JACKSON AND DERBY. — FERROUS IODIDE. 219 



The idea that the white substance was a second modification gained some 

 support from the observations of Thomson, Carius and Wanklyn, and 

 De Luca, who describe ferrous iodide as white or gray, and also from the 

 fact that the white compound was formed in atmospheres containing very 

 little water, although in none of these cases had moisture been rigorously 

 excluded. We, therefore, tried a number of experiments to settle this 

 point. In all of these it was necessary to select specimens of the salt 

 which show this change from red to white, such as rather thin red plates 

 or the brown amorphous powder obtained by sublimation, since thicker 

 plates or masses of the iodide show the phenomenon with difficulty or not 

 at all. We see no reason, however, to ascribe these differences in behav- 

 ior to anything except the size of the crystals. Two similar portions of 

 the ferrous iodide were exposed, the one to dry carbonic dioxide, the 

 other to moist carbonic dioxide, the experiments being carried on at the 

 same time and under parallel conditions. The specimen in the moist 

 gas turned white quickly, while that in the dry gas remained unaltered. 

 Other similar experiments showed that the change of color took place 

 much more slowly in a gas containing little moisture than in one contain- 

 ing more. In all these experiments the amount of moisture must be 

 kept small, as, if too much aqueous vapor is used, the salt deliquesces so 

 rapidly that the formation of the white body cannot be observed. A sam- 

 ple of ferrous iodide was immersed in absolute ether (dried with sodium), 

 and underwent no change of color even after long standing ; when, how- 

 ever, a minute fraction of a drop of water was added, it rapidly turned 

 white. Upon heating a specimen of the white body in a tube filled with 

 dry carbonic dioxide a cloud appeared which looked like moisture and 

 condensed in the cold part of the tube, the substance turning red at the 

 same time. As the tube cooled this moisture was quickly reabsorbed, 

 and the white body was formed again. This experiment was repeated 

 several times with the same specimen. These experiments leave no 

 doubt that the white substance is not a second isomeric modification of 

 the ferrous iodide, but is formed by the action of water on the red anhy- 

 drous salt. 



The question next arises. Is the white body a mixture of the red anhy- 

 drous and the green hydrous ferrous iodide, or is it a crystallized salt 

 with a definite amount of water ? Our experiments are not sufficiently 

 conclusive to answer this question with certainty, but we would present 

 the following argument in favor of the second view with all necessary 

 reserve. A sample of ferrous iodide was exposed at ordinary temper- 

 atures (in this case about 22°) to an atmosphere of hydrogen kept moist 



