The rays are invisible, but they affect a photographic plate in the same way that light does. They also make air into a conductor of electricity and if allowed to impinge upon a surface coated with a suitable substance they cause it to glow.
This spontaneous giving off of rays is now spoken of by the general term of "radio-activity," and it has grown into an important branch of science. A number of other substances have been found to exhibit the same peculiar ray-forming powers, notably Thorium, one of the components of the incandescent
gas mantle by the prolonged application of a fragment of which to a photographic plate an impression can be obtained due to the rays.
What, then, are these rays? It is found that they are of three kinds, not that they vary from time to time, but that they can be sorted out into three different sorts of rays which are given off simultaneously all the time.
The first sort are stopped by a sheet of paper, the second passing easily through a thick metal plate, while the third appear to be identical with X-rays.
For convenience the three sorts are termed Alpha, Beta and Gamma rays, respectively, after the first three letters of the Greek alphabet.
Further, the Alpha rays prove to be a torrent of tiny particles about the size of atoms, indeed if they be collected the gas Helium is obtained, so that evidently they are helium atoms, and since that is one of those substances whose molecules consist of a single atom each they are also molecules of helium. No doubt the reason why they are so easily stopped by a piece of paper is because being complete atoms they are large, huge indeed, compared with the particles which form the Beta rays, for they are apparently those same electrons which are found in the X-ray tube, and which are at least 2000 times smaller than the smallest atom.
When the electrons in the vacuum tube are suddenly brought to a standstill X-rays are given off and in like manner X-rays no doubt would be given off when they start on their journey, providing that they started suddenly enough. Hence it is the starting
or sudden explosion-like ejection of the Beta particles which is believed to give rise to the Gamma rays.
The strength or intensity of the rays can be measured very conveniently by their action in making air conductive to electricity, for which purpose a very beautiful but simple instrument called an Electroscope is employed. It consists generally of a glass-sided box or else a bottle with a large stopper, consisting of sulphur or some other particularly good insulator. Through this a wire passes down into the inside of the vessel terminating in a vertical flat strip to the upper end of which is attached a similar strip of gold leaf or aluminium foil. Normally the leaf hangs down close to the strip, but if the wire above the stopper be electrified by touching it with a piece of sealing-wax rubbed lightly against the coat sleeve the charge of electricity passes down into the inside and causes both strip and leaf to become so electrified that they repel each other.