Columns 7–14 contain the composition of the saline compounds of the elements, placed according to their forms, RX, RX2 to RX8 (in the 14th column). If the element R has a metallic character like H, Li, Be, &c., then X represents Cl, NO3, ½ SO4, &c., haloid radicles, or (OH) if a perfect hydrate is formed (alkali, aqueous base), or ½ O, ½ S, &c. when an anhydrous oxide, sulphide, &c. is formed. For instance, NaCl, Mg(NO3)2, Al2(SO4)3, correspond to NaX, MgX2, and AlX3; so also Na(OH), Mg(OH)2, Al(OH)3, Na2O, MgO, Al2O3, &c. But if the element, like C or N, be of a metalloid or acid character, X must be regarded as (OH) in the formation of hydrates; (OM) in the formation of salts, where M is the equivalent of a metal, ½ O in the formation of an anhydride, and Cl in the formation of a chloranhydride; and in this case (i.e. in the acid compounds) Z is put in the place of X; for example, the formulæ COZ2, NO2Z, MNO2Z, FeO2Z2, and IZ3 correspond to CO(NaO)2 = Na2CO3, COCl2, CO2, NO2(NaO) = NaNO3, NO2Cl, NO2(OH) = HNO3; MnO3(OK) = KMnO4, ICl, &c.
The 15th column gives the compositions of the peroxides of the elements, taking them as anhydrous. An asterisk (*) is attached to those of which the composition has not been well established, and a dash (—) shows that for a given element no peroxides have yet been obtained. The peroxides contain more oxygen than the higher saline oxides of the same elements, are powerfully oxidising, and easily give peroxide of hydrogen. This latter circumstance necessitates their being referred to the type of peroxide of hydrogen, if bases and acids are referred to the type of water (see Chapter XV., Note [7] and [11 bis]).
The 16th column gives the composition of the lower hydrogen compounds like N3H and Na2H. They may often be regarded as alloys of hydrogen, which is frequently disengaged by them at a comparatively moderate temperature. They differ greatly in their nature from the hydrogen compounds given in columns 1–4 (see Note [12]).
Column 17 gives the specific gravity of the elements in a solid and a liquid state. An asterisk (*) is placed by those which can either only be assumed from analogy (for example, the sp. gr. of fluorine and hydrogen, which have not been obtained in a liquid state), or which vary very rapidly with a variation of temperature and pressure (like oxygen and nitrogen), or physical state (for instance, carbon in passing from the state of charcoal to graphite and diamond). But as the sp. gr. in general varies with the temperature, mechanical condition, &c., the figures given, although chosen from the most trustworthy sources, can only be regarded as approximate, and not as absolutely true. They clearly show a certain periodicity; for instance, the sp. gr. diminishes from Al on both sides (Al, Mg, Na, with decreasing atomic weight; and Al, Si, P, S, Cl, with increasing atomic weight, it also diminishes on both sides from Cu, Ru, and Os.)
The same remarks refer to the figures in the 18th column, which gives the so-called atomic volumes of the simple bodies, or the quotient of their atomic weight and specific gravity. For Na, K, Rb, and Cs the atomic volume is greatest among the neighbouring elements. For Ni, Pd, and Os it is least, and this indicates the periodicity of this property of the simple bodies.
The last (19th) column gives the melting points of the simple bodies. Here also a periodicity is seen, i.e. a maximum and minimum value between which there are intermediate values, as we see, for instance, in the series Cl, K, Ca, Sc, and Ti, or in the series Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge.
PRINCIPLES OF CHEMISTRY
CHAPTER XV
THE GROUPING OF THE ELEMENTS AND THE PERIODIC LAW
It is seen from the examples given in the preceding chapters that the sum of the data concerning the chemical transformations proper to the elements (for instance, with respect to the formation of acids, salts, and other compounds having definite properties) is insufficient for accurately determining the relationship of the elements, inasmuch as this may be many-sided. Thus, lithium and barium are in some respects analogous to sodium and potassium, and in others to magnesium and calcium. It is evident, therefore, that for a complete judgment it is necessary to have, not only qualitative, but also quantitative, exact and measurable, data. When a property can be measured it ceases to be vague, and becomes quantitative instead of merely qualitative.