CHEMICAL ACTION 181 



Experiment 115. Grind a mixture of one half ounce of iron 

 filings and one ounce of sulphur in a mortar. Examine carefully 

 and draw a magnet through the mass. Is it a compound or a 

 mixture still? 



Into an old test tube put a little of the mass, and heat it slowly 

 but well. When solid, allow the mass to cool. Break the tube 

 and examine the substance. Is it iron? Is it sulphur? Is it a 

 compound or a mixture ? 



208. Symbols. For convenience, a system has been 

 devised so that names of elements and compounds, and 

 even chemical changes, may be expressed by symbols. 



The names of elements are generally expressed by 

 their first letter, or two letters : hydrogen, H ; oxygen, 

 O ; carbon, C ; calcium, Ca ; zinc, Zn, etc. Moreover, 

 the symbol for any element (e.g. C, O, or H) means also 

 one atom of that element. To express more than one 

 atom, a small figure is placed after the letter; thus H 2 

 means " two atoms of hydrogen"; O 3 means " three 

 atoms of oxygen." 



The symbol for a compound is made by writing the 

 symbols of its elements in order, each showing the num- 

 ber of its atoms in the substance. Thus HC1 means that 

 in the compound hydrochloric acid one atom of hydro- 

 gen is combined with one of chlorine ; HNO 3 (nitric 

 acid) is a compound in which one atom of hydrogen, one 

 of nitrogen, and three atoms of oxygen are combined. 



The symbol of a compound (HC1, HNO 3 , etc.) of course repre- 

 sents one molecule of the substance. Two molecules would be thus 

 written, 2 HC1, 2 HNO 3 , etc.; three molecules, 3 HC1, etc. The 

 number (2, 3, etc.) so written belongs to the whole group of ele- 

 ments, and means that in the whole quantity represented the 

 quantity of each element is taken just that number of times. For 



