Ernest Rutherford (1871–1937) was a New Zealander, the son of an odd-job man, who won a scholarship to university and came to England to work under J. J. Thomson at Cambridge. His famous gold foil experiment was carried out in 1909, when he was Professor at Manchester. This extract is from C. P. Snow’s The Physicists (1981). Snow, a scientist turned novelist, caused a furore in 1959 with his Rede Lecture The Two Cultures, which argued that scientists ‘have the future in their bones’, whereas ‘intellectuals, in particular literary intellectuals, are natural Luddites’.

If any scientist had a nose for, to use Medawar’s phrase, ‘the solution of the possible’, Rutherford had. His attack was simple and direct, or rather he saw his way, through the hedges of complication, to a method which was the simplest and most direct.

An example is the most dramatic event of his career, the experiments by which he proved the existence of the atomic nucleus. The Curies had shown that radium emits various kinds of ‘radiation’, and one of these was now known to consist of a stream of electrically charged particles. These ‘alpha particles’ were identical to helium atoms with their electrons removed; but they originated not from helium gas but sprang spontaneously from the radium atoms as they disintegrated.

Even though atomic disintegration was still little understood, Rutherford saw these high-speed alpha particles as useful projectiles. He intercepted them with a thin sheet of gold foil, to see what happened as they passed through. If atoms were diffuse spheres of electrical charge, as Thomson had imagined, then most of the alpha particles should have gone straight through; a few should be deflected slightly. But some of the alpha particles bounced straight back again. It was like firing artillery shells at a piece of tissue paper, and getting some of them returning in the direction of the gun.

Rutherford could only explain this by postulating that these alpha particles were hitting small, massive concentrations within the atoms. He thus concluded that most of an atom’s mass resided in a minute, positively charged nucleus at the centre, while the electrons went around the outside – very much like the planets orbiting the massive sun. Most of the atom was just empty space. If an atom were expanded to the size of the dome of St Paul’s Cathedral, virtually all its mass would lie within a central nucleus no larger than an orange. The large majority of alpha particles passed the atoms’ emptiness and carried on through the foil; but just occasionally one would hit a nucleus head-on, and rebound along the way it had come.

Positive, like all Rutherford’s physics. He said that he knew it was convincing, and maintained that he was completely surprised. One wonders if he hadn’t had a secret inkling. He was superlatively good at making predictions about nature.

In 1919, he started firing alpha particles at nitrogen atoms. Nothing much should have happened. A great deal did.

As in his earlier experiments, the alpha particles came from radium. This time he was directing them down a tube filled with nitrogen gas. At the far end, he found he was detecting not just alpha particles, but also particles with all the properties of hydrogen nuclei. There was, however, no hydrogen in the tube. With his high-speed alpha-particle projectiles, Rutherford had actually broken them off the nuclei of the nitrogen atoms.

The discovery of radioactivity had earlier shown that certain, rare types of atom could spontaneously disintegrate. Now Rutherford had shown that ordinary atoms were not indestructible. By knocking out a hydrogen nucleus (later called a proton) from the nucleus of nitrogen he had converted it into another element, oxygen. Rutherford had, to a limited extent, achieved the dream of the alchemists and changed one element to another.