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713 If the periodic generalization give any clue whatever to weight of gallium be over-estimated by about two units, the nature of the elements and the character of their inter- progression might occur as in the previous period. The relationship, it certainly tends to justify the conclusion subsequent values are arranged so as to secure the proper that a genetic relationship exists between them. It is, in position for bromine, the atomic weight of which is fact, impossible for any one fully imbued with the facts of certainly known with a close approach to accuracy. organic chemistry to avoid being struck by the obvious, Arsenic is in consequence placed in the fourteenth group although often distant, analogies presented by the elements instead of in the fifteenth, next to selenium, and the to genetically-related carbon compounds—analogies which weight assigned to this latter element is two units below are the more remarkable in that the exact mass relation- the value at present accepted, which is certainly an ships met with in the case of such compounds are not anomalous value, bearing in mind the fact that the apparent among the elements. But as so many arguments difference between sulphur and chlorine is more than three have been advanced in support of the view that the units. Little can be said of the fifth period as it contains elements undergo decomposition at high temperatures, it so few elements. In the sixth period the conditions is perhaps well to note here that the evidence of genetic j appear to be similar to those in the fourth, but the relationship among them is entirely inferential. Judging tendency to the development of extended families is even from the chemist’s point of view, no valid proof of the more marked. The value 124 is assigned to tellurium in occurrence of elementary dissociation under terrestrial order to bring iodine into position, and antimony is in conditions has yet been given, while as to its occurrence consequence thrown somewhat out of its usual position; under solar or stellar conditions, until the interpretation in addition, the atomic weight assigned to it is a unit which it is sought to put upon spectroscopic observations higher than the preferred value deduced from Cooke’s is accepted by those who should be best able to appreciate determination. In the seventh period the probable their force, the subject is one on which we can only hold existence of families of immediate homologues is again our judgment entirely in abeyance. The discovery of forced upon the attention. Thus caesium, like rubidium, helium justifies the belief that several new elements does not follow in the immediate order of progression; may yet become known having lower atomic weights even nor does barium, but like zinc, strontium, and cadmium, than oxygen. Therefore supposing that genetic relation- seems to belong to an extended family of which the ships exist, since in a large proportion of cases corre- superior terms are missing. The mystic elements of the sponding elements among those having small atomic cerium group closely follow barium. Whether the period weights differ in atomic weight by very nearly 16 units,, terminates shortly afterwards with a final term having an and since successive elements differ in so many cases by atomic weight near to 154, or whether a great developabout a single unit, the assumption may be made that the ment of the rare-earth elements occur in it, so that the series from hydrogen to oxygen should comprise a period period terminates with tantalum and tungsten, it is of 16 terms. It then becomes a question whether the impossible to say. The eighth (or ninth) period comremaining elements can be arranged below these. An prises the heaviest of the platinum metals and a number attempt to answer this is made in the preceding Table; of other heavy metals; it clearly comes to an early close. the results are both remarkable and encouraging. The last period (the ninth or tenth) has at present only The first line or period calls for no special comment. two representatives, and of these uranium must be almost Noteworthy points in the second period are the appearance the final term; perhaps radium belongs to this period. of magnesium in a column in advance of that containing Such a treatment of the subject obviously brings into beryllium, and of phosphorus as the analogue of an unknown prominence the possible existence of a far larger number element coming between nitrogen and oxygen. No serious of elements than has hitherto been contemplated, and objection can be taken to such results, as beryllium in attention is specially directed by it to the probable some respects more closely resembles aluminium than occurrence of numerous sets of closely-related elements magnesium, and nitrogen and phosphorus, though they besides the long-recognized three triads which appear in are in many ways analogous elements, also differ in im- the fourth group (Fe, Co, Ni—Ru, Rh, Pd—Os, Ir, Pt). AnevK portant particulars. In the third period the The extreme importance of extending the search for new periodic homology difference 16 changes at the twelfth elements in every possible direction is thus brought into arrange- term, and owing to a sudden increase in the pro- view. gression difference becomes 20 between silicon Atomic Weights. and titanium. Proceeding by units, vanadium and chromium take up their proper positions, but scandium, it As atomic weight is the ultimate criterion of position, will be noticed, occupies position 43, whereas the atomic everything depends on the exact determination of the weight at present assigned to it is 44; as it occurs to- proportions in which the elements combine. The gether with elements of much higher atomjc weight it is field of investigation open to the chemist in this ^xfct, not unlikely that the value is over-estimated. In the direction is immense, and there is none m which tions of fourth period many points of interest are brought out. he can labour with greater advantage; it is a for- atomic Proceeding from chromium by units, manganese and iron tunate circumstance that an enthusiastic and most ^tded* take their proper places, but it then becomes impossible skilful band of workers in America is engaging to proceed by units, as copper and zinc would be thereby in such labours with rare devotion. If the arrangement of entirely thrown out of position. If, however, the as- elements above adopted be in any way justified, it is clear sumption be made that cobalt and nickel are members of that considerable alterations must be made in the atomic the iron group, i.e., immediate homologues of iron instead weights of elements in a number of instances—even in cases of terms in the progression series (also that perhaps they in which the determinations have been carried out with are “ isomeric ” elements), order may be restored by pro- such care that the results are commonly regarded with ceeding from them, copper being thus brought into the confidence. When the methods which have been adopted eighth or magnesium group, of which it is not an inappro- are studied, it is obvious that in not a few cases, although priate occupant; but zinc must then be reckoned in the extreme care may have been taken in the actual detersame family. It will be seen that “steps down” again mination of the numerical values, the proof obtained that occur in two (the eleventh and twelfth) groups in place of the material dealt with has been free from all admixture is the one in the previous period; if, however, the atomic often far from complete. Much more must be done in S. II. — 90