394 SCIENCE PROGRESS 



and Kraus, 1 and of liquid sulphur dioxide, according to Walden. 2 

 The latter investigator has also determined the conductivity of 

 about forty carefully purified organic substances, 3 and in the 

 majority of cases has obtained minimum values which vary 

 from i — 5 x io -7 reciprocal ohms at 25 , whilst Kohlrausch 

 found 0*4 x io -7 for the purest water. A few examples may be 

 given. The value for ethyl alcohol is 2 x io -7 , for acetone 

 2*3 x io -7 , methyl alcohol 1*45 x io -6 . A few substances give 

 exceptionally high values: thus for formic acid 1*5 x io -5 was 

 found, and for acetamide 29 x io -5 . When one takes into 

 account the impossibility of obtaining absolutely pure sub- 

 stances, it is clear that the values cited can only be regarded as 

 upper limits. 



Many fused salts, as is well known, have a high conductivity, 

 as have the filaments of Nernst lamps at high temperatures ; the 

 latter, however, are not pure substances. A reference to these 

 interesting questions must here suffice. 



Complex Ions 



Our knowledge of complex ions is due, in the first instance, 

 to Hittorf. In the course of his classical experiments on the 

 migration of the ions, he observed that in the electrolysis of 

 solutions of potassium ferrocyanide the iron was transported, 

 along with the cyanogen, to the anode. This can be most 

 readily accounted for by the presence in solution of negatively 

 charged ions containing iron and cyanogen. In the same way 

 Hittorf observed that in the electrolysis of solutions of cadmium 

 chloride, part of the cadmium moved towards the anode ; in 

 this case it must be assumed that cadmium forms one of the 

 components of a negatively charged complex ion, perhaps 

 CdCl/ / . 



As an illustration of the methods employed in the investi- 

 gation of complex ions, the experiments of Bodlander and 

 Fittig, by which it was shown that solutions of silver chloride 

 in ammonia contain the ion Ag(NH 3 )./ may be cited. If this ion 

 is present it must, according to the laws of chemical dynamics, 

 be in equilibrium with its components according to the equation 



1 Amer. Chein. Jonm. 1898, 20, 520. 



2 Zeit. physikal. Chein. 1902, 39, 513. 



3 Ibid. 1903, 46, 103. 



