But there is one other advance of fundamental importance which came with the study of the properties of gases ionized by X-rays. For up to this time the only type of ionization known was that observed in solution and here it is always some compound molecule like sodium chloride (NaCl) which splits up spontaneously into a positively charged sodium ion and a negatively charged chlorine ion. But the ionization produced in gases by X-rays was of a wholly different sort, for it was observable in pure gases like nitrogen or oxygen, or even in monatomic gases like argon and helium. Plainly, then, the neutral atom even of a monatomic substance must possess minute electrical charges as constituents. Here we had the first direct evidence (1) that an atom is a complex structure, and (2) that electrical charges enter into its make-up. With this discovery, due directly to the use of the new agency, X-rays, the atom as an ultimate, indivisible thing was gone, and the era of the study of the constituents of the atom began. [Pg 41] And with astonishing rapidity during the past twenty-five years the properties of the subatomic world have been revealed. Physicists began at once to seek diligently and to find at least partial answers to questions like these: 1. What are the masses of the constituents of the atoms torn asunder by X-rays and similar agencies? 2. What are the values of the charges carried by these constituents? 3. How many of these constituents are there? 4. How large are they, i.e., what volumes do they occupy? 5. What are their relations to the emission and absorption of light and heat waves, i.e., of electromagnetic radiation? 6. Do all atoms possess similar constituents? In other words, is there a primordial subatom out of which atoms are made? The partial answer to the first of these questions came with the study of the electrical behavior of rarefied gases in so-called vacuum tubes. This field had been entered and qualitatively explored with amazing insight as early as 1879 by Sir William Crookes, who in describing in that year some of his experiments said: The phenomena in these exhausted tubes reveal to physical science a new world—a world where matter exists in a fourth state.... In studying this fourth state of matter we seem at length to have within our grasp and obedient to our control the little indivisible particles which with good warrant are supposed to constitute the physical basis of the universe.[24] [Pg 42] Further, by 1890 Sir Arthur Schuster[25] had gone a step farther and shown how the ratio of the charge to the mass of these same hypothetical particles might be determined. Indeed he had experimentally evaluated this ratio, obtaining, however, a value very much too small, namely, .