Page:Popular Science Monthly Volume 29.djvu/542

526 after a laborious search, found that it belonged to yttrium. Subsequent studies showed this modification of spectrum analysis to exceed in delicacy all known tests for the rarer earths; yttrium can be detected when present in one millionth part. Within a twelvemonth, Crookes has made known the application of radiant matter spectroscopy to samarium; the delicacy of this test surpasses that for yttrium, and the anomalous behavior of the mixed earths yields phenomena "without precedent."

When Dalton, the Manchester schoolmaster, added to the atomic theory of the Greeks the laws of definite and of multiple proportions, he transformed an "interesting intellectual plaything" into an exact scientific theory capable of experimental demonstration. The importance of ascertaining the atomic weights of the elements with the utmost accuracy has stimulated chemists to apply to the problem their best endeavors; and as the methods of analysis become more refined, the determinations are again and again repeated, every ascertainable and imaginable source of error being carefully eliminated. Besides the experimental repetitions, the figures obtained by various observers have recently been submitted to careful recalculations by Clarke in this country, and soon after by Lothar Meyer and Seubert, in Germany. Their labors give chemists the latest and most reliable constants.

For many years chemists have dimly perceived the probable correlation of the properties of the elementary bodies and their atomic weights. Dumas pointed this out for certain marked groups, Newlands emphasized it; but it remained for a Russian chemist, Mendelejeff, to establish, in 1869, a law of great importance. Mendelejeff showed that if the elements are grouped in the order of their atomic weights, it will be found that nearly the same properties recur periodically throughout the entire series. This so-called Periodic Law is more concisely stated thus: The properties of the elements are periodic functions of their atomic weights. The accuracy of the deductions based on this law is strikingly shown by the fact that Mendelejeff, finding an unfilled blank in the periodic system, boldly announced the general and special properties of the element awaiting discovery; six years later, Lecoq de Boisbaudran discovered gallium, an element which proved to have properties almost identical with those of the hypothetical eka-aluminium described by Mendelejeff. And in 1879 the accuracy of Mendelejeff's prophecy was further confirmed by Nilson's discovery of scandium, the counterpart of the hypothetical ''ekabor. Eka-silicon,'' though yet to be discovered, may almost be regarded as a known element, so fully have its properties been predicted.

The correlation between atomic weights and physical properties is being extended, and now embraces the fusibility, boiling-points, general affinities, color, occurrence in nature, physiological functions, and many other factors. Dr. Carnelley, who has been active in