580 
ON RECENT CHANGES IN 
that in a great number of cases formulae written in accordance with the two 
systems will not correspond in the number of atoms forming the molecule. 
For example: — 
K O, C 0 9 — K C O, 
k 9 co 3 
iio,c 4 H 3 o 3 =c 4 H 4 d 4 
C 2 H 4 0 2 
(O — 8, C= 6) 
(O = 16, C = 12) 
(O — 8, C= 6) 
(O = 16, C = 12) ; 
whilst in others the appearance of the formula which has undergone modifi¬ 
cation is preserved ; this occurs notably in metallic oxides,— 
Old Numbers. 
Fe 2 0 3 
CuO 
MnOo 
New Numbers. 
Fe 2 0 
CuO 
MnO 
3 
6 } • 
This arises, of course, from the fact that the atom, both of oxygen and metal, 
is now supposed to be double the weight formerly assigned to it; the number 
of atoms therefore is the same, but the molecular weight double. 
The translation of old formula) into those which accord with modern views 
as to atomic weights is a very simple operation: it consists in throwing the 
formula into the empirical shape, and then according to circumstances either 
halving the number of those atoms whose weights have been doubled, or 
doubling the number of those which remain as heretofore. One example will 
suffice. 
Sulphuric acid. Formula hitherto usually employed H O, S 0 3 . Empirical 
formula corresponding, IIS 0 4 . The values of O and S are now doubled, 
hence the quantity of hydrogen combined with S0 4 is now H 2 instead of H ; 
the new formula, therefore, is Id 2 S0 4 ; and so in other cases. 
From the allusion already made (page 533) to the modern application of 
the term ‘ equivalent,’ it will immediately be seen that the elements differ in the 
number of atoms of hydrogen their atoms are respectively capable of repre¬ 
senting ; in other words the) 7- have different replacing-power, atomicity, or 
quantivalence, as it has been variously named. It will be necessary to in¬ 
quire into this point more minutely, since upon these differences are founded 
many important theoretical deductions ; in fact, upon the question of quan¬ 
tivalence depend all our ideas of chemical constitution. 
I shall best render the matter intelligible by selecting examples from one 
or two of the groups into which the elements have been divided in conse¬ 
quence of this property. 
Chlorine combines with hydrogen in the proportion of 355 parts by weight 
to 1 (equal volumes); hydrochloric acid gas is the result. This compound is 
decomposable by sodium, yielding exactly the same amount of hydrogen as 
was employed in its production, and using up for this purpose 23 parts by 
weight of sodium. From this it appears that 23 parts of sodium combine 
with the same quantity of chlorine as 1 part of hydrogen; we say, therefore, 
that 23 of sodium and 1 of hydrogen are equivalent to each other. It 
happens that 23 is the atomic weight of sodium ; one atom of sodium then is 
equivalent to 1 atom of hydrogen, or as it is otherwise expressed, sodium is 
monatomic or univalent. 
Oxygen is a good example of a bivalent or diatomic element. The oxygen 
in water, as already explained, exists in combination with hydrogen in the 
proportion of 16 parts to 2 ; and we say that the quantity of hydrogen in 
one molecule of water consists of two atoms, because we know it can be 
divided into two equal parts, each of which is independently replaceable; on 
the other hand, the oxygen in one molecule of water, or of any of its deriva- 
