METHOD OF INVESTIGATION. 9 



the seventh is held more tenaciously, and is not given off until heated 

 to 210. 



However, in cases like these, where successive portions of water are 

 given off at different temperatures, it is difficult to make a distinction 

 between water of crystallization and water of constitution. In the min- 

 erals just quoted, however, it has been found that the heat of hydration 

 of the last molecule of water is different from that of the molecules of 

 water which pass off at a lower temperature, which confirms the belief 

 of a difference in the bonding in the two cases. 



As a whole, the question of the association of the atoms of oxygen 

 and hydrogen in certain compounds is far from settled, while the diffi- 

 culties involved in its investigation are very great. The existing data 

 bearing upon the subject is practically nil. 



PRESENT METHOD OF INVESTIGATION. 



In a preliminary paper bearing upon this subject 1 the writer described 

 the infra-red absorption spectra of two numerals, selenite (CaSO 4 -(- 

 2H 2 O) and brucite (Mg(OH) 2 ), in which the atoms of oxygen and 

 of hydrogen are thought to be combined in a different manner. It was 

 the application of the results of previous work, in which it was abun- 

 dantly proven that certain groups of atoms have characteristic absorp- 

 tion bands. Hence, if the oxygen and hydrogen, which enter into the 

 composition of certain minerals, are united as they are in a molecule of 

 water, then one would expect to find the absorption spectra of such 

 minerals to be a composite of the bands of water and of the bands 

 caused by the other constituents. 



The only previous investigation bearing on this subject, from the 

 standpoint of infra-red absorption spectra, is that of Konigsberger, 2 

 who studied the pleochroism of several minerals, including selenite. 

 Unfortunately his plate of selenite was too thick, so that no energy was 

 transmitted beyond 2.5 /x. He calls attention to the fact that a small 

 absorption band at 1.5 /A coincides with that of water, from which it 

 would appear that in selenite the absorption of the "water of crystalliza- 

 tion" does not appear to be different from that of water. 



Thinking that it would be fairer to select the large water bands at 

 3.0, 4.75, and 6 /x, as a criterion, the writer (loc. cit.) examined thin sec- 

 tions of several minerals and found, as had been anticipated, that min- 

 erals containing water of crystallization have large absorption bands 



1 Physical Review, 20, p. 252, 1905. 



- Konigsberger : Ann. der Phys., 61, p. 703, 1897. 



