CHAPTERS FOR STUDENTS. 
725 
H 2 0 represents 18 grams, or 22’4 litres of water in vapour. 
HC1 represents 36‘5 grams, or 22*4 litres of hydrochloric acid. 
NH 3 represents 17 grams, or 22-4 litres of ammonia. 
C 2 H 6 0 represents 46 grams, or 22-4 litres of alcohol in vapour. 
That weight of a chemical compound which thus, when in the gaseous state, 
fills 22-4 litres, is called shortly its molecule. 
The molecule of water is expressed by the formula H 2 0 ; it is formed by com¬ 
bining 2 atoms of hydrogen, each weighing 1 and measuring 1 unit of volume 
(i. e. 1T2 litres) with 1 atom of oxygen, weighing 16 and measuring 1 volume. 
Before combination, the 2 volumes of hydrogen and 1 of oxygen together make 
3 volumes, but in combining they contract into 2 volumes. This contraction 
of volume occurs in the majority of cases of gaseous combination. 
There are reasons for supposing that the atoms, even in elementary sub¬ 
stances, are not altogether isolated and independent of each other, like the 
bricks in a wall, each being surrounded on all sides by mortar, but are united 
to each other in pairs, thus forming molecules. Then, since each atom fills 
11-2 litres, the space occupied by the molecule formed of 2 atoms is 22 4 litres. 
According to this system, all substances, elements as well as compounds, are re¬ 
duced to the same standards of measurement. The formula; of the elements in 
their ordinary uncombined condition are, then, 0 2 , H 2 , Cl 2 , Br 2 , I 2 , etc. 
Symbols and formulae are employed to show the changes which are brought 
about by letting different bodies act on each other, as in the following ex¬ 
amples. 
Hydrochloric acid may be formed by combining hydrogen and chlorine 
gases :— 
H 3 -fiCl 2 =2HCl. 
This equation shows that one molecule of hydrogen and one molecule of 
chlorine give two molecules of hydrochloric acid. 
Hydrochloric acid may also be made by acting upon common salt with 
sulphuric acid:— 
2 Na C1+H 2 S 0 4 =Na 2 S 0 4 +2 H Cl. 
Two molecules of chloride of sodium, and one molecule of sulphuric acid, 
produce one molecule of sulphate of sodium and two molecules of hydro¬ 
chloric acid. 
Oxide of silver is made by decomposing nitrate of silver with lime (oxide 
of calcium):— 
2AgN0 3 -fCa0=Ag 2 0 + Ca2N0 3 . 
Two molecules of nitrate of silver, and one molecule of oxide of calcium, 
give one molecule of oxide of silver and one molecule of nitrate of calcium. 
In the last equation, the two atoms of silver in the nitrate exchange places 
with the one atom of calcium in the lime. The two silver atoms are, there¬ 
fore, equivalent to the one calcium atom. Becollecting that the composition 
of water is represented by the formula H 2 O, and comparing this with the 
formula of lime, CaO, and that of oxide of silver, Ag 2 0, we find that H 2 is 
equivalent either to Ca or to Ag 2 . Two atoms of silver are therefore equiva¬ 
lent to two atoms of hydrogen, or one atom to one atom. Hydrogen being, 
as usual, taken as the standard for comparison, we say that Ag is univalent, 
or, in other words, that each atom of it has the same combining or replacing 
value as one atom of hydrogen. Calcium, for similar reasons, is bivalent. 
In the last equation, it will be seen that the group of symbols N 0 3 is on 
the left-hand side combined with Ag, whilst on the right-hand side 2iN r 0 3 is 
combined with the Ca. This tells us that the group JN T 0 3 possesses a univa- 
