Mr. M. M. P. Muir on Chemical Notation. ?') 



statements occur as these : — 



K 2 + 2H 2 S0 4 = 2KHS0 4 + H 2 ... (1) 



Ca 3 +2H 2 S0 4 =2CaS0 4 + 2H 3 ... (2) 



Na 2 + 2HC1 = 2NaCl + H 8 .... (3) 



Mg + 2HC1 =MgCl 2 + H 2 (4) 



By comparing equations (1) and (2), it is evident that the 

 quantity of potassium expressed by the symbol K has replaced, 

 in sulphuric acid ? a quantity of hydrogen represented by the 

 symbol H, while twice this quantity of hydrogen has been 

 replaced by the symbol Ca. The conclusion, therefore, to be 

 deduced from these equations is, that, so far as the power of 

 replacing hydrogen in sulphuric acid is concerned, the symbol 

 Ca is equivalent to the symbol K taken twice. Similarly 

 from a comparison of (3) and (4) it may be concluded that 

 Mg is equivalent, in hydrogen-replacing power, to 2Na. 

 From a consideration of such equations as these the idea of 

 valency takes its origin. This idea has been largely developed ; 

 and chemists have been able to fix the valency of a large number 

 of radicles, whether simple or compound. The valency of a 

 chemical substance expresses a certain function which that 

 substance is capable of performing ; it tells us that the sub- 

 stance under consideration is capable of doing a certain 

 amount of work; but inasmuch as the circumstances under 

 which this work is done vary, the valency also will vary with 

 the circumstances. Nevertheless we find that for each ele- 

 mentary radicle there is a fixed maximum valency ; we have 

 therefore a most useful means of classifying these radicles 

 according to their power of doing a certain work. It is true 

 that the unit of work done is not very strictly defined ; it is 

 only the unit weight of hydrogen displaced, or combined with. 

 Yet this classification is in practice found to be most useful. 

 Given the valency of a metallic radicle, and the basicity of an 

 acid upon which that radicle exercises an action, it is possible 

 to foretell what the composition of the resultant of that action 

 will be. Hence those quantities of two or more radicles 

 which are able to replace the same quantity of hydrogen may 

 be truly said to be " equivalent" quantities. 



14. Dr. Mills (Joe. cit.) objects to this use of the word equi- 

 valent, and avers that " we might as well say that the values 

 of the loads in a railway truck are always equivalent, whether 

 the load consists of air, of hay, or bullion." They are equi- 

 valent in one respect, viz. in that they have displaced equal 

 volumes of air. We have nothing to do with the general 

 value of a radicle when we speak of its valency in the com- 



