It is because the force which a positively-charged atom exerts on an electron is so great and because the electron is so light and easily moved compared to an atom that the electron has not been isolated at will until recent years. The isolation in fact depends upon the electron being endowed with a sufficient speed to carry it through or past the action of an atom which is seeking to absorb it into its system. A lump of matter flying in space might enter our solar system with such speed as to be able to pass through and go on its way almost undeflected. Or again, it might have a much lower speed and go so much nearer the sun that it was seriously deflected in its course, as we see in the case of comet visitors. But if for some reason or other the lump of matter found itself inside the solar system without the endowment of high velocity it would certainly be absorbed. Just so an electron can pass through an atom with or without serious deviation from its line of motion, provided that motion is rapid enough. Only recently have we been able to exert electric forces of sufficient strength to set an electron in motion with the speed it must have if it is to maintain an individual existence Now we can gather electrons at will, dragging them from the interior of solid bodies, and hurl them with tremendous speed like a stream of projectiles. Since in the open air the speed is soon lost by innumerable collisions with the air-molecules, the effect can only be studied satisfactorily in a glass bulb from which the air has been evacuated. Crookes made great improvements in air-pumps during an investigation on thallium, and consequently was able to obtain the high vacuum required for the experiment with the electron streams. It was afterwards found by Roentgen that when an electron stream in an evacuated bulb was directed upon a target placed within the bulb, a remarkable radiation issued from the target. Thus arose the so-called X or Roentgen rays. As you all know they have for many years played a most important part in surgery and medicine. You may have heard that during the war they were also used to examine the interior of aeroplane constructions and to look for flaws invisible from without. Although X-Rays are of the same nature as light rays they can penetrate where light rays cannot, passing in greater or less degree through materials which are opaque to visible light and allowing us to examine the interior which is hidden from the eye.