22 John White 



lead iodid was always formed. The most reasonable explana- 

 tion for the conversion of lead iodid into the basic iodid, then, 

 is to suppose reaction 2, above, to be reversible; the first stage, 

 i. e., 



would naturally take place very readily on account of the tend- 

 ency to form the difficultly soluble iodid; later, however, under 

 the gradual action of water, with the formation of a still less 

 soluble basic iodid, the reaction reverses itself, i. e., goes in the 

 sense 



and then water acting upon the ^^^r fT D would con- 

 vert it into the basic iodid as shown in reaction 3 above. The 

 conversion of the iodid into the basic iodid would of course be 

 slow, owing in part to the slight solubility of lead iodid, but 

 more to the fact that the active mass of lead acetate is too 

 small to force the original reaction to reverse itself. That this 

 is the case was demonstrated by the addition of lead acetate, 

 when the formation of the basic iodid took place quite 

 rapidly. Indeed it was found that freshly precipitated lead 

 iodid, which is but slightly affected by cold water, can be read- 

 ily and completely transformed into the basic iodid by simply 

 adding lead acetate and shaking. Other acetates produce a like 

 effect, but not quite so readily as the lead acetate. The fact 

 that acetic acid does not, in small quantities, prevent the reac- 

 tion is probably because it is so slightly ionized in comparison 

 with its salts as to produce little or no effect. It is hoped to 

 test reaction 3 above in the near future. The impossibility of 

 preparing the iodid-acetate has so far prevented this. 



From the study of the reactions involved in the formation 

 of these double salts, as well as those just considered in their 

 decomposition, the conclusion may be drawn that in these we 

 must regard the iodin as united directly to the lead, and that 

 in all probability the double salt is of the type commonly desig- 



328 



