PAPERS OX CHEMISTRY AND PHTS1 119 . 



One notes that (a) an iodine has been removed, and 

 replaced by hydrogen: and (b) the iodine thus removed 

 retains the power of resubstituting in the bezene ring, 

 a power which negative iodine does not have. A con- 

 siderable amonnt of evidence collected since 1921 has 

 confirmed the idea suggested at that time, that a some- 

 what similar behavior was to be expected on the pai 

 any substance containing io'dine in a position ortho- or 

 para- to an amino- or to an hydroxyl-group, these two 

 groups being among the most strongly negative known. 



The rates at which iodine is removed under the condi- 

 tions used (boiling with 10^c hydrochloric acid, unless 

 otherwise specified) vary a y with the constitution 



of the substance examined. In many cases, too, resub- 

 stitution takes place less readily, and the iodine which 

 then accumulate ::i oxidi: ortion of the sub- 



stance. It is therefore often convenient, particularly 

 when carrying out approximate quantitative measure- 

 ments, to prevent both resubstitution and oxidation by 

 the addition of a mild reducing agent. For this purpose 

 stannous chloride has been used, and it may be empha- 

 sized that this has never as yet been found to act on any 

 halogen which could not also be removed by acid alone. 



From the relative stabilities of their inorganic com- 

 pounds with oxygen and with hydrogen, it is obvious 

 that iodine should show positive reactions more readily 

 than bromine, and this in turn more readily than chlo- 

 rine. This is found to be the case. TThen bromine occu- 

 pies positions such as those specified for positive iodine, 

 it is also removable under the same conditions, though 

 some eight or ten times more slowly. The evidence for 

 a similar reaction of chlorine, in the aromatic series, 

 is indirect, though probably sufficiently definite, and for 

 a single case only, namely, 2. 4. 6-triaminochloroben- 

 zene. 



More recent work has concerned itself with substances 

 containing two amina- or hydroxyl-groups. and halogen 

 ortho- or para- to at least one of them. Such halogens 

 mine or iodine) are removed much more rapidly 

 when the two negative groups are meta with respect to 

 each other, and more slowlv when these are other 



