APPENDIX 161 



for chlorine, oxygen and ammonia. The number of 

 atoms of each element required to enter into combina- 

 tion with another element is called valence, or "that 

 property of an element which determines the number 

 of atoms of another element which its atom can hold 

 in combination." One atom of hydrogen can never hold 

 in combination more than one atom of another element. 

 For this reason hydrogen is the standard of measure 

 for valence as well as for atomic weight. Those ele- 

 ments which combine with one atom of hydrogen are 

 called monads, or univalent, with two, diads, or diva- 

 lent, with three, triads or trivalent etc. 



In writing out the result of chemical changes one 

 finds that all the elements entering into the change can 

 be expressed by an equation in which the signs -f- or , 

 and of equality ==, may be employed, every atom in the 

 substances on the left of the sign of equality must be ac- 

 counted for on the right of that sign, if the change is 

 correctly interpreted. Thus : nitrate of silver, when 

 acted on by hydrochloric acid, is changed into chloride 

 of silver and nitric acid expressed by the following 

 equation : 



AgN0 3 + HC1 == AgCl + HN0 3 



On the left we have one atom of silver, one of nitrogen 

 and three of oxygen, combined to form nitrate of silver. 

 To this we add one atom of hydrogen and one of 

 chlorine, combined to form hydrochloric acid. The 

 change which takes place is that the chlorine displaces 

 the molecule N0 3 which is joined by the hydrogen and 

 the result is that 011 the right we now have one atom 

 of silver combined with one of chlorine, forming chloride 

 of silver, while one atom of hydrogen has joined one of 

 nitrogen and three of oxygen to form nitric acid; but, 



