The very first step into this newly discovered territory made the riddle still more bewildering. As we have said, Maxwell's use of a material analogy as a means of formulating mathematically the properties of electro-magnetic fields of force had led to results which brought electricity into close conjunction with light. In his own way Crookes focused, to begin with, his attention entirely on the light-like character of electric effects in a vacuum. It was precisely these observations, however, as continued by Lenard and others, which presently made it necessary to see in electricity nothing else than a special manifestation of inert mass.
The developments leading up to this stage are recent and familiar enough to be briefly summarized. The first step was once more an accident, when Röntgen (or rather one of his assistants) noticed that a bunch of keys, laid down by chance on top of an unopened box of photographic plates near a cathode tube, had produced an inexplicable shadow-image of itself on one of the plates. The cathode tube was apparently giving off some hitherto unknown type of radiation, capable of penetrating opaque substances. Röntgen was an experimentalist, not a theorist; his pupils used to say privately that in publishing this discovery of X-rays he attempted a theoretical explanation for the first and only time in his life - and got it wrong!
However, this accidental discovery had far-reaching consequences. It drew attention to the fluorescence of minerals placed in the cathode tube; this inspired Becquerel to inquire whether naturally fluorescent substances gave off anything like X-rays, and eventually - yet again by accident - he came upon certain uranium compounds. These were found to give off a radiation similar to X-rays, and to give it off naturally and all the time. Soon afterwards the Curies succeeded in isolating the element, radium, an element which was found to be undergoing a continuous natural disintegration. The way was now clear for that long series of experiments on atomic disintegration which led finally to the splitting of the nucleus and the construction of the atomic bomb.
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A typical modern paradox emerges from these results. By restricting his cognitive powers to a field of experience in which the concept of force as an objective reality was unthinkable, man has been led on a line of practical investigation the pursuit of which was bound to land him amongst the force-activities of the cosmos. For what distinguishes electric and sub-electric activities from all other forces of physical nature so far known to science, is that for their operation they have no need of the resistance offered by space-bound material bodies; they represent a world of pure dynamics into which spatial limitations do not enter.
Equally paradoxical is the situation of theoretical thinking in face of that realm of natural being which practical research has lately entered. We have seen that this thinking, by virtue of the consciousness on which it is founded, is impelled always to clothe its ideas in spatial form. Wherever anything in the pure spatial adjacency of physical things remains inexplicable, resort is had to hypothetical pictures whose content consists once more of nothing but spatially extended and spatially adjacent items. In this way matter came to be seen as consisting of molecules, molecules of atoms, and atoms of electrons, protons, neutrons, and so forth.
In so far as scientific thought has held to purely spatial conceptions, it has been obliged to concentrate on ever smaller and smaller spatial sizes, so that the spatially conceived atom-picture has finally to reckon with dimensions wherein the old concept of space loses validity. When once thinking had started in this direction, it was electricity which once more gave it the strongest impulse to go even further along the same lines.
Where we have arrived along this path is brought out in a passage in Eddington's The Nature of the Physical World. There, after describing the modern picture of electrons dancing round the atomic nucleus, he says: 'This spectacle is so fascinating that we have perhaps forgotten that there was a time when we wanted to be told what an electron is. This question was never answered. No familiar conceptions can be woven round the electron; it belongs to the waiting list.' The only thing we can say about the electron, if we are not to deceive ourselves, Eddington concludes, is: 'Something unknown is doing we don't know what.'4
Let us add a further detail from this picture of the atom, as given in Eddington's Philosophy of Physical Science. Referring to the so-called positron, the positive particle regarded as the polar opposite of the negative electron, he remarks: 'A positron is a hole from which an electron has been removed; it is a bung-hole which would be evened up with its surroundings if an electron were inserted. ... You will see that the physicist allows himself even greater liberty than the sculptor. The sculptor removes material to obtain the form he desires. The physicist goes further and adds material if necessary - an operation which he describes as removing negative material. He fills up a bung-hole, saying he is removing a positron.' Eddington thus shows to what paradoxical ideas the scientist is driven, when with his accustomed forms of thought he ventures into regions where the conditions necessary for such forms no longer exist; and he concludes his remarks with the following caution: 'Once again I would remind you that objective truth is not the point at issue.'
By this reminder Eddington shows how far science has reconciled itself to the philosophic scepticism at which man's thinking had arrived in the days of Hume. In so far as the above remark was intended to be a consolation for the bewildered student, it is poor comfort in the light of the actions which science has let loose with the help of those unknown entities. For it is just this resignation of human thought which renders it unable to cope with the flood of phenomena springing from the sub-material realm of nature, and has allowed scientific research to outrun scientific understanding.