42 Messrs. A. C. and A. E. Jessup on the 
Consequently, spectroscopically, sodium belongs to both these 
families, a fact which confirms our supposition. Precisely 
the same holds in the case of magnesium. 
Copper affords an example of special interest. As above, 
its spectrum is seen to resemble that of sodium, but there are 
also lines which are similar in type to those of tin, lead, and 
some other heavy metals. The explanation of this fact is that 
the outer copper ring is of the same shape as the outer sodium 
ring, but yet the internal structure of the two is different, 
that of copper being similar to tin and lead. 
The properties of an element will, however, not depend 
solely on its outer ring, but also upon two other main causes. 
These are (a) the actual number of rings in an atom, and (b) 
the way in which the rings are superimposed on each other. 
The effect of the first cause upon the properties of the atom 
is simply that the larger the number of rings, the more 
unstable the atom, or we may perhaps say, the greater its 
tendency to disintegration. The second cause has been 
already touched on ; it may be mentioned, however, that 
wherever a whole atom is composed of rings of the same kind, 
the outer ring will behave normally ; but where the rings are 
not of the same kind, there will be what may be looked upon 
as distortion, and consequently abnormal properties are to be 
expected, and the greater the valency of a ring the more 
marked the distortion will be when such a ring is super- 
imposed on one dissimilar to itself. 
We are now in a position to discuss the influence of the 
constitution of the elements on their chemical reactions. We 
will, for the sake of clearness, limit ourselves mostly to simple 
combinations of two elements. 
Chemical compounds are formed entirely by an exchange 
of electrons between the combining atoms ; but the number of 
electrons taking part in the exchange, and the ease with 
which they do so, depends on the structure of the atom. 
As before stated, the chief tendency of the electrons is to 
form into systems of eight, and when this occurs, what may 
be called the principal valency is exerted. As an example, 
calcium with two electrons will give them up to oxygen 
which has six ; when this occurs the calcium will become 
positively, and the oxygen negatively charged with " elec- 
tricity/' and combination results. To secure uniformity we 
may say that in this change calcium is divalent and oxygen 
hexavalent, the principal valency being the number of 
electrons the atom can give, or eight minus the number it 
can receive. Thus nitrogen will take three electrons from 
hydrogen, or give five to oxygen, being in both cases penta- 
valent. Simple changes such as the above always take place 
