VOL.8, No. 9
ATOMIC MODELS
1851
flat file if necessary, and the channels for the wires around which the ball is to be cast may be cut out with a pocket knife. After thorough drying give the mold a good soaking in melted vaseline and grease well with the same material before each use. Before use, molds should be paired by placing the ball bearing between two halves and trimming the edges t o fit. A groove or two made across the edges on one side will serve t o identify a pair and to enable the halves t o be fitted together in the same way each time. A battery of eight or nine molds can conveniently be filled a t one time w i t h a mix of 45 g. of plaster of Paris to 25 g. of water. After all the halves are filled the first will probably he thick enough so that it will not spill when inverted for joining t o its mate around the desired wire. Rubber bands hold the halves together. An hour is ample time to allow before unmolding.
Literature Cited (I)
MELLOR."A Comprehensive Treatise on Inorganic and Theoretical Chemistry." Vol. X, Longmans, Green and Co., New York City. 1930, pp. 238-9.
Radium Rays Live Only Fraction of Second. A short and adventurous life of a hundred millionth of a second is enjoyed by an alpha ray from radium after being imprisoned in the nucleus ~f the atom for a period which may range from less than a yrar to one thousand million years. Lord Rutherford recently described here the results of his long researches in radioactivity in these terms in a lecture to the Royal Institution. Despite its hectic life after escaping, he continued, the brief period of activity of the alpha cartick has been so thoroughly studied that we are better acquainted with its character and advcntrre.9 than we could ever hcpe to be with the character and personality of a distinguished citizen. "The alpha particle has played a great part in the development of physics," said Lord Rutherford. " I t provided the first evidcnce of the transmutation of elements. . . . I t first afforded the opportunity of detecting effects due to a single atom, thus furnishing, for the first time, definite proof of the truth of the atomic theory. By its aid, also, an early. . . estimate w a made of the dimensions of the atom. . . " I t was, moreover, by studying the scattering of alpha particles that the nuclear structure of the atom had been revealed, a peculiarity which constituted the basis of all modern views on the nature of the atom. The alpha particle had also proved invaluahlc in investigating the forces reigning a t the heart of the atom, and had been applied with grcat success to the artificial transmutation of the light elements. . . . . "The nucleus of an atom constitutes a minute world of its own. If an atom of radium were magnified to the size of the lecture room, its nudeus would be smaller than a pin head. . . . In some sense we might regard thc nucleus as analogous to a drop o l water which was held together by surface tension, but the analogy was a very rough one. I n the case of the nucleus, the surface tension of the water drop was replaced by a potential barrier of grcat height, through which the particles inside were unable to escapc . . The height of the barrier at its crest might be as much as 20,00U.000 volts."-Science Scnncc
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