572 PRINCIPLES OF CHEMISTRY 



CHAPTER XIV 



THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM, 

 CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM , 



IT is easy by investigating the composition of corresponding com- 

 pounds, to establish the equivalent weights of the metals that is, the 

 quantity which replaces one part by weight of hydrogen. If a metal 

 directly, decomposes acids, with the evolution of hydrogen, then the 

 equivalent weight of the metal may be determined by taking a definite 

 weight of it and measuring the volume of hydrogen evolved by its 

 action on an excess of acid ; then it is easy to calculate the weight 

 of the hydrogen from its volume. 1 The same result may be arrived at 

 by determining the composition of the normal salts of the metal ; for 

 instance, by finding the weight which combines with 35 '5 parts of 

 chlorine or 80 parts of bromine. 2 The equivalent of a metal may be 

 also learnt by simultaneously (i.e., in one circuit) decomposing an acid 

 and a fused salt of a given metal by an electric current and determin- 

 ing the relation between the. amounts of hydrogen and metal separated, 

 because, according to Faraday's law, electrolytes (conductors of the 

 second order) are always decomposed in equivalent quantities. The 

 equivalent of a metal may even be found by simply determining the 

 relation between the weight of a metal and of its salt giving oxide, 

 as by this weight we know the weight of the metal which combines 

 with 8 parts by weight of oxygen, and this will be the weight of an 

 equivalent, because 8 parts of oxygen combine with 1 part by weight of 



1 Under favourable circumstances (by taking all the requisite precautions), the weight 

 of the equivalent may be accurately determined by this method. Thus Reynolds and 

 Ramsay (1887) determined the equivalent of zinc to be 32'7 by this method (from the 

 average of 123 experiments), whilst by other methods it has been fixed (by different 

 observers) between 82'55 and 33'95. 



The difference in their equivalents may be demonstrated by taking equal weights of 

 different metals, and by collecting the hydrogen evolved by them (under the action of an 

 acid or alkali). 



2 The most accurate determinations of this kind were carried on by Stas, and will be 

 described in speaking of silver. 



