Before with a discussion of the de tails of the table, it may he well to inquire what significance can he attached to this periodic varia bility of properties as functions of the atomic.
the respective points are undulatory, with well defined maxima and minima, whieh occur at regu lar intervals. The curves for most of the other properties which are capable of precise meas urement are found to have a similar char acter; the maxima and minima, of course, do not always coincide with the same elements in one curve as in another, but the elements which oc cupy similar positions on one curve are also found to be similarly located on another. It is especially noticeable, moreover, that such curves indicate a relationship between the groups of elements, as well as between the elements them selves of each single group. Thus the properties of the alkaline-earth metals are always found to be intermediate between those of the alkalies and those of the aluminum group. Breaks in the con tinuity of the curves indicate lack of sufficient ex perimental data.
The arrangement of the elements, as shown in the accompanying table, is the one generally adopted at present, and includes all the well known elements. An asterisk marks the ele ments discovered since 1StM. Hydrogen oc cupies a nnique position. and is generally omitted from the classification. Argon, heli um, neon, and krypton cannot be properly included as yet, because their chemical be havior is still unknown. The vertical columns in chide the elements most closely associated with one another, and are known as (;rniip.c I., 11., rte.: horizontally we have the 1. 2. 3. etc., in which the similarities are not great, excepting that a parallelism exists between the elements of one series as compared with those of another. The elements in odd-numbered series bear a closer resemblance to one another than they do to the elements of the intervening even-numbered series, and vice versa, so that it has been found expedi ent to make two divisions of each group, as will be seen in the table, the odd-numbered series being set to one side, the even-numbered to the weight. The many attempts to connect the atomic masses themselves in arithmetical relations would indicate a widespread opinion that the substances now called elements are really compounds of sim pler substances, whose particles have a finite mass and represent individuals of distinct chemi cal properties. so that the chemical elements in each of the periodic groups might be likened to one of the 'homologous series' of organic com pounds. (see, for example, IlvnnocAnnoNs.) This yiew really antedates the periodic law, but fails in large measure to account for the re semblance existing between adjacent members of different groups. Others, especially Sir William have held that the atoms are really for tuitous agglomerates of an indifferent primordial clement, and that atoms of approximately the same mass behave similarly because they vibrate similarly. while atoms of greater mass might vi brate harmoniously with the smaller ones. It is difficult to explain, according to this hypothesis of the 'genesis of the why their number should be as limited as it is. But sour facts are known vaguely pointing to the idea that the atoms of elements within the same periodic group are capable of vibrating at harmonically related rates, and that the great majority of chemieal and physical properties depend upon atomic vi brations. It may, however, be argued that just
as violin-strings may be composed of different materials and yet vibrate together according to common laws. so may the elements he composed of as Illany individual materials and "till exhibit. a perio?lie recurrence of properties, if the latter depend upon the harmonic vibrations of the atoms. Until much additional proof has been brought, the periodic law, while furnishing a vague earund be taken as positive evidence of the qualitative unity of matter.
In the table it will be found that the first group contains the univalent elements. the second gnaw those which are divalent, and so on up to the seventh, where the maximum valency is seven. The maximum valency of the elements of the eighth group may be set at eight, but their compounds rarely exhibit so high a valency, and in many other respects this eighth group is rather anomalous and is taken as a transition group between the seventh and the first. Thus the three elements copper, silver, and gold be long, with respect to ninny of their properties, especially when uneombined, in the eighth group; but their valency is usually low, and many of their salts are so similar to those of sodium that it is often found expedient to place them in the first group, in the positions occupied in the table by their names inclosed in parentheses. These valencies refer especially to the stable oxides. Stable compounds of hydrogen occur only in the fourth, fifth, sixth, and seventh groups, four atoms of hydrogen combining with one of each element of the fourth group, and this amount decreasing until we find the halogens in the seventh group univalent toward hydrogen. The first group includes the most electro-positive ele ments, and there is a steady transition toward the electronegative end of the series in the seventh group, while the eighth group shows a rather sudden return tors.ard the eleetro-positive side. The majority of the compounds derived from elements at the left end of the table are soluble, colorless, and volatile, whereas these properties change from left to right until we find the maximum of insolubility, color. and resist ance to heat in the lower right hand of the table. It is also possible to select analogous compounds of the different elements and find those of similar properties fall within a well-marked zone upon the chart. Mendel6e1I, in his original essay, added the following: (1) The elements which have the lowest atomic weights are those most widely distributed in nature and also represent the most typical characteristics found in the second series of the table: (2) the atomic weight determines the character of an element; (3) from a consideration of their position in the system new analogies can be discovered between ele ments; (4) it may be expected that new ele ments should be discovered to fill blank spaces within the table, and their properties can he Pre dicted from a consideration of those of the ad jacent elements: (5) errors in the assumed atomic weights may be detected through an ir regularity in the position of the element in the periodic system.