100 BUTLER ART. D 



values taken negatively; therefore 



aMa + ^Mb + etc - KMk — \Mi - etc =0, 



or, 



aMa + ^Mb + etc = KMk + \Mi -\- etc (84) [33] 



The relation between the quantities Ma, Mb, etc., is thus of 

 the same form as that between the units of the component 

 substances (82). These relations take a very simple form if we 

 employ as the unit quantity of each substance, its formula- 

 weight in grams. Thus if we take as unit quantities of water, 

 sodium chloride and the hydrate, NaCl-H20 the quantities in 

 grams represented by the symbols H2O, NaCl and NaCl • H2O, 

 the relation between these substances is represented qualita- 

 tively and quantitatively by the equation. 



H2O ^- NaCl = NaCl-HaO. 



With this choice of units, (84) becomes 



■^HjO + -^NaCl = -^NaClHzO- 



Therefore the values of mhjOj MNaCi ^^^ A'NaCiH20 for these sub- 

 stances, in parts of the system of which they are present as 

 actual components, are related by the equation 



MH2O + MNaCl = MNaClHjO' 



Similarly, if the substances hydrogen chloride, oxygen, water 

 and chlorine are components of a system when the unit of quan- 

 tity of each substance is the quantity (in grams) represented by 

 its chemical formula (82) becomes 



2HC1 + ^02 = H2O -f CI2, 



and equation (84) takes the form 



2Mhci + hMo, = Mu,o + ^ch- 



Thus the values of /i in parts of the system of which these sub- 

 stances are present as actual components, are related by the 



