The corpuscular theory of matter, in its modern sense, originated in connection with the study of the discharge of electricity through rarefied gases. Previous to the experimental investigation of this subject it was customary to regard positive and negative electricity as being of the same general nature, but differing from each other somewhat as a right-handed helix or spiral differs from a left-handed one, or (more accurately) as a positive number differs from a negative one. The study of vacuum-tube phe nomena indicated, however, that there is a far more profound difference than this between the two kinds of electricity. It was shown by the researches of Plucker, Hittorf, Crookes and others that when electricity is passed through a tube containing air or any other gas in an ex tremely rarefied condition, the discharge from the negative electrode (or "cathode") is wholly different from the discharge that takes place at the positive electrode (or °anode"). The nega tive discharge (when the vacuum in the tube is high enough) takes place along straight lines that are everywhere at right angles to the sur face of the cathode from which they proceed, and the phenomena observed at the positive electrode are altogether different and far less striking.
The negative discharge that proceeds from the cathode (or negative electrode) at right angles to its surface, in a "vacuum tube," is called the °cathode ray," and special attention was naturally paid to this ray, in an effort• to discover its nature. The most strikingly obvi ous fact about it is, that it excites a vivid fluor escence in the glass wall of the tube, where it strikes it. Hittorf, in 1869, showed that a solid object, placed in the course of the ray, inter cepts it and casts a shadow, its outline being plainly visible because there is no fluorescence on the part of the tube that is shielded by the obstacle. Crookes, following Hittorf, took up the study of vacuum phenomena in a fascinat ing and masterly way and obtained many re sults that were not only beautiful and striking, but also exceedingly suggestive and stimulating to further inquiry. By placing a very light pad dle-wheel in the tube, so that itspaddles were within the cathode stream (or ray) on one side, and out of it on the other side, he obtained mechanical rotatory effects. By giving the negative electrode a concave form, and thereby bringing the cathode ray to a focus at a point within the tube, he showed that marked heat ing effects could be produced by it. These phe nomena, together with many others that were observed, suggested that the cathode ray con sists of a stream of material particles, nega tively electrified by contact with the cathode and then repelled from the cathode on account of the charge they have acquired. This hail in fact been strongly urged by Varley (in 1871), and very likely by others also, as it was a fairly obvious inference, though it was not the only explanation possible. (In Germany the cathode ray was quite generally believed to be due to some form of wave-like disturbance in the ether, and this view was held by Goldstein, who first introduced, in 1876, the name "Ka thodenstrahlen," or °cathode rays"). One dif
ficulty was, to identify the nature of the charged particles that were thus repelled. There were reasons for believing that they are not ordinary molecules or atoms. It was known, for example, that when an electrically-charged liquid is evaporated, the vapor does not carry away the electrical charge with it, and this ap peared to indicate that the individual atoms or molecules of a gas cannot be separately electri fied. Moreover, if the cathode ray consists merely of electrified molecules, it was hard to understand why the effects that were observed in connection with the cathode were not also manifested in connection with the anode, or positive electrode. Crookes, as a result of his researches, concluded that the projectile the ory (or charged-particle theory) of the cathode ray is correct, but as he fully realized the dif ficulties in the way of that theory, he announced his belief that in vacuum-tube phenomena we are dealing with matter in a previously unknown state, which he called the °radiant state.' It is fair to say that his views appeared to physicists in general as rather too mystical, though the eminence and ability of their author ensured them a respectful reception.
Following the experiments of Crookes there was a lull in the activity with which the phe nomena of vacuum tubes were studied, but in tense interest in the subject was again aroused by two exceedingly striking discoveries. Len ard, in 1894, showed that the cathode ray can be made to emerge from the tube and pass into. the outside air of the laboratory, if a "win dow' of very thin aluminum (instead of glass) is provided at the point at which the cathode ray strikes the tube. Two years later (namely in 1896) Röntgen discovered that a previously unknown form of radiation is emit ted from the point at which the cathode ray strikes against the tube, or against any other solid obstacle. The prospective usefulness of the Rontgen rays (or °X-rays") to the surgeon gave them an intense practical interest, in ad dition to the interest that they had for purely physical reasons; and from this time onward the study of the electric discharge was prose cuted with renewed vigor and earnestness, and by a large number of physicists—further stimulation being presently added by the dis covery of radioactivity (1896) and of polonium and radium (1898). Exceedingly prominent among the physicists who took UD the study of the cathode discharge at about this time was Sir J. J. Thomson. Beginning his researches by investigating the general phenomena attend ing the passage of electricity through gases, and guided by a wonderful scientific imagina tion, supplemented by a profound knowledge of mathematics and marked experimental skill, he established the soundness of Crookes' views, generalized them amazingly, developed a new "corpuscular theory" of matter and placed that theory on a firm foundation.