IN C. P. SNOW's novels the scientists are usually heroic -- magnanimous rationalists almost to a man: This in contrast to the rest of us, with whom his patience seemed to shorten as he grew older. His last novel, A Coat of Varnish, is almost unrelievedly pessimistic.

As a mediator between these two worlds -- the hopeful and rational world of science and the bleak world of fallen man -- he was, however, unsurpassed. It is perhaps a fact of some encouragement that his last book, drafted not long before his death, is this short but brilliant history of 20th-century physics.

The Physicists, brief and clearly illustrated, is the sort of book which in its excellence might have been produced by any fine popularizer of science. But in this case its special merit derives from Snow's great authority and sure insight. It was a story so intimately familiar to him -- not only the people involved but the significance of their work -- that he wrote this book from memory.

The tale itself is familiar in a rough way to almost anyone who cares to inform himself about modern science -- the story of how the atom (thought, a century ago, to be the ultimate constituent of matter) was gradually dissected. It was, in one of Snow's homely but telling images, like the peeling of an onion, but also as if the first man to peel an onion had thought it a solid and layerless object. There was first the development of the electron theory, later the gradual disclosure of the nucleus and the discovery of the gigantic kinetic forces locked in its minute heart. It is a story that even scientific tyros may grasp, in some fashion, from any elementary textbook.

But C. P. Snow was too imaginative, too astute in his observation of human types, to fail to see that even in this lofty enterprise the quirks of human nature would assert themselves. Excellent as The Physicists is as a resume and primer, it is even better as a portrait gallery of the figures who unraveled the atomic mystery. Snow knew most of them personally and his touches of portraiture are engaging.

Thus Niels Bohr, the great Danish physicist, "though one of the deepest minds of his century . . . was a talker as hard to get to the point as Henry James in his later years." Ernest Rutherford, the magisterial figure of British experimental physics, spoke in a voice so booming that a sign in his laboratory at Cambridge ("Talk Softly Please") was designed to protect the equipment against vibrations. Paul Dirac, the bilingual son of a Swiss father and a British mother, "turned out abnormally taciturn in both languages," French and English. Sir John Cockcroft, who designed the first big accelerator, "in about the only magniloquent gesture of a singularly modest and self-effacing life . . . walked with soft-footed games player's tread through the streets of Cambridge and announced to strangers. 'We've split the atom. We've split the atom.' "Einstein, in his later years, bore "considerable resemblance to a handsome and inspired golliwog."

More than a history or a gallery of arresting miniatures, The Physicists is a vehicle of Snow's considered reflections on the social and political context of physics. Those who know his magisterial essays ("Science and Government," for instance) will recognize the themes.

The first is that experimental science is the one human enterprise that actually develops, that truly shows "the direction of time's arrow," although its flight is paradoxically towards greater complexity and mystery. It is an enterprise freighted with extremes of promise and threat, having produced both the theoretical prospect of limitless energy (through fusion) and the shadow of thermonuclear weaponry.

On the development of the latter, which is really the climax of the story, Snow has wise and revealing things to say -- for instance, that since physicists in the '30s already possessed the requisite theoretical knowledge, the development of the first atomic bomb as a collaborative wartime project in the 1940s was a feat of "amateur engineering," with scientists as the amateurs.

This book may not solve the enduring dilemma of secrecy. But it casts startling light on the inevitable differences that divide statesmen and scientists, in wartime and today. After it became virtually certain that the Manhattan Project would succeed, Niels Bohr became obsessed with the view that the Russians should be informed. This was many months before Potsdam. But he was, in this, harshly (Snow says insensitively) rebuffed by Winston Churchill.

In fact, most of the professional physicists working on the atomic weapon predicted, from their own intimate knowledge of the state of Soviet science, that it would take about four years for the Russians to develop their own bomb. In this they were apparently right, although, as Snow acknowledges, the factor of espionage is imponderable. But Snow's larger point is that physical science at this exalted level is inescapably international and collaborative, that the obsession of its political overseers with secrecy is scientifically pointless. It was not through want of "secrets" but because of misplaced priorities and the failure to mass resources that the German scientists failed to duplicate Manhattan. Or so Snow argues.

These specifically political judgments, though certainly authoritative, will be disputed. As will be Snow's optimism about the promise of fusion power. What will not be disputed is that Snow, in this as in other highly readable and vivid works, had more to say about the place of science in 20th-century democratic society, and said it better and more wisely, than anyone else.