396 SCIENCE PROGRESS 



and an electric charge is a considerable drawback to an 

 extended use of the theory. 



One further application of Abegg and Bodlander's theory 

 may now be referred to. The electro-affinity of a complex ion 

 is greater than that of its component simple ion ; in other words, 

 the electro-affinity of an ion is strengthened by association with 

 a neutral molecule. On this basis we can understand how weak 

 ions, which are forced to dissociate to some extent when combined 

 with oppositely charged strong ions, will strive to strengthen them- 

 selves by entering into combination with neutral molecules. As 

 an example, we may consider acetic acid and ammonium acetate. 

 As the hydrogen ion is comparatively weak the acid is only 

 slightly ionised, but by association with the NH 3 group its electro- 

 positive character is strengthened, so that the salt is strongly 

 ionised. 1 We have also examples of the strengthening of the 

 electro-negative character of cations by association with 

 certain groups, as in the case of the hydrated platinic 

 chloride investigated by Hittorf and Salkowski. 2 This hydrate 

 has distinctly acid properties in aqueous solution, so that 

 hydrogen ions and negatively charged PtCl 4 OH' or PtCl 4 0" 

 ions must be present. In this case, according to the theory, 

 the negative O" or OH' ion has been strengthened by associa- 

 tion with the neutral group PtCl 4 . Reference has already been 

 made to the occurrence in solutions of cadmium chloride of 

 CdCl/' ions ; for such phenomena the term auto-complex forma- 

 tion has been suggested. Further contributions to this subject 

 have been made by Steele 3 and others. Donnan awd Bassett 4 

 have recently adduced evidence to show that solutions of cobalt 

 chloride contain complex CoCI/' ions. 



Considerations of space preclude reference to many other 

 interesting applications of the electrolytic dissociation theory. 



Some Criticisms of the Theory 



It seems appropriate to conclude with a short account of the 

 criticisms to which the theory has been subjected from time 

 to time, and to consider, in the light of what has already 



1 Werner has arrived at the same conception of the nature of the NH4 group 

 in another way. Neuere Anschaitungen, p. 97. 

 3 Zeit. physikal. Chem. 1899,28, 546. 



3 Ibid. 1902, 40, 722. 



4 Journ Chem. Soc. 1902, 81, 939. 



