3 i4 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



proportions are particularly common in compounds of three 

 or more elements, e.g. 



LITHARGE . . lead : oxygen =103*55 : 8 



and 



SULPHURIC ANHYDRIDE . sulphur : oxygen = 16-03 : 3 x 8 

 unite in these proportions to form 



LEAD SULPHATE . . lead : sulphur : oxygen 



= 103-55 : J 6'03 14x8 



Equivalents and atomic weights. The possibility of 

 making use of a series of equivalents to express the propor- 

 tions in which elements combine together finds an obvious 

 explanation in the atomic theory. But, whilst the equivalents 

 and the atomic weights are closew related to one another, 

 they are not necessarily identicafl. Thus, in the case of 

 hydrogen and oxygen the weights in which the elements 

 combine together to form water are approximately i to 8. 

 These are, therefore, the equivalents, or combining weights, 

 of the two elements. But whilst Dalton preferred to regard 

 water as a compound of i atom of hydrogen with i atom 

 of oxygen, Berzelius regarded it as a compound of 2 atoms 

 of hydrogen with i atom of oxygen. On Dalton's hypo- 

 thesis the equivalents and the atomic weights are both 

 expressed by the numbers i : 8, i.e. water is composed of 

 one atom of hydrogen of weight i, and one atom of oxygen 

 of weight 8. On Berzelius's hypothesis the equivalents are 

 still in the ratio i : 8, but the atomic weights were given as 

 6*2177 : 100 = i : 16, i.e. water is composed of two atoms 

 of hydrogen of weight i, and one atom of oxygen of weight 

 16, the combining ratio being 2 x i : 16 = i : 8 as before. 

 The numbers expressing the equivalent and the atomic weight 

 of an element must, however, be in a simple integral ratio to 

 one another, e.g. in the above instance 2 : i, or, in general, 

 m : n. 



It may be noted that analytical data, such as those em- 



