Page:Popular Science Monthly Volume 8.djvu/482

466 as to outweigh many times over all seeming variations. All this evidence is, however, inadequate in one respect: the relations thus far pointed out cannot be simply expressed in figures. Are there, then, any numerical relations connecting the elements? This question may be answered, partly by studying their atomic weights, and partly by an examination of their specific volumes.

The regularities which connect the elementary atomic weights have been examined and discussed by many investigators from widely differing points of view. Some chemists have contented themselves with the naked facts; others have considered the bearing of those facts upon chemical theories; and a third class, with less caution than ignorance, have speculated upon them in the wildest and most reckless manner. Of course a full summary of the whole subject, however interesting it might prove, would be out of place in a condensed argument like this. All we can do here is to glance at a few of the many relations known, and afterward consider them in their connection with our main subject. The general reader who cares to go deeper into the question will do well to consult the original papers of Dumas, Gladstone, J. P. Cooke, Kremers, Mendelejeff, and others.

Of the relations now under consideration, the one most frequently cited is as follows: Many elements are most naturally arranged in threes, of which the middle member has an atomic weight very nearly a mean between the atomic weights of the other two. Thus we have calcium, atomic weight, 40; strontium, 87.5; and barium, 137. Here, if the value of strontium were 88.5, it would be an exact mean. Again, chlorine has the atomic weight 35.5; bromine, 80; and iodine, 127; the second being almost precisely midway between the first and third. A still closer agreement with theory is furnished by lithium, sodium, and potassium, whose values are respectively 7, 23, and 39.1. A fourth example is afforded by potassium, 39.1; rubidium, 85.4; and cæsium, 133; while a fifth case is offered by phosphorus, 31; arsenic, 75; and antimony, 122. To be sure, these illustrations afford only an approximation to regularity; but then the variations are themselves somewhat regular. In each of these twos the middle term is just a little too low to be an absolute mean between its associates; that is, the variations from theory are all in one direction. It is hardly possible at present to say whether this means anything, or is only ascribable to accident. One more example of regularity among atomic weights is worth noting, namely, the relation which connects the members of the oxygen group. Here we have oxygen, 16; sulphur, 32; selenium, 79.5; and tellurium, 128. These higher numbers are simple multiples of the lowest; there being only a variation of half a unit (minus) in the case of selenium. Since these elements are very similar in their chemical relations, this regularity is extremely significant. Can it be due to chance, and void of real meaning?

But all these relations prove nothing—they merely suggest.