Page:Popular Science Monthly Volume 59.djvu/166

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156
THE POPULAR SCIENCE MONTHLY.

wonderful closeness to exact multiples. This is shown by the following table, taken from the report of the Committee on Atomic Weights of the American Chemical Society for 1899:

 
Arsenic 75 .0 Lead 206 .92 Phosphorus 31 .0
Boron 11 .0 Lithium 7 .03 Rhodium 103 .0
Bromin 79 .95 Manganese 55 .0 Silver 107 .92
Carbon 12 .0 Mercury 200 .0 Sodium 23 .05
Cerium 139 .0 Nitrogen 14 .04 Sulfur 32 .07
Cobalt 59 .00 Osmium 191 .0 Tin 119 .0
Gallium 70 .0 Oxygen 16 .00 Yttrium 89 .0
Iron 56 .0 Palladium 107 .0
 

In addition to these at least nine others have atomic weights differing not more than 0.1 from whole numbers. By the law of probabilities this close approach to whole numbers cannot be the result of chance, but no satisfactory explanation has as yet been offered.

Prout's hypothesis was not unique in concerning itself with an effort to show a unity of matter. Very early there was noticed a connection between the atomic weights and the properties of certain groups of elements. Attention was first called to this by Professor Döbereiner, of Jena, and an account was given of it in print in 1816, just after the first enunciation of Prout's hypothesis. Döbereiner noticed that the equivalent weight of strontium was 50, while the values then accepted for calcium and barium were respectively 27.5 and 72.5. Fifty is the mean of 27.5 and 72.5, and the properties of strontium may be looked upon as being an average of those of calcium and barium. It was soon clear, however, that strontium was as much entitled to recognition as an element as calcium or barium. Hence it appeared that there was a numerical relation between the weights of the atoms of these three elements, barium, strontium and calcium, which corresponded to both the chemical and the physical properties of the elements. Several other similar groups of three were discovered by Döbereiner, and this, which was really the earliest germ of the Periodic Law, became known as Döbereiner's law of triads. It is of especial interest, as having enabled its author to predict the atomic weight of bromin, which was later confirmed by experimental investigation. In this respect it anticipated the Periodic Law, and may be said to represent a phase of this later and greater generalization.

Little attention was attracted by the speculations of Döbereiner, and a quarter of a century or so later the subject was taken up anew by the great French chemist, Dumas. He developed to some extent the law of triads, though he made little actual advance beyond the point attained by Döbereiner. Dumas's work was, however, widely noticed, and proved very stimulating to the chemists of his day. It