148 DR. B. D. STEELE, DR. D. McINTOSH AND DR. E. H. ARCHIBALD 



PART IV. 



The Abnormal Variation of Molecular Conductivity, etc. 



i 



By B. D. STEELE, D. MC!NTOSH and E. H. ARCHIBALD. 



IN discussing the nature of those inorganic liquids which are able to act as " ionising" 

 solvents, WALDEN (' Zeit. fur anorg. Chemie,' 1900, 25, p. 209) states that "a 

 measurable dissociation (ionisation) occurs only in combinations of the elements of the 

 5th and 6th groups of the periodic table and in compounds of these elements with 

 hydrogen and the halogens." We have shown in Part I. of this investigation that 

 the hydrides of the halogen elements and of sulphur belong to the class of " ionising " 

 solvents, so that this class consists of compounds of the elements of the 5th, 6th, and 

 7th groups amongst themselves and with hydrogen. Attempts have been frequently 

 made to arrive at some generalisation connecting the so-called " ionising" power with . 

 certain physical constants of the pure solvents. 



Thus, according to NERNST and THOMSON, a close relationship exists between the 

 dissociating power and the dielectric constant. These investigators were led to look 

 for this relation by the consideration that the force with which two electrically 

 charged bodies attract or repel each other depends on the magnitude of the dielectric 

 constant of the separating medium, and as the ions are to be regarded as electrically 

 charged bodies, the force attracting two unlike ions will be more weakened, and 

 dissociation aided, in a solvent of high than in one of low dielectric constant. 



This expectation is only partially realised in the parallelism which exists for a great 

 number of solvents between the two properties in question ; thus liquefied ammonia 

 which possesses a low dielectric constant is a better dissociating solvent for some 

 substances than water which has a high dielectric constant ; moreover, the majority 

 of electrolytes are far more dissociated in water than in hydrocyanic acid or in 

 hydrogen peroxide, although the dielectric constant of water is less than that of 

 either of these liquids. No measurements of the dielectric constant of the halogen 

 hydrides or of sulphuretted hydrogen have yet been made. 



DUTOIT and ASTON (' C. R.,' 1897, 125, p. 240) have attempted to show that ionic 

 dissociation occurs only in solvents in which the molecules are associated, but, 

 although numerous instances occur in which this parallelism obtains, it is by no 

 means a general rule. Thus, although both ammonia and sulphur dioxide are 

 unassociated liquids, both are able to form conducting solutions ; and although the 

 hydrides of bromine, iodine, and sulphur are unassociated, and hydrogen chloride is 



