ON THE HALOGEN HYDRIDES AS CONDUCTING SOLVENTS. 139 



amount of substance which has been carried to the electrode by the current can then 

 be calculated. The current is usually measured by a silver voltameter placed in the 

 same circuit as the electrolytic cell. The calculation, neglecting certain small 

 corrections, is as follows : 



If x grammes of the substance, whose equivalent weight is n, be transported by 

 the current which deposits y grammes of silver on the cathode of the voltameter, 



then the transport number of the cation is given by p = . p represents the 



u 

 fraction of the total current which is carried by the cation, on the assumption that 



one unit charge of electricity is associated with one equivalent of the dissolved 

 substance. It is probable that this condition is fulfilled only in solutions of salts of 

 the simplest type (STEELE, loc. cit.). 



Preparation of Solutions. 



The most convenient refrigerant which was available was a mixture of carbon 

 dioxide and acetone, and as at the temperature of this mixture hydrogen chloride 

 is a gas and hydrogen iodide a solid, the choice of solvent was limited to 

 hydrogen sulphide and hydrogen bromide. Solutions in the former solvent are very 

 much more difficult to analyse than those in the latter, and accordingly hydrogen 

 bromide only has been used as solvent during the investigation. 



The hydrogen bromide was prepared and purified by the method described in 

 Part I. In order to make the solutions for electrolysis, the gas was condensed in a 

 graduated vessel in which a sufficient quantity of the substance under examination 

 had been placed, the condensation being stopped as soon as the desired volume of 

 solution had accumulated. 



A quantity was usually made sufficient for two experiments, and by placing the 

 receiver in a good silvered vacuum vessel with a stiff paste of the carbon dioxide and 

 ether, the solution could be kept for a period of twenty hours without renewal of the 

 refrigerant. The apparatus in which the electrolysis was carried out was immersed in 

 a bath of solid carbon dioxide and acetone contained in a large cylindrical silvered 

 vacuum vessel. This mixture can be maintained at a practically constant tempera- 

 ture by blowing a steady stream of air through it ; the temperature, moreover, may 

 be varied within certain limits by altering the rapidity of the air current. 



The Validity of FAEADAY'S Law. 



The measurement of the transport number depends on FARADAY'S law, and although 

 this is known to hold rigidly for aqueous solutions, there is no evidence as to its 

 validity for solutions such as those under investigation. Experiments were therefore 

 undertaken with the object of testing the law. 



This was accomplished by comparing the weight of silver deposited in a voltameter 



T '2 



