86 THE REALITIES OF MODERN SCIENCE 



needles or spears, which transfixed the particles of 

 the substances upon which the acid acted. Newton 

 explained the actions in terms of forces of attraction 

 which were negligible except when the atoms between 

 which they acted were very close to each other. With 

 the discovery of Volta 1 in 1800 of a relation between 

 chemistry and electricity, theories like that of Berzelius 

 arose. Although, as we shall see in the next chapter, 

 present theories point to an electrical cause, the details 

 of the mechanism are largely unknown. 



It became convenient to speak of the combining 

 ability of atoms without particular reference to any 

 theory of affinity. The term " valence " thus came into 

 use. One atom of chlorine combines with one of 

 hydrogen, and no combinations have been formed, 

 the molecules of which contain only hydrogen and 

 chlorine atoms, in which more than one atom of each 

 element was involved. These two substances are 

 said to have similar power of combination or valence. 

 Oxygen has twice the valence of hydrogen or chlorine, 

 for one atom of oxygen combines with two of hydrogen. 



Valence is always referred to that of hydrogen, which 

 is taken as unity. Chlorine is thus seen to be mono- 

 valent while oxygen is divalent. A monovalent atom 

 may for the moment be likened to a one-armed man 

 who may grasp the hand of another one-armed man, 

 as in HC1. An oxygen atom in such a picture has two 

 hands and may grasp those of two monovalent atoms 

 such as hydrogen. Carbon has four bonds. When it 

 burns incompletely we have the formation of the 

 extremely poisonous carbon monoxide, CO. Two of 



1 Cf . Chapter XIV. 



