If you didn't know, you might have thought that the earnest handful of senators from the Appropriations Committee was sitting around at the height of the defense budget debate, rehearsing the familiar arguments about nuclear weapons.

But committee Chairman Mark O. Hatfield (R-Ore.) had convened the little gathering yesterday to discuss binary weapons, or nerve gas. Binaries have a lot in common with nukes.

In the first place, they are so unimaginably lethal that Richard Nixon, no dove, instituted a freeze on their production and signed a chemical warfare pact with the Soviets in 1972.

In the second place, as with the nukes, the crucial question is: "How much is enough?"

The administration says that the Soviets have stored more than we have. This contention is disputed by critics such as Sen. David H. Pryor (D-Ark.), who proffers estimates showing that we have enough nerve gas on hand to kill every man, woman and child on earth.

But that is not enough for Ronald Reagan, who proposes to manufacture binary weapons, which carry in a single shell two chemicals that are harmless if kept apart but form a deadly mix on firing. The Pentagon promotes them as another deterrent--deterrence not being guaranteed by the possession of thousands of nuclear weapons and the manufacture of thousands more.

Certain facts about binary weapons are not in doubt. One is that they would cause massive civilian deaths. The cost of protecting civilians would be, as in the case of nuclear attack, prohibitive. It might cost as much to provide masks and shelters for noncombatants as it would to build blast shelters strong enough to withstand nuclear obliteration. As in the case of nukes, plans to evacuate noncombatants are dismissed by sensible people.

We have deployed a large supply of chemical weapons in Europe. To venture to deploy more would set off demonstrations of a nuclear scale and would, according to one expert, provide the Soviet Union with a "propaganda weapon it does not deserve." Recently, the Social Democratic Party of West German Chancellor Helmut Schmidt passed a resolution calling for removal of all stocks of poison gas from its soil and renunciation of any further deployment.

Reagan's posture on use of chemical weapons is exactly what it is on nukes. We plan no first strike, but we know the Soviets are perfectly capable of one.

And as with nukes, the administration says we should build up first and negotiate later.

For openers, Ronald Reagan is asking for $700 million, a trifling figure in a weapons procurement budget of $180.3 billion, in the new chemical weapons race. Before the poison-gas window of vulnerability can be shut, the bill will be in the neighborhood of $9 billion.

About the best thing that could be said during an afternoon of contemplating the return of the monster came from James F. Leonard, who formerly ran the chemical warfare section of the Arms Control and Disarmament Agency. "Nuclear weapons are far worse," he said.

But the resemblances are so compelling--from strategic, political and lethal viewpoints--that the discussion stalled once in a heated exchange about SALT I. Sen. Jake Garn (R-Utah), an "anything goes" zealot on national security, reminded Retired Rear Adm. Thomas D. Davies, a veteran of the Pentagon, NATO and the arms control agency, of Soviet perfidy in regard to the nuclear buildup and of the folly of "negotiating from weakness."

"We were talking about binary weapons," Chairman Hatfield said gently.

The administration might have turned back the clock on poison-gas production anyway, but it pounced on the occasion of alleged chemical use by the Soviets in Afghanistan and by their client in Cambodia to sound the trumpet for binaries.

Harvard Professor Matthew Meselson, regarded as the ultimate authority in these matters, said, without mentioning those cases, that the evidence was "highly questionable."

Meselson advocates a buildup, not of untested binary shells but of protection against chemicals. Soldiers can be almost fully protected, while civilians are dying, horribly, by the millions. The final futility of chemical warfare is that it cannot win a war, because it cannot kill a properly equipped enemy, only slow him as he lumbers about in heavy protective gear.

The Soviets have perfected a bulky butyl rubber outfit. We are competing with a lighter model, according to congressional testimony last year by Army Maj. Gen. Niles J. Fulwyler, although problems remain.

Do Americans, when suited up, "have any way of taking care of bodily functions?" he was asked. The general said no.

"How about the Russians?"

"I don't believe they do either," was the reply.

At last, in the arms race, one small instance of "rough parity."