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I pointed down at the tanks。 〃What cells are you growing？〃
〃Theta…d 5972；〃 he said。
〃Which is？〃
〃A strain of E。 coli。〃
E。 coli was a mon bacterium； found pretty much everywhere in the natural environment； even in the human intestine。 I said； 〃Did anyone think it might not be a good idea to use cells that can live inside human beings？〃
〃Not really；〃 he said。 〃Frankly that wasn't a consideration。 We just wanted a well…studied cell that was fully documented in the literature。 We chose an industry standard。〃
〃Uh…huh 。。。〃
〃Anyway；〃 Ricky continued； 〃I don't think it's a problem； Jack。 It won't thrive in the human gut。 Theta…d is optimized for a variety of nutrient sources…to make it cheap to grow in the laboratory。 In fact； I think it can even grow on garbage。〃
〃So that's how you get your molecules。 Bacteria make them for you。〃
〃Yes；〃 he said； 〃that's how we get the primary molecules。 We harvest twenty…seven primary molecules。 They fit together in relatively high…temperature settings where the atoms are more active and mix quickly。〃
〃That's why it's hot in here？〃
〃Yes。 Reaction efficiency has a maxima at one hundred forty…seven degrees Fahrenheit； so we work there。 That's where we get the fastest bination rate。 But these molecules will bine at much lower temperatures。 Even around thirty…five； forty degrees Fahrenheit； you'll get a certain amount of molecular bination。〃
〃And you don't need other conditions；〃 I said。 〃Vacuum？ Pressure？ High magnetic fields？〃 Ricky shook his head。 〃No； Jack。 We maintain those conditions to speed up assembly； but it's not strictly necessary。 The design is really elegant。 The ponent molecules go together quite easily。〃
〃And these ponent molecules bine to form your final assembler？〃
〃Which then assembles the molecules we want。 Yes。〃
It was a clever solution； creating his assemblers with bacteria。 But Ricky was telling me the ponents assembled themselves almost automatically； with nothing required but high temperature。 What； then； was this plex glass building used for？ 〃Efficiency； and process separation；〃 Ricky said。 〃We can build as many as nine assemblers simultaneously； in the different arms。〃
〃And where do the assemblers make the final molecules？〃
〃In this same structure。 But first； we reapply them。〃
I shook my head。 I wasn't familiar with the term。 〃Reapply？〃
〃It's a little refinement we developed here。 We're patenting it。 You see； our system worked perfectly right from the start…but our yields were extremely low。 We were harvesting half a gram of finished molecules an hour。 At that rate； it would take several days to make a single camera。 We couldn't figure out what the problem was。 The late assembly in the arms is done in gas phase。 It turned out that the molecular assemblers were heavy； and tended to sink to the bottom。 The bacteria settled on a layer above them； releasing ponent molecules that were lighter still； and floated higher。 So the assemblers were making very little contact with the molecules they were meant to assemble。 We tried mixing technologies but they didn't help。〃
〃So you did what？〃
〃We modified the assembler design to provide a lipotrophic base that would attach to the surface of the bacteria。 That brought the assemblers into better contact with the ponent molecules； and immediately our yields jumped five orders of magnitude。〃
〃And now your assemblers sit on the bacteria？〃
〃Correct。 They attach to the outer cell membrane。〃
At a nearby workstation； Ricky punched up the assembler design on the flat panel display。 The assembler looked like a sort of pinwheel； a series of spiral arms going off in different directions； and a dense knot of atoms in the center。 〃It's fractal； as I said；〃 he said。 〃So it looks sort of the same at smaller orders of magnitude。〃 He laughed。 〃Like the old joke； turtles all the way down。〃 He pressed more keys。 〃Anyway； here's the attached configuration。〃 The screen now showed the assembler adhering to a much larger pill…shaped object； like a pinwheel attached to a submarine。 〃That's the Theta…d bacterium；〃 Ricky said。 〃With the assembler on it。〃
As I watched； several more pinwheels attached themselves。 〃And these assemblers make the actual camera units？〃
〃Correct。〃 He typed again。 I saw a new image。 〃This is our target micromachine； the final camera。 You've seen the bloodstream version。 This is the Pentagon version； quite a bit larger and designed to be airborne。 What you're looking at is a molecular helicopter。〃
〃Where's the propeller？〃 I said。
〃Hasn't got one。 The machine uses those little round protrusions you see there； stuck in at angles。 Those're motors。 The machines actually maneuver by climbing the viscosity of the air。〃
〃Climbing the what？〃
〃Viscosity。 Of the air。〃 He smiled。 〃Micromachine level； remember？ It's a whole new world； Jack。〃
However innovative the design； Ricky was still bound by the Pentagon's engineering specs for the product； and the product wasn't performing。 Yes； they had built a camera that couldn't be shot down； and it transmitted images very well。 Ricky explained it worked perfectly during tests indoors。 But outside； even a modest breeze tended to blow it away like the cloud of dust it was。 The engineering team at Xymos was attempting to modify the units to increase mobility； but so far without success。 Meanwhile the Department of Defense decided the design constraints were unbeatable； and had backed away from the whole nano concept； the Xymos contract had been canceled； DOD was going to pull funding in another six weeks。 I said； 〃That's why Julia was so desperate for venture capital； these last few weeks？〃
〃Right；〃 Ricky said。 〃Frankly； this whole pany could go belly up before Christmas。〃
〃Unless you fix the units； so they can work in wind。〃
〃Right； right。〃
I said； 〃Ricky； I'm a programmer。 I can't help you with your agent mobility problems。 That's an issue of molecular design。 It's engineering。 It's not my area。〃
〃Um； I know that。〃 He paused； frowned。 〃But actually； we think the program code may be involved in the solution。〃
〃The code？ Involved in the solution to what？〃
〃Jack； I have to be frank with you。 We've made a mistake；〃 he said。 〃But it's not our fault。 I swear to you。 It wasn't us。 It was the contractors。〃 He started down the stairs。 〃e on； I'll show you。〃
Walking briskly； he led me to the far side of the facility； where I saw an open yellow elevator cage mounted on the wall。 It was a small elevator； and I was unfortable because it was open； I averted my eyes。 Ricky said； 〃Don't like heights？〃
〃Can't stand them。〃
〃Well； it's better than walking。〃 He pointed off to one side； where an iron ladder ran up the wall to the ceiling。 〃When the elevator goes out； we have to climb up that。〃 I shuddered。 〃Not me。〃
We rode the elevator all the way up to the ceiling； three stories above the ground。 Hanging beneath the ceiling was a tangle of ducts and conduits； and a network of mesh walkways to enable workers to service them。 I hated the mesh； because I could see through it to the floor far below。 I tried not to look down。 We had to duck repeatedly beneath the low…hanging pipes。 Ricky shouted over the roar of