Page:Russell - An outline of philosophy.pdf/119

Rh as developed by Rutherford. This was a most curious discovery, introducing, in a new field, a certain type of discontinuity which was already known to be exhibited by some other natural processes. No adage had seemed more respectable in philosophy than "natura non facit saltum", Nature makes no jumps. But if there is one thing more than another that the experience of a long life has taught me, it is that Latin tags always express falsehoods; and so it has proved in this case. Apparently Nature does make jumps, not only now and then, but whenever a body emits light, as well as on certain other occasions. The German physicist Planck was the first to demonstrate the necessity of jumps. He was considering how bodies radiate heat when they are warmer than their surroundings. Heat, as has long been known, consists of vibrations, which are distinguished by their "frequency", i.e. by the number of vibrations per second. Planck showed that, for vibrations having a given frequency, not all amounts of energy are possible, but only those having to the frequency a ratio which is a certain quantity h multiplied by 1 or 2 or 3 or some other whole number, in practice always a small whole number. The quantity h is known as "Planck's constant"; it has turned out to be involved practically everywhere where measurement is delicate enough to know whether it is involved or not. It is such a small quantity that, except where measurement can reach a very high degree of accuracy, the departure from continuity is not appreciable.

Bohr's great discovery was that this same quantity h is involved in the orbits of the planetary electrons in atoms, and that it limits the possible orbits in ways for which nothing in Newtonian dynamics had prepared us, and for which, so far, there is nothing in relativity-dynamics to account. According to Newtonian principles, an electron ought to be able to go round the nucleus in any circle with the nucleus in the centre, or in any ellipse with the nucleus in a