THE VOLATILE PART OF PLANTS. 49 
The molecular weight of a compound is the sum of the 
weights of the atoms that compose it. For example, wa- 
ter being composed of 1 atom, or 16 parts by weight, of 
oxygen, and 2 atoms, or 2 parts by weight, of hydrogen, 
has the molecular weight of 18. 
The following scheme illustrates the molecular compo- 
sition of a somewhat complex compound, one of the car- 
bonates of ammonia. 
Ammonia gas results from the union of an atom of 
nitrogen with three atoms of hydrogen. One molecule 
of ammonia gas unites with a molecule of carbonic acid 
gas and a molecule of water, to produce a molecule of 
carbonate of ammonia. 
oe ee eee = tat? parts 
Carbonate Garnonie acid | Carbon, hte aaa 
eee mol. = Oxygen, 2 “ = go {=H parts; —70 parts, 
Water, a I ele aa . 4g }=18 parts 
1mol. ~{ Oxygen, 1“ =16/— °° P 
Notation of Compounds,—For the purpose of express- 
ing easily and concisely the composition of compounds, 
and the chemical changes they undergo, chemists have 
agreed to make the symbol of an element signit'y one atom 
of that element. 
Thus H implies not only the light, combustible gas hy- 
drogen, but one part of it by weight as compared with other 
elements, and S suggests, in addition to the idea of the 
body sulphur, the idea of 32 parts of it by weight. Through 
this association of the atomic weight with the symbol, the 
composition of compounds is expressed in the simplest 
manner by writing the symbols of its elements one after 
the other, thus: carbonic oxide is represented by C O, 
oxide of mercury by Hg O, and sulphide of iron by Fe S. 
C O conveys to the chemist not only the fact of the 
existence of carbonic oxide, but also instructs him that its 
molecule contains an atom each of carbon and of oxygen, 
and from his knowledge of the atomic weights he gathers 
the proportions by weight of the carbon and oxygen in it, 
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