16 CHEMISTRY OF FARM PRACTICE 



is produced and this compound with its three oxygen 

 atoms shows that the sulphur atom develops six bonds. 

 In the table of elements given on page 6 the usual valence 

 of each element is given. The theory of valence is used 

 with advantage in fixing in the mind the formulas of com- 

 pounds. 



17. Formulas. The attraction existing between atoms 

 binds them into molecules. These may be represented by 

 formulas which are made up of a combination of symbols. 

 The formula HC1 denotes one atom of hydrogen chemically 

 combined with one atom of chlorine. It also indicates a 

 weight of hydrogen chloride equal to the combined weight 

 of its atoms. This is the molecular weight and is equal 

 to 1+35.46 or 36.46. Two molecules of hydrogen chloride 

 are expressed 2HC1. The molecule of water is expressed by 

 the formula H^O, indicating two atoms of hydrogen united 

 with one atom of oxygen. A molecule of sulphuric acid 

 is H2SO4. This indicates that it is composed of two atoms 

 of hydrogen, one of sulphur, and four of oxygen. Three 

 molecules of sulphuric acid would be expressed by the 

 term 3H 2 SOi. In this latter formula there are in all six 

 atoms of hydrogen, three of sulphur, and twelve of oxygen. 

 The formula 2Caa(PO4)2 means two molecules of calcium 

 phosphate. The two molecules contain six atoms of cal- 

 cium, four atoms of phosphorus, and sixteen atoms of 

 oxygen. It will be seen that each of the coefficients 

 3 and 2 in these two examples multiplies each of the sym- 

 bols following it, taken as many times as is indicated by 

 the subscript following each symbol. Each of the symbols 

 within the parenthesis is in turn multiplied by the sub- 

 script following the parenthesis. 



18. Hydrates. Many salts are loosely combined with 

 water to form hydrates; thus, when copper sulphate is 

 dissolved in water and the water is slowly evaporated there 

 will form blue crystals which contain five hydrating mole- 

 cules, and this formula of the resulting blue vitriol is written 



