leather rollers, and when the machine tries to spin the threads together they fly apart and refuse to join up the minute hooks with which the wool fibres are furnished.  The spinning operation would come to an end were there not means provided by which the air can be so filled with moisture that the fibres become damp and the action ceases.  So in some cases a stream of air filled with positive and negative ions is made to play upon the fibres; the fibres select what ions they want, and so neutralizing themselves, spinning can proceed again.

When a current of electricity runs along a wire there is in fact nothing more than a procession of electrons.  The stream of electrons that runs through the filaments in the lamps that light this room, raising the filaments to a white heat, are set in motion by the dynamos in the city.  There is a complete wire circuit, including the dynamo, the conductors, and the lamps.  When the dynamos are not working the electrons do not as a whole move either way, though they are always there.  When the dynamo begins to turn, the electrons set out on their continuous journey.

Electrons are involved in the emission of wireless signals, and in their receipt.  The so-called ‘valve’, which multiplies minute electric signals and was so greatly improved during the war, depends entirely on the action of electrons, and the brilliant experimental work was based on the newly-acquired knowledge of their properties.

I have told you that under certain circumstances a stream of electrons may generate X-Rays, in reality a form of light rays.  This action is a very common one, and it is curious that the faster the electron goes the shorter is the wave-length of the radiation.  A very fast electron generates an X-Ray of so short a wave-length that the penetrating power of the ray, which goes with the shortness of the wave, is excessive, and in this way we may have rays which go right through the human body or even through inches of steel.  As the speed of the exciting electron becomes less, the X-Rays are less penetrating.  With still slower electrons we may generate ordinary light, and it will take a slower electron to generate red than to generate blue.  The slowest electrons we use in this way have a speed of many hundred miles per second; the fastest have a speed which nearly approaches that of light, or 186,000 miles a second.

And conversely radiation can set electrons in motion.  When X-Rays are driven into a patient’s body electrons are set in motion within, and moving over certain minute distances, initiate chemical actions which are necessary to some cure.  Or they may go right through the body and fall on a photographic plate, setting in operation chemical action which forms a picture on the plate.

There is another occasion of an entirely different kind when the electron is greatly in evidence and displays effects which are most astonishing and significant.  Every atom of radium or other radio-active substances sooner or later meets with the catastrophe in which its life as radium ends and atoms of other substances are formed.  At that moment occurs the emission which is the characteristic property of the substance.  One of the radiations emitted consists of high-velocity electrons, moving, some of them, nearly as fast as light.