394 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 18 
of these hydrates and experiments by the writers indicate that an 
anhydrous product is not obtained at room temperature. It must 
consequently be concluded that the specimen employed in this 
second determination was not the anhydrous compound. 
This conclusion is fortified in three directions. In the first place 
it has been shown! that the interatomic distances in all of the other 
alkali halides (rubidium fluoride was not investigated) are purely 
additive and can, if desired, be calculated through the medium of 
“atomic radii’’*for these particular cystals. An acceptance of this 
second structure? for lithium iodide would show it (see, however, 
the discussion of rubidium fluoride given below) to be the only devi- 
ation from a closely exact agreement with the requirements of this 
additive rule: The nearest approach of lithium and iodine atoms 
according to this structure is 3.537A, that required by the rule of 
additive interatomic distances, which is probably accurate for the 
rest of the alkali halides at least to within 1 per cent, is 3.01A. 
_In the second place the structure of lithium iodide as found! upon 
fused material gives a ‘‘sodium chloride arrangement” of atoms in 
which the distance from lithium to iodine atoms is in substantial 
agreement with the additive rule (Li to I = 3.015A). The density 
calculated from this structure (9 = 4.03) likewise agrees with that 
determined’ by the usual methods (p = 4.061). In. the originally 
published results of this study it was indicated that the calculated 
intensities of reflection are not in so good accord with the intensities 
of the observed powder lines as is generally the case. A careful 
reéxamination of these films shows that the strongest line of the 
hydrated product has practically the same position as the 100 (2) 
reflection from the anhydrous lithium iodide; when a correction is 
made for the intensity contributed by this extraneous line there is 
complete agreement of calculated intensities with those observed 
upon the films. The data upon which this assignment of structure 
was based are shown in Table I. Other lines appeared upon the 
photographs but their accurate measurement was prevented by the 
nearness of hydrate reflections. _No corrections’? were applied for the 
absorption of lithium iodide and great accuracy is not claimed for 
the spacing measurements. 
The observed intensities of reflections in the photogtaphes of the 
second determination? disagree with those to be expected from the 
5W. L. Braaa, Phil. Mag. 40: 169. 1920. 
°P. G. Baxter, Am. Chem. Journ. 31: 558. 1904. 
7Q. Pauut, Zeitsch. f. Krystal 66: 591. 1921-22. 
