io INFRA-RED TRANSMISSION SPECTRA. 



coinciding with those of water. The selection of selenite (CaSO 4 -f 

 2H 2 O) and brucite (Mg(OH),) as types of water of crystallization and 

 of constitution was a fortunate one, for the two following reasons: 

 (i) The selenite curve showed all of the absorption bands of water in 

 their proper positions and intensities, except the 4.65 p band, which is 

 shifted and too deep for the thickness of water contained in the plate 

 under examination. The fact that the band is shifted caused me to 

 suspect that it may be complex, and, from the fact that it lies in the 

 region where the NCS radical and certain sulphur compounds have a 

 strong absorption band, that it may be due to the SO 4 radical a sur- 

 mise that since then has been unexpectedly verified. (2) Although the 

 brucite curve did not contain the water bands, thus showing the differ- 

 ence between water of constitution and water of crystallization, it con- 

 tradicted my previous work, 1 in which it was shown that the OH radical 

 in the alcohols has an absorption band of 2.95 /A (true value is 3/u.). 

 From the brucite curve the conclusion was drawn that the OH radical is 

 inactive. Since then I have studied the chemical side of the question 

 more thoroughly, from which it appears that there is no marked differ- 

 ence 2 in the activity of the OH radical in the hydroxides studied, and, 

 hence, that the brucite curve should have a band at 3.0 p, instead of the 

 one shifted to 2.5 p.. In other words, the present research, which is in 

 part the outcome of the foregoing discrepancies, has to a very unex- 

 pected degree furnished us with an abundance of new proof that certain 

 groups of atoms have definite absorption bands, which really means that 

 these groups of atoms, or "ions," enter into the various compounds in a 

 similar manner. 



To sum up, in the present investigation the criterion for distinguish- 

 ing water of crystallization from water of constitution is the presence 

 of absorption bands at 1.5, 2, 3, 4.75, and 6 p., which is the location of 

 the absorption bands of water. If there are no other absorption bands 

 near by, then the intensity of these bands should be somewhat like that 

 of water, viz, the bands at 1.5, 2, and 4.75 /* are weak, while the bands 

 at 3 and 6//, are very strong. 



A hydroxyl group will also cause an absorption band at 3 p.. Silicates 

 may have a small band shifting from 2.9 to 3.1 p., but since it is weak, 

 there is no danger of confusing it with the strong water band in the 



same region. 



1 Investigations of Infra-red Spectra, Washington, 1905 ; Phys. Rev., vol. 22, 1905. 



2 From the chemical standpoint, however, the OH in alcohol and in H 2 SO 4 is 

 more active, since it is replaceable by a metal (more acid) than the OH in brucite 



, which is not replaceable (more basic). 



