some of the electrons out of them, and thus leaves behind it a trail of electrified wrecks. Mr. Wilson deposits drops of water on these wrecks, and thus the path of the electrified atom or electron is marked out by a trail of drops of water which can be seen and photographed. We can map out in this way the path of even one atom.
I think every worker at the Atomic Theory must have looked at these photographs with feeling akin to those of Adams and Leverrier when they first saw Neptune. Confident as one may be in the truth of a theory, there are few whose faith is so robust that they do not feel relieved when they see the conclusions to which they have been led by theory verified by experiment. Seeing is believing. Let me quote on this point a sentence by the great man who fills our thoughts to-day. Roger Bacon says, 'Argument may conclude a question but it cannot make us feel certain, except the truth be also found to be so by experience.'
To illustrate what this method can do, let me take two examples. It has been known ever since the discovery of Röntgen rays that when these rays pass through a gas they produce electrified atoms and electrons; if we take by Wilson's method a photograph of air when the Röntgen rays are passing through it, we find that the drops of water are not uniformly distributed over the photograph, but are strung together in fine lines giving the appearance of an untidy spider's web. This shows that when the atoms are struck by the Röntgen rays some of them give off electrons moving at a high speed; the paths of the electrons are indicated by the fine lines along which the water drops are arranged. Thus the electrons liberated by Röntgen rays start off at a speed which carries them a considerable distance through the air. Now let us take another case when electrified atoms and electrons are