xvn MOLECULAR ARCHITECTURE 425 



350-355) published later in the same year, Williamson 

 suggested that : 



" Formulae may be used ... as an actual image of what 

 we rationally suppose the arrangement of constituent 

 atoms in a compound, as an orrery is an image of what we 

 conclude to be the arrangement of our planetary system ; 

 and decompositions may be actually effected between them 

 by the exchange of a molecule of one group for a molecule 

 in another." 



" The adoption of such a method will of course neces- 

 sitate the adoption of types, from which, by the replacement 

 of certain elements or molecules, we can deduce the con- 

 stitution of more and more complex groups. I believe that 

 throughout inorganic chemistry, and for the best known 

 organic compounds, one single type will be found sufficient ; 

 it is that of water, represented as containing 2 atoms of 



hydrogen to i of oxygen, thus rrO. In many cases a multi- 

 ple of this formula must be used, and we shall presently see 

 how we get thereby an explanation of the difference between 

 monobasic and bibasic acids, etc." (loc. at., pp. 351-352). 



Gerhardt (1856) suggests four inorganic types. In the 



fourth volume of his Treatise, published in 1856, Gerhardt 

 accepted Williamson's " water - type " as of dominant 

 importance in chemistry, but suggested that the number 

 of types should be increased to four, namely, WATER, 



HYDROCHLORIC ACID, AMMONIA, and HYDROGEN. Unlike 



Williamson, however, he preferred to regard these types as 

 expressing the double decompositions which a compound 

 might undergo, rather than the actual arrangement of the 

 atoms in the molecule. 



Gerhardt expressed his views as follows : 



" My types are types of double decomposition. Water, in a 

 large number of double decompositions, may exchange its 

 oxygen and its hydrogen for other elements (simple radicals) 

 or for groups of elements (compound radicals). But I refer 



