team face their final challenge. They must take a memento of the mill and send it, by air, along a valley to Kate. Mike and Hermione decide to take a silver-nitrate photograph while Ellen and Jonathan hope to make hydrogen balloons to get the image to Kate.
As silver compounds were used in early photography, Hermione hopes that some of the rock taken from the mine they explored last time will contain enough silver for their photograph. But when she casts an expert eye over the rock, Hermione can’t find any silver ore. So she decides to search for more promising rocks in an old silver mine.
Mike’s job is to make nitric acid to react with Hermione’s silver ore and he chooses an unlikely source, horse manure. But can a combination of manure and stones produce a recognisable photograph?
If the photo does get taken, there's still the problem of delivery. Jonathan and Ellen decide to make a hydrogen balloon with a timed release mechanism. Jonathan sets about making hydrogen using bits of zinc and acid while Ellen figures out how to use the wind to their advantage. Having made a wind speed detector from an old fan and voltmeter, Ellen has to figure out how to make the photograph drop into Kate’s eagerly waiting hands.
It's touch and go as to whether the delivery system will work - and if there's something to deliver. Join the team to discover if they end their time in Colorado with a victory - or if they're heading home defeated.
First broadcast: Wednesday 2 Nov 2005 on BBC TWO