xvi ATOMIC WEIGHTS OF THE METALS 373 



or potassium without producing any change of crystalline 

 form. Again, silver sulphide, Ag 2 S, as argentite, is isomor- 

 phous with lead sulphide, PbS, as galena, and is nearly 

 always present as an isornorphous impurity in this mineral, 

 in spite of the unequal numbers of atoms in the two 

 molecules. Cases of this sort show that the law of 

 isomorphism, although of great value, cannot be used 

 as a final test in establishing the atomic weights of the 

 elements. 



D. FORMULA OF METALLIC COMPOUNDS. 



Empirical and molecular formulae. In the preceding 

 chapter (p. 352) a method has been described by which the 

 formula' of a volatile compound may be deduced from its 

 vapour-density and percentage composition, when the atomic 

 weights of its constituents are known. This method can- 

 not be applied to substances, such as sugar or potassium 

 chlorate, which decompose when heated, or to substances, 

 such as silica, which boil at so high a temperature that their 

 vapour-densities cannot be determined. There is, however, 

 no difficulty in deducing from the analysis of the substance 

 an EMPIRICAL FORMULA, which expresses its composition in 

 the simplest way, but gives only a minimum value for its 

 molecular weight. 



The method used in calculating an empirical formula 

 may be illustrated by using Stas's figures for the analysis of 

 cinnamic acid (Works, I. 281). 



On combustion, 0*900 gram of acid gave 0*444 gram 

 of water and 2*402 grams of carbonic anhydride. The 

 composition of the acid was therefore : 



C 7278 H 5-47 O 2175%. 



Taking the atomic weights as 



C 1 2 'oo H i '008 O 1 6, 



