568 MAN IS MASTKRING HIS MATKRIAL WORLD family. As Newlands expressed it, "the eighth element starting from a given one is a kind of repetition of the first, like the eighth note of an octave of music." Independently of each other and in ignorance of Newlands' work, Lothar Meyer in Germany and Dmitri Ivanovitch Mendeleeff in Russia made a thorough study of the properties of the elements and noted a similar relationship. They proposed the law, known as the periodic law, which states that the properties oj the elements are periodic Junctions of their atomic weights. Periodicity is very common in nature. The growth of trees, the cycle of droughts, periods of large sunspots, the tides, and the full moon all occur periodically. A flattened wheel on a railroad car produces a periodic clatter which is a function of the distance traveled, because every full turn of the wheel brings the flattened portion of the wheel into contact with the track. Mendeleeff prepared a table of the elements based upon their periodicity in properties. He left a number of gaps in this table in places where the progression in properties seemed to demand it, arguing that there must be elements which had not yet been discovered. The known character of the elements above and below these gaps made it possible to predict the properties of the missing elements. Mendeleeff made these predictions in 1870 for three elements, calling them eka-boron, eka-aluminum, and eka-silicon. These elements, discovered in 1879, 1875, and 1886, and named scandium, gallium, and germanium, showed properties that checked remarkably closely with those predicted by Mendeleeff. This strong support of the periodic law was strengthened later by the discovery of all but two of the remaining missing elements partly through the aid furnished by the periodic table. The periodic table is one of the most useful generalizations in chemistry, because it is of great value in predicting the properties of elements and in classifying chemical information. How shall one explain this very remarkable law? One generally expects to find a common origin or a common building material for people, houses, or materials which are similar in properties. It is the fact that houses are made of smaller units, such as bricks, that makes possible such a wide variety of patterns. This complexity of structure also makes it possible to build large or small houses of the same design, using different numbers of bricks. If the atoms of the elements are ultimate, no relationships should exist. The chief significance of the periodic law is that it suggests the existence of atoms of similar, and therefore complex, structure.
ATOMS ARE COMPLEX 569 Discharge-tube Experiments Suggested the Electrical Nature of Atoms. It was observed that the electrons produced in discharge tubes have the same charge and mass, regardless of the nature of the metals used for the electrodes or of the nature of the gas in the tube. It was also observed that a stream of positively charged particles, the canal rays, was produced in discharge tubes in addition to the cathode rays. Sir J. J. Thomson developed the hypothesis that the atoms of all the elements are composed of these positively charged particles in combination with electrons. This evidence of the electrical nature of atoms, furnished by the discharge tube, was confirmed by other observations. The emission of electrons by certain metals when they are heated or exposed to light, the production of an electromotive force by chemical cells or static methods, and Faraday's laws of electrolysis became intelligible with the advent of the electron theory of atomic structure. The Discovery of Radioactivity Was the Next Major Evidence of the Complexity of Atoms. The discovery of radioactivity was made in 1896 by Antoine Henri Becquerel while he was engaged in the study of the phosphorescence of certain substances. Becquerel selected certain compounds of uranium for the experiments because they are very phosphorescent. The thought occurred to him that uranium compounds might emit X rays, so he exposed a uranium compound to sunlight and placed it above a metal cross which rested on a photographic plate wrapped in a piece of heavy black paper. With the development of the plate the image of the cross became visible, thus showing that a penetrating radiation had been emitted by the uranium. Becquerel continued his study of this interesting phenomenon and, by a lucky accident, made one of the most momentous discoveries of physical science. He had placed his photographic plate in a dark cupboard with some uranium salt until he could find time on a sunny day to carry out some more experiments. Nearly a month went by before he got around to further work on this problem. It occurred to him to develop the plate and see if it had been affected by the unexposed uranium. This he did, and the plate had been affected; here was a substance that emitted penetrating rays without any excitation. Pierre Curie (1859-1906) and his Polish wife, Madame Marie Curie, became interested in these Becquerel rays and started an investigation to learn whether or not there were other substances besides uranium salts that would emit these rays. They discovered that uranium ore, pitchblende, showed this property of emitting rays even more than