250 THE NUMERICAL RELATION 



The Three (and last) Series is composed of Hydrogen and the metals of the alkalies. 

 The analogies between Lithium, Sodium, and Potassium are very close, as is well 

 known, and there can be no doubt in regard to the propriety of classing them together. 

 It may be said, however, in regard to Hydrogen, that it resembles as closely some of the 

 metals of the Four Series as it does those of the alkalies. Though this cannot be denied, 

 yet the fact that the atomic weight of Hydrogen is the nucleus of the series, and the 

 great solubility of the alkalies in water, may be urged as reasons for placing it at the 

 head of the Three Series. 



The isomorphism of Lithium, Sodium, and Potassium is fully established ; but I can 

 find no data which prove Hydrogen isomorphous either with them or with the metals 

 of the other group. 



The unit of the atomic weights which has been used thus far throughout the table, 

 is the double atom of Hydrogen ; but the nucleus of the Three Series is the weight of 

 the single atom, so that the unit in this series is one half of the unit of the weights in 

 all the other series. This fact must be kept in mind in comparing the atomic weights 

 of this mth those of the other series. All the weights might have been made uniform 

 by doubling them throughout ; but as this would not have changed the relation, and 

 would have been departing from the general custom, it was thought best to confine the 

 doubling to the Three Series, into which alone Hydrogen enters. The general symbol 

 of this series is 1 -J- ?j 3, where of course the unit is one half of that of the symbols at 

 the head of the other series. The observed atomic weights will be found to correspond 

 very closely with the theoretical numbers ; indeed, the two coincide, except in the case 

 of Potassium, where the difference is 0.6. This, however, it must be remembered, is 

 0.6 of the single Hydrogen atom. Compared with the double atom, as the weight of 

 Potassium is generally given, the difi'erence amounts to but 0.3. 



One of the most remarkable points of the classification which has been now ex- 

 plained, is the affiliation of the series. We find in Chemistry, as in other sciences, that 

 Nature seems to abhor abrupt transitions, and shades off her bounding lines. Many 

 of the elements, while they manifestly belong to one series, have properties which ally 

 them to another. Several examples of this have already been noticed. In such cases, 

 we find invariably that there is a similar affiliation of the atomic weight. Of all the 

 elements Chromium and Manganese are the most protean. Two atoms of these ele- 

 ments unite with seven atoms of Oxygen and form acids analogous to Perchloric Acid, 

 and, as has already been shown, the weight of two atoms of either element falls into 

 the Nine Series. INIoreover, one atom of Chromium or of Manganese unites with 

 three atoms of Oxygen to form Chromic or Manganic Acid. Chromic Acid is a 



