20 



INFRA-RED TRANSMISSION SPECTRA. 



In fig. 3 is illustrated the effect of dehydrating the selenite. This 

 was accomplished with difficulty on account of warping and shrinking 

 of the plate, which necessitated dismounting the specimens for each 

 heating. It was found necessary to clamp the specimen between two 

 metal plates in order to prevent it from warping and breaking. Curve a 

 gives the transmission of a clear piece having a thickness of 0.204 mm - > 

 curve b shows the transmission after partial dehydration. 



7O% 



S 6 7 8 



FIG. 4. Selenite. 



It is of considerable interest in showing the permanence of the 4.55 /* 

 band. Beyond 6 ^ the transparency increases and becomes 1.5 times as 

 great as selenite at 6.9 p. The dehydrated section, which is an opaque 

 white mass, was then moistened with water and allowed to stand over 

 night to dry and set, just as in the case of plaster of Paris. The result 

 is shown in curve c, where the transparency at 6.9 p. has decreased to 

 almost the original value found for the transparent crystal. It will of 

 course be understood that the great opacity up to 5 p. for the dehydrated 

 section is due to the lack of homogeneity introduced in expelling the 

 water, which has the well-known property of scattering the radiation 

 of short wave-lengths. At 7/x the increase in absorption due to the 

 presence of water can not be doubted. 



In fig. 4 the transmission curves of a very much thinner partially 

 dehydrated section are given. In curve a the section was subtrans- 

 parent and in curve b it was translucent. The curves are given to show 

 the increased transparency beyond the band of metallic reflection in 

 the region of 9 /x. 



