The Outline of Science, Volume 1

But these wonders of the atom are only a prelude to the more romantic and far-reaching discoveries of the new physics—the wonders of the electron. Another and the most important phase of our exploration of the material universe opened with the discovery of radium in 1898. In the discovery of radio-active elements, a new property of matter was discovered. What followed on the discovery of radium and of the X-rays we shall see. As Sir Ernest Rutherford, one of our greatest authorities, recently said, the new physics has dissipated the last doubt about the reality of atoms and molecules. The closer examination of matter which we have been able to make shows positively that it is composed of atoms. But we must not take the word now in its original Greek meaning (an "indivisible" thing). The atoms are not indivisible. They can be broken up. They are composed of still smaller particles. The discovery that the atom was composed of smaller particles was the welcome realisation of a dream that had haunted the imagination of the nineteenth century. Chemists said that there were about eighty different kinds of atoms—different kinds of matter—but no one was satisfied with the multiplicity. Science is always aiming at simplicity and unity. It may be that science has now taken a long step in the direction of explaining the fundamental unity of all the matter. The chemist was unable to break up these "elements" into something simpler, so he called their atoms "indivisible" in that sense. But one man of science after another expressed the hope that we would yet discover[Pg 254] some fundamental matter of which the various atoms were composed—one primordial substance from which all the varying forms of matter have been evolved or built up. Prout suggested this at the very beginning of the century, when atoms were rediscovered by Dalton. Father Secchi, the famous Jesuit astronomer said that all the atoms were probably evolved from ether; and this was a very favoured speculation. Sir William Crookes talked of "prothyl" as the fundamental substance. Others thought hydrogen was the stuff out of which all the other atoms were composed. The work which finally resulted in the discovery of radium began with some beautiful experiments of Professor (later Sir William) Crookes in the eighties. It had been noticed in 1869 that a strange colouring was caused when an electric charge was sent through a vacuum tube—the walls of the glass tube began to glow with a greenish phosphorescence. A vacuum tube is one from which nearly all the air has been pumped, although we can never completely empty the tube. Crookes used such ingenious methods that he reduced the gas in his tubes until it was twenty million times thinner than the atmosphere. He then sent an electric discharge through, and got very remarkable results. The negative pole of the electric current (the "cathode") gave off rays which faintly lit the molecules of the thin gas in the tube, and caused a pretty fluorescence on the glass walls of the tube. What were these Rays? Crookes at first thought they corresponded to a "new or fourth state of matter." Hitherto we had only been familiar with matter in the three conditions of solid, liquid, and gaseous. Now Crookes really had the great secret under his eyes. But about twenty years elapsed before the true nature of these rays was finally and independently established by various experiments. The experiments proved "that the rays consisted of a stream of negatively charged particles travelling with enormous velocities from 10,000 to 100,000 miles a second. In addition, it was found[Pg 255] that the mass of each particle was exceedingly small, about 1/1800 of the mass of a hydrogen atom, the lightest atom known to science." These particles or electrons, as they are now called, were being liberated from the atom. The atoms of matter were breaking down in Crookes tubes. At that time, however, it was premature to think of such a thing, and Crookes preferred to say that the particles of the gas were electrified and hurled against the walls of the tube. He said that it was ordinary matter in a new state—"radiant matter." Another distinguished man of science, Lenard, found that, when he fitted a little plate of aluminum in the glass wall of the tube, the mysterious rays passed through this as if it were a window. They must be waves in the ether, he said. From "Scientific Ideas of To-day." DETECTING A SMALL QUANTITY OF MATTER In the left-hand photograph the two pieces of paper exactly balance. The balance used is very sensitive, and when the single word "atoms" has been written with a lead pencil upon one of the papers the additional weight is sufficient to depress one of the pans as shown in the second photograph. The spectroscope will detect less than one-millionth of the matter contained in the word pencilled above. Reproduced by permission of X-Rays Ltd. THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER WOUNDED IN THE GREAT WAR Note the pieces of shrapnel which are revealed. Photo: National Physical Laboratory. AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT CORE Reproduced by permission of X-Rays Ltd. A WONDERFUL X-RAY PHOTOGRAPH Note the fine details revealed, down to the metal tags of the bootlace and the nails in the heel of the boot.