454 Mr. J. Brown on the Role of 



precipitated on it. Zinc immersed in the dilute solution 

 immediately received a brown deposit of iodine — the zinc no 

 doubt combining with the chlorine. Some gas also, no doubt 

 hydrogen, was evolved at the zinc, especially in a moderately 

 dilute solution, where the action was more rapid. That the 

 zinc in this case should not combine with all the iodine nascent 

 at its surface is remarkable. Does it suggest a polarization of 

 the IC1 molecules in which the chlorine end is attracted and 

 the iodine end turned outwards? After a time the iodine 

 deposited in long thin threads. These grew down from the 

 under surface of the zinc with considerable rapidity, giving a 

 curiously animated appearance to the process. 



13. The following modification of this experiment is also 

 interesting. When dilute iodine-chloride solution was care- 

 fully poured on to strong zinc-chloride solution, there was 

 formed at the dividing line a narrow well-defined brown band 

 which looked like deposited iodine. 



I wished to try if the immersion of metallic zinc in the 

 zinc-chloride solution would increase this, and arranged the 

 solutions in the opposite limbs of a V-tube, suspending a 

 piece of sheet zinc in the zinc chloride. The iodine- chloride 

 solution mixed rather quickly with the zinc chloride. In one 

 case, in a flat Y-shaped cell, whose limbs were 6 centim. long, 

 I observed that it did so by eddying in long striae down, round 

 the bend of the tube, and up on the other side. The com- 

 paratively rapid motion of these striae first down and then up 

 through the zinc chloride was very striking. I do not find 

 a ready explanation of it. 



In about an hour the zinc-chloride solution had taken a 

 uniform brown tint, and then a black cloud with a curiously 

 sharply defined lower edge formed at the bend of the tube 

 about 2 centim. from the lower end of the zinc. Under the 

 microscope this was seen to consist of minute acicular double- 

 pointed black crystals, probably iodine. It is remarkable that 

 although the zinc in this form of experiment is enveloped by 

 the brown solution, almost none of the iodine crystals are de- 

 posited on it as in the case of § 12 ; but in about a week or 

 two there are deposited on the glass, usually at some distance 

 from the zinc, very pretty crystals of iodine. 



In one case where a rather dilute iodine-chloride solution 

 had been used, after eight weeks a very fine group of pointed 

 crystals one or two centim. long (a kind of iodine tree) formed 

 in the limb of the tube opposite to that containing the zinc. 

 These formed in a beautiful brown-crimson solution, consisting 

 probably of iodine dissolved in zinc-iodide solution. 



In this particular form of experiment the action takes place 

 slowly. The zinc seems to combine chiefly with the chlorine, 



