`It's called nerve gas,' Nordmann said, `because it kills by interfering with transmission of nerve impulses. Nerves work electrically, but the impulses jump from nerve cell to nerve cell - across gaps called synapses - by chemical means. Nerve gases such as tabun, sarin, and ZV interfere with that jumping process. The result is difficulty in breathing, respiratory paralysis, and death. Now let's talk about potency.'

Graves lit a cigarette and glanced at Phelps. Phelps was smiling, nodding his head as Nordmann talked. Faintly from the TV they heard: `He's our man, he's our man, he's our man…' And loud cheers.

'Tabun and sarin,' Nordmann said, `or the American gas GB, must be inhaled to kill. Therefore gas masks provide an adequate defence. These gases are also relatively weak. They're not produced any more. But there is another family of gases, like VX, which can kill by absorption through the skin as well as by inhalation. The smallest fraction of an ounce is lethal. Am I clear?'

`You're clear,' Graves said.

`VX is terribly powerful,' Nordmann said. `The lethal dose is estimated to be between two and ten milligrammes, or a few thousandths of an ounce. But powerful as that is, it's nothing compared to ZV. ZV, like VX, is an oil. It's sticky, it clings to things, it hangs around the environment. But a tenth of a milligramme is a lethal dose. In other words, it's about a hundred times as powerful as VX. We're now talking about extraordinary potency.'

Phelps was still smiling, still nodding. Graves felt very cold.

`It is so potent that it has never been manufactured as a single gas. Instead, it's a binary - that is, it's produced as two separate gases, each harmless by itself. But when they mix, they're deadly. The gases are designated Binary 75 and Binary 76 respectively. They're generally stored in yellow and black tanks. The film you are going to see is a French Army

training film showing the effects of ZV on a condemned prisoner.'

Phelps got up and turned on the projector.

On the screen they saw a man in denim clothing standing in an enclosed room. The man was looking around nervously.

`This subject,' Nordmann said, `is going to be exposed to the LD500 dose of the gas, that is, five tenths of a milligramme. It is a fully lethal dose.'

Faintly from the other room they heard, `My fellow Americans, it is with great pleasure that I -'

At the bottom of the screen appeared the words GAS 75 INTRODUCED. The prisoner did not react. Moments later: GAS 76 INTRODUCED. The prisoner responded instantly. He placed his hand on his chest, coughed, and wiped his nose.

`- join you in this great tradition, this reaffirmation of the democratic process -'

`- and I must urge you to follow the vision of our great land, to seek the promise, to fulfil the expectations. Let me make one thing perfectly clear '

'Will somebody shut that door?' Graves said.

On the screen the prisoner appeared in close-up. His nose was running profusely; the liquid dripped down to his shirt. His eyes were hard black dots.

A full-body shot showed the man bent over in evident pain.

The prisoner vomited explosively, and the caption stated unnecessarily, VOMITING.

Very faint now from the living-room they heard the sound of prolonged applause.

On the screen the prisoner was clearly confused and in great pain. A dark stain appeared on his trousers.

The man staggered and leaned against the wall. His legs and arms twitched and jerked spastically.

The prisoner looked around briefly, but his face was contorted in agony. He lost his balance and fell, twitching and jerking, to the floor. The camera panned down to follow him. From the other room the applause continued.

The man was not really awake. He lifted his head from the floor in short jerks and finally flopped down, not moving.

The camera remained on the man. His chest was still moving slightly. Then it stopped.

A moment later.

The screen faded to dark. And the last letters appeared. TIME FROM INTRODUCTION OF BINARY GAS TO DEATH: 1.7 MINUTES.

The film ran out. The screen was white. Phelps turned the room lights back on.

`Jesus Christ,' Graves said. He lit a cigarette and noticed that his hands were shaking.

`As I said, that man received five times the minimum lethal dose,' Nordmann said quietly. `Had he got less, he would still have died - more slowly.'

`How much more slowly?'

`From tests on animals, it may take as long as an hour or two.'

`That same progression?'

`The very same.'

`Jesus Christ,' Graves said again.

He walked back into the living-room, which seemed glaringly bright. Through the windows he could look out over the downtown area of the city. He stood with his back to the television and listened to the familiar voice saying, `My fellow Americans, and my fellow Republicans, we have come to a momentous time for our nation. We face great problems, and we face great challenges. We must act now to -'

The set abruptly clicked off. Graves turned and saw that Phelps had done it. `I hope you understand now,' Phelps said. 'Wright has half a ton of that gas. In the San Diego area there are a million people. Plus some very distinguished visitors. We can't afford cat-andmouse games any longer.'

`I agree,' Graves said, staring out at the street below. There were no trees. He wondered why they hadn't put any trees in downtown San Diego. Trees made a difference.

Behind him Phelps picked up the telephone and dialled a number. He said, `Phelps here. I want 702.' There was a pause.

Nordmann came over to stand by Graves and look down at the street. `You know,' he said, `I told the Army four years ago if they kept transporting this crap all around, it was only a matter of time before somebody -'

`You have?' Phelps said into the phone. His voice was excited. `Where?'

Graves turned. Phelps was nodding, his head bobbing up and down like a mechanical bird.

`Yes, yes… yes… good work. We'll be there in five minutes.' He hung up and turned to Graves. `702 followed the limousine back to Wright's old apartment house. The van split off and went somewhere else, but the limo went back to Avenue B.'

`And?'

`They arrested John Wright as he stepped from his car.'

Graves nodded and tried to feel the same excitement that Phelps so clearly showed. But he still had a nagging sense of defeat, as if he had cheated at the game - or had quit too early.

`Come on,' Phelps said. `You can introduce him to me.'

At the apartment house two men were standing up facing the wall, guarded by the men from car 702. Phelps and Graves hurried over.

One of the men was George, the chauffeur. He was muttering something under his breath. Wright was beside him, neatly dressed in his English-cut suit.