THE VOLATILE PAET 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 w eight 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 onion 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. 



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 signify 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. 

 3 



