o families, each
containing four or five members, between which there is a great
similarity.

~Family resemblances.~ Let us now inquire more closely in what respects
the elements of a family resemble each other.

1. _Valence._ In general the valence of the elements in a family is the
same, and the formulas of their compounds are therefore similar. If we
know that the formula of sodium chloride is NaCl, it is pretty certain
that the formula of potassium chloride will be KCl--not KCl_{2} or
KCl_{3}. The general formulas R_{2}O, RO, etc., placed below the
columns show the formulas of the oxides of the elements in the column
provided they form oxides. In like manner the formulas RH, RH_{2}, etc.,
show the composition of the compounds formed with hydrogen or chlorine.

2. _Chemical properties._ The chemical properties of the members of a
family are quite similar. If one member is a metal, the others usually
are; if one is a non-metal, so, too, are the others. The families in the
first two columns consist of metals, while the elements found in the
last two columns form acids. There is in addition a certain regularity
in properties of the elements in each family. If the element at the head
of the family is a strong acid-forming element, this property is likely
to diminish gradually, as we pass to the members of the family with
higher atomic weights. Thus phosphorus is strongly acid-forming, arsenic
less so, antimony still less so, while bismuth has almost no
acid-forming properties. We shall meet with many illustrations of this
fact.

3. _Physical properties._ In the same way, the physical properties of
the members of a family are in general somewhat similar, and show a
regular gradation as we pass from element to element in the family. Thus
the densities of the members of the magnesium family are

Mg = 1.75, Zn = 7.00, Cd = 8.67, Hg = 13.6.

Their melting points are

Mg = 750 deg., Zn = 420 deg., Cd = 320 deg., Hg = -39.5 deg..

~Value of the periodic law.~ The periodic law has proved of much value in
the development of the science of chemistry.

1. _It simplifies study._ It is at once evident that such regularities
very much simplify the study of chemistry. A thorough study of one
element of a family makes the study of the other members a much easier
task, since so many of the properties and chemical reactions of the
elements are similar. Thus, having studied the element sulphur in some
detail, it is not n