220 PROCEEDINGS OF THE AMERICAN ACADEMY. 



by contact with dilute sulphuric acid containing 33 per cent of acid, and 

 the absorption of water followed by successive weighings at intervals of 

 about a day. Upon plotting these results a curve was obtained, which 

 showed a change in tlie rate of absorption at a certain point, and a calcu- 

 lation of the amount of water absorbed at this point gave the following 

 result: 



0.4289 gram of ferrous iodide absorbed 0.0491 gram of water. 



Calculated for Fel22H20. Found. 



Water 10.40 10.28 



The coincidence of these numbers is certainly striking, and, so far as 

 it goes, tells in favor of a definite composition for the white body ; but 

 little weight can be given to a single determination, and in this case the 

 result must be regarded with the more suspicion, because the ferrous 

 iodide, after it had turned from red to white in this experiment, became 

 gray from decomposition with 'liberation of iodine. 



When one of the red plates was viewed under the polarizing micro- 

 scope in ordinary air, it appeared at first granular and then white, at 

 the same time assuming a lower order of symmetry, as extinction was 

 observed at certain angles when the stage was rotated, an effect which 

 could not have been produced by the original crystal, as it was a hexa- 

 gonal plate bounded by two basal planes. This observation shows little 

 more than that the white substance was crystalline. 



The anhydrous ferrous iodide dissolves rapidly and freely in water 

 with a considerable evolution of heat. The dilute solutions are colorless, 

 the stronger ones green. The hot saturated solutions deposit green 

 crystals of Fel24H20. Our analysis of this salt gave results which agreed 

 only approximately with those required by the theory, but this is not 

 strange, as it is very deliquescent and rapidly decomposed by the oxygen 

 of the air with liberation of iodine. The water cannot be determined by 

 loss, as the salt gives off hydriodic acid when it is heated. Fortunately 

 it was not necessary for us to spend our time in attempting to obtain 

 better analytical results, as Volkmann * has made a careful study of 

 crystallized ferrous iodide, and has isolated it with four, six, or nine mol- 

 ecules of water of crystallization. From his description of the prepara- 

 tion of these salts there can be no question that the salt obtained by us 

 was Fel24H20. 



The solution of ferrous iodide, when exposed to the air, is decomposed, 



* Ber. d. chem. Ges. 1894, R. 618. 



