270 HOLMES. 



These detenninaTions show that the effect of adding acid to the 

 solution after the admixture of alkaloidal and iodine solutions, is merely 

 to arrest the reaction and prevent the further formation of periodides, 

 rather than to destroy to any considerable extent the compounds already 

 formed. Moreover, in the experiments with free codein (numbered 73, 

 74, and 75) the filtrates after titration with thiosulpjiate were acidified 

 with sulphuric acid and no iodine was liberated after several hours, so 

 that the addition of acid after equilibrium has been reached would 

 seem to be without effect. 



It has been shown that morphine, codeine, and heroin are precipitated 



by Wagner's reagent from their aqueous solutions even when no acid, 



either bound or free, is present. It was now found that a solution of 



iodine in water, or in water to which a little aeetone or alcohol had been 



added to increase the solubility of the halogen, would also give alkaloidal 



precipitates which were apparently lower periodides. A saturated solu- 



N 

 tion of iodine in -t: sulphuric acid also gave a codein periodide when 



added to a concentrated solution of free codeine. It is known that 

 periodides may be formed in the absence of water by using organic 

 solvents. Finally it will be shown that it is unnecessary even to dis- 

 solve the iodine and base, and that these alkaloids, or their salts, will 

 combine with the vapor of iodine in the entire absence of moisture with 

 the formation of compounds which are plainly periodides. 



EXPERIMENTS "WITH lODIXE VAPOB. 



The behavior of the alkaloids toward iodine is in general similar, 

 whether the latter is in gaseous form or in solution. For example, mor- 

 phine, codeine, and heroin, both as salts and in the free state, take up 

 iodine readily when exposed to its vapor. Both caffeine and theobromine 

 in the free state exhibit no affinity- for iodine under similar circumstances, 

 either in the presence or absence of moisture. On the other hand if 

 thev are exposed simultaneously to the vapors of iodine and moist hydro- 

 chloric acid they are readily converted into periodides. The experiments 

 carried out with caffeine indicated that the periodide formed in this 

 manner is identical with the weU-known pentaiodide which is always 

 the result of reactions in aqueous solutions. 



The following table gives the results of the experiments with the 

 free alkaloids. These were dried to constant weight, morphine and 

 codeine at 9-5° and heroin in a vacuum desiccator over sulphuric acid. 

 Thev were then exposed in a desiccator to the vapor of iodine. 



