Page:Popular Science Monthly Volume 87.djvu/119

Rh changes of energy in chemical reactions without the assumption of any definite theory of the constitution of matter. This tendency advanced so far that text-hooks of chemistry were written in which the word atom or molecule was taboo, and chemistry was based instead on the law of combination in multiple proportion. At that time, it did undoubtedly appear that there was little, if any, hope of finding a concrete proof of the validity of the atomic hypothesis, or of detecting by its effects a single atom of matter or a single electron, for it was known that the smallest fragment of matter visible under a high power microscope must still contain many millions, or even billions, of atoms.

The march of science has, however, been so rapid in this direction that we have been able in recent years to show in a definite and concrete way the independent existence of atoms and also of electrons in rapid motion.

We shall first of all consider the method devised by Rutherford and Geiger for detecting and recording the effects of single alpha particles from radium. At this stage, it is unnecessary to enter into details of the nature of the transformations occurring in radioactive matter. It suffices to say here that the atoms of a radioactive substance are unstable and occasionally break up with explosive violence. In many cases, the explosion is accompanied by the ejection of a charged body, called the alpha particle, with a velocity of about 10,000 miles a second. These alpha particles are known from other investigations to consist of charged atoms of the rare gas helium. The presence of these rays is simply shown by the marked phosphorescence they set up in certain substances. I have here a fine glass tube which was filled about a week ago in Manchester with purified emanation released from about one fifth of a gram of pure radium. In the interval of its journey across the Atlantic, the activity of the emanation has decayed to about one quarter of its original value. The glass walls of the tube are made so thin—about 1/100 millimeter—that the alpha rays are able to escape freely into the surrounding air. They produce a small phosphorescence in the walls of the glass tube which is just visible in the darkened room. On bringing near, however, a screen covered with zinc sulphide, a brilliant phosphorescence is observed which increases in intensity as we approach the tube. Similar effects are seen to be produced in this crystal of willemite, while the crystal of kunzite is seen to be translucent and emit a ruddy light. This phosphorescence of zinc sulphide and willemite is due mainly to the alpha rays, and from the present emanation tube about 5,000,000,000 of these particles are projected each second.

In their passage through air or other gas, the alpha particles produce from the neutral molecules a large number of negatively charged