308 HISTORY OF CHEMISTRY. [LECTURE XV. 



have been proposed in order to bring these exceptions as far 

 as possible into harmony with the system, but neither of them 

 wholly gets rid of the difficulty. 



One party, under the leadership of Kekule, 57 adheres to the 

 definition of valency given above, but admits that there is a 

 large class of substances to which it is not applicable. This 

 class is composed of the molecular compounds, the smallest 

 particles of which consist of aggregates of molecules held to- 

 gether by means of molecular forces. Examples of the class 

 are the compounds containing water of crystallisation (also 

 alcohol, benzene, etc., of crystallisation), the majority of double 

 salts, the ammonium salts, phosphorus pentachloride, iodine 

 trichloride, etc. No precise definition of them can be given, 

 but they are characterised generally by the facts that they can- 

 not pass, undecomposed, into the state of vapour (although 

 Thorpe found phosphorus pentafluoride to be an exception to 

 this rule 58 ), and that they are easily formed from and decom- 

 posed into their molecular constituents. 



The adherents of constant valency are further obliged to 

 recognise the un'saturated compounds as exceptions. Even 

 although there are not a very great many of these compounds, 

 still their existence constitutes a serious objection to the doctrine; 

 and fruitless endeavours have been made to weaken this objec- 

 tion on the ground of the tendency exhibited by such substances 

 to become saturated. 59 



The opponents of these views, whose first representatives 

 are Frankland and Couper, 60 define the valency of an element as 

 its maximum saturating capacity, and under this definition the 

 unsaturated compounds cease to occupy an exceptional posi- 

 tion. In view of the fact that they further assume the valency 

 in the case of many elements to be considerably higher than 

 had previously been assumed nitrogen and phosphorus, for 

 example, as quinquivalent, sulphur as sexivalent, iodine as 



57 Lehrbuch der Chemie. I, 142, 443 ; Comptes Rendus. 58, 510. 

 58 Annalen. 182, 204. 9 Compare p. 269 and Horstmann, Theoretische 

 Chemie. 295. M Compare pp. 231 and 254. 



