8 INFRA-RED TRANSMISSION SPECTRA. 



as the radiation from the moon. The exact determination of the data 

 presented involves the use of one of the most sensitive heat-measuring 

 devices yet produced, viz, the radiometer. The region examined is a 

 vast one compared with the visible spectrum, and has heretofore been 

 little explored. 



From the intimate relation between refractive index and reflecting 

 power, the data obtained gives us some idea of the dispersion of the 

 substances examined. The bands of selective reflection, especially the 

 very sharp ones, such as the new one found in quartz at 12.5 /x, will be 

 useful as a monochromatic source of energy, for example, in inter- 

 ferometer work. 



Many chemical compounds contain both oxygen and hydrogen, which, 

 on applying heat, pass off in the form of water. That the water is not 

 united so tenaciously as the other constituents is evident from the fact 

 that in many instances it can be more easily removed ; many salts give 

 up their water if exposed to dry air at ordinary temperature. From 

 the circumstance that many of these compounds are crystalline, the 

 water is said to be present as "water of crystallisation." The manner 

 in which the water exists in the crystal is not understood. By some it 

 is considered a part of the chemical molecule ; by others it is thought 

 that the molecules of water exist in their entirety among the molecules 

 which constitute the crystal. It is characteristic of water of crystalliza- 

 tion that it is expelled at a temperature far below red heat, and fre- 

 quently below 100 C. Another characteristic of minerals containing 

 water of crystallization is their property of reabsorbing water after it 

 has been removed. Copper sulphate is an excellent example ; on apply- 

 ing heat, the blue crystal becomes a crumbling white mass, which, if 

 permitted to stand in dry air, absorbs water and resumes its blue color 

 and crystalline structure. 



On the other hand, the water which is given off only at a red, or even 

 a white heat, can scarcely be present in the compound in the same man- 

 ner as the water of crystallization, and is distinguished as "water of 

 constitution." In this case the water is not supposed to exist as such 

 in the mineral, but to result from the union of oxygen and hydrogen or 

 from the hydroxyl groups contained in the compound. 



Of course there are minerals which contain water in both these 



\ 



forms. In copper sulphate (CuSO 4 +5H 2 O), four molecules of water 

 are given off at 100 and the fifth at 200, so that the latter is possibly 

 combined in some manner different from the former. In epsomite 

 (MgSO 4 -f-7H 2 O), six molecules of water are given off at 132, while 



