34 HOW CROPS GEOW. 



bonates of ammouium, which consists of four elements, 

 ten atoms, and has a molecalar weight of seventy-nine. 



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 carbon dioxide 

 gas and a molecule of water to produce a molecule of 

 ammonium carbonate. 



Atoms. Atomic Molecular 

 weights, weights, 

 f Ammonia _( Hydrogen, 3 = 3 ) _i7~) 

 Imol. —) Nitrogen, 1 = 14 ( ^^' 



Ammonium 



J Carbon dl- i Carbon, 1 =12 



Cai-bonate =^ ^^^V^-i ^:, = Xt'"""' i - " =44 ^=79 



Tmoi -1 oxide Imol.-) Oxygen, 2 

 imoi. Water, _| Hydrogen, 2 _ ^ 



(. 1 mol.~ I Oxygen, 1 =16 



Notation and Formulas of Compounds. — For the 

 purpose of expressing easily and concisely the composi- 

 tion 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 

 hydrogen, but also 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 33 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 their elements one after the other. Thus, carbon 

 monoxide is represented by CO, mercuric oxide by HgO, 

 and iron monosulphide by FeS. The symbol CO con- 

 veys to the chemist not only the fact of the existence 

 of carbon monoxide, but also instructs him that its mole- 

 cule 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. 



When a compound contains more than one atom of an 

 element, this is shown by appending a small figure to the 

 symbol of the latter. For example : water consists of 

 two atoms of hydrogen united to one of oxygen, and its 



