CHAPTERS FOR STUDENTS. 
655 
One of the most important differences between the two cases is this : the 
gases may be mixed together in any proportions at will, but the analysis of water 
always gives a measure of hydrogen twice as great as that of the oxygen. This 
invariable character extends through all cases of chemical combination. 
Since only sixty-five elements are known, it is plain that, to form the appa¬ 
rently innumerable different compounds of them with which we are acquainted, 
some must unite in several proportions. Many examples of this may easily be 
found in the various compounds of oxygen with the metals; e. g. 25 parts of 
mercury with 1 part of oxygen form the black oxide, whilst 25 parts of mercury 
with 2 parts of oxygen form the red oxide. 
When a compound is taken, and one of its constituents is removed, the residue 
or remainder is generally capable of combining with other bodies. The ratio 
between the quantity of the constituent replaced and that of the body replacing 
is always the same when the same pair of bodies is concerned, and is that in 
which they will themselves, under other circumstances, combine together. 
Thus, when water is converted into hydrochloric acid by the action of chlorine, 
71 parts of this element take the place* of 16 parts of the oxygen in combining 
with the hydrogen. 71 parts of chlorine are therefore “equivalent” to 16 parts 
of oxygen, and will combine with them when placed under suitable conditions. 
The three laws deduced from the above general statements may be put more 
concisely as follows :— 
Law of Definite Proportions .—A chemical compound is always formed of the 
same elements united in the same proportion. 
Law of Multiple Proportions— When one element unites with another in 
severed proportions, the quantities so uniting bear to each other a simple rela¬ 
tion. _ 
Law of Equivalents .—“ When a body, A, unites with other bodies, B, C, i>, 
the quantities of B, C, D which unite with A represent the relations in which 
they unite among themselves, in the event of union taking place.’’ 
In order to exhibit more plainly the changes which bodies operate upon each 
other, and their mode of combining together, symbols are employed. The sym¬ 
bol of an element is simply the initial of its name. But chemists employ these 
symbols, not only as a kind of shorthand, but to express certain weights aud 
volumes of the various elements. The symbol for chlorine being Cl, this stands 
for 35‘5 grams weight of chlorine. Similarly, II represents 1 gram weight of 
hydrogen ; O represents 16 grams of oxygen ; S, 32 grams of sulphur; and so 
on. These numbers are found by noticing the proportions in which the different 
elements combine together. 
The symbols have, beyond this, an additional significance ; they stand for 
11-2 litres of the gases which would be formed by taking any of the elements, 
and, if they are not already in the gaseous state, bringing them into that con¬ 
dition, and measuring them all at the same temperature. So that— 
H stands for 1 gram or 11*2 litres of hydrogen gas. 
Cl 
O 
Br 
I 
»» 
?» 
r> 
r> 
chlorine gas.- 
oxygen gas. 
bromine gas. 
iodine gas. 
etc. 
35‘5 grams or 1F2 
16 „ 11*2 
80 „ 11-2 
127 „ 11*2 
etc. etc.. 
We see from this two important things. First, when different gases combine 
they do so in volumes which are either equal, or there is some very simple ratio 
between them, such as 2 to 1, or 3 to 1, or 2 to 3, and the like. Secondly, we 
see that the specific gravities of the different elements, taken when in the state 
of gas, are proportional to their combining weights. 
