Text: Next, an analysis was made of the radium emanations themselves to ascertain their exact nature. Although these radiations are invisible to the eye nevertheless they were made to appear visible by various ingenious contrivances. By placing a small quantity of radium into a prepared cavity in a solid lead block, the rays were then permitted to pass through a tiny aperture in the lid and made to graze along a wall which had been covered with a phosphorescent substance such as zinc sulphide. The radium rays brushing against this wall produced a faintly illuminated streak. Now a magnet of known strength was held against the wall a few inches to one side of the streak. Immediately the streak divided into three parts, one portion bending toward the magnet another portion bending away from the magnet, while a third portion retained the original position, being neither attracted or repelled. This proved that three distinct streams are emitted, one of which consists of positively charged particles, another consists of negative particles, while another is of such a nature as to absolutely defy the magnetic field. The positive stream they called Alpha rays, the negative stream Beta rays and the independent stream Gamma rays, after the first three letters of the Greek alphabet. These are usually referred to by the Greek characters themselves, as a, b and y rays. By letting these rays fall squarely upon a phosphorescent target held at variable distances from their source, minute sparks, plainly visible through a telescope were produced by the a and b streams thereby proving that they consist of infinitesimal particles like leaden shot from a shotgun. The y rays were found to be of the nature of X-rays but far more penetrating. After determining the existence of the three different kinds of emissions from radium various experiments were then undertaken to determine their exact nature. First their respective velocities were ascertained. This was accomplished by noting the amount of deflection that could be produced by magnets of known strength acting upon the charged particles. Prof. Schuster proposed a mathematical formula by which velocity may be computed where the amount of curvature and the strength of the magnetic field are known. But his equations required certain assumptions and were therefore not entirely satisfactory until later experiments supplied the missing data. Accordingly Profs. Thomson, Wilson and others experimentally determined the velocity; energy and charge of both the a and b particles without indulging in any assumptions whatsoever. It was found that an electric current flowing from negative to positive possesses all the properties of the b rays from radium, except velocity; hence it was possible to make a very close study of b particles under most favorable circumstances in vacuum tubes. Such currents are called Cathode rays, because they flow the negative pole (or cathode) to the positive pole (or anode). By boring a hole through the centre of the anode some of the current or ray would pass directly through it because of its velocity, and when it thus fell upon a phosphorescent screen behind the anode the same scintillations were produced as in the case of the radium emanations, thereby showing that the cathode current or stream actually consists of multitudes of individual particles (electrons) which were found to be identical with those which comprise the b rays from radium. It was arranged that the negative particles (electrons) which pass through the anode would be deflected by a magnet and caused to fall into an insulated hollow vessel. An electrometer connected to the vessel was therefore able to record the aggregate charge of the particles collected within a given time. By a similar arrangement their aggregate energy was measured by means of a galvanometer. With these quantities known, together with the amount of curvature produced by a given force, a simple algebraic equation then yielded the information sought, viz., the velocity. In like manner the velocity of any current or any radioactive emanation may be definitely ascertained. This experiment, and various others of a more elaborate nature, enabled scientists to determine the following facts about radioactive radiations: a rays are almost identical with helium atoms, having a mass 7,000 times greater than b particles. An a particle is not, therefore, an individual proton, but is an aggregation of protons and electrons in which the protons predominate and thus give it a positive charge. These a particles, because of their relatively larger size, are far less penetrating than are the b particles. They are unable to pass through an ordinary sheet of paper. Their velocity is about 20,000 miles a second. b rays consist of individual electrons which are, of course, negatively charged. They have a mass of about 1/1700th of an atom of hydrogen (the smallest atom known) and can easily penetrate thin sheets of aluminum or iron. They have a maximum velocity of 170,000 miles a second, which is over nine-tenths the speed of light itself. y rays consist in nature to X-rays but have greater penetrating powers, and like the latter they possess the same velocity as ordinary light, viz., 186,000 miles a second. They carry no electric charge and therefore cannot be deflected by a magnetic or electrostatic field. They are emitted only by those radioactive substances which also emit b rays (some substances emit only a particles). They do not consist of particles such as make up the a and b streams, but are pulsations of energy evidently produced by the atomic "explosion" when the b particles are shot forth. Owing to the unusual penetrabillity of the y rays it is difficult to utilize them efficiently in the study of radio-chemical effects. There remains much yet to be determined regarding these powerful y rays. Prof. Thomson's apparatus, mentioned above, proved that b particles carry a considerable charge; yet their mass was so excessively minute that it could not be measured by any means then employed. He estimated that it would take a century to collect a weighable amount of electrons in his insulated vessel, viz., one-thirtieth of a milligram. Of course the mass, and consequently the size, of these electrons could have been mathematically computed from their aggregate charge, energy and known speed, provided the number of particles constituting the aggregation were known. Being unable to determine this by means of the aforementioned apparatus, he undertook an entirely different experiment and was happily rewarded with success. The details of this brilliant experiment are quite interesting. (Dr. Abrahms' Electron Theory card id 5603)

See Also: TRIPLE FLOWS, TRIPLE CURRENTS, LAW OF TRIANGLE, LAWS OF BEING

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