SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 53 



same group, and, therefore, a shifting of the center of gravity of the absorp- 

 tion of the group itself. This may be true also of the uranyl bands, since it is 

 known that the uranyl bands break up into much finer bands at low tempera- 

 tures. At any rate, the changes in the uranyl bands at ordinary tempera- 

 tures, when chemical reactions take place, are comparatively simple compared 

 with the same changes in the case of the neodymium bands. 



It is very important that a careful study be made of a series of chemical 

 transformations of neodymium salts (especially at low temperatures). For 

 instance, it has been shown that different anhydrous neodymium salts pos- 

 sess very different absorption spectra. Comparisons between the spectro- 

 photographs of dry chemical reactions should be made and these compared 

 with the same reactions in various solvents. As soon as we know what 

 physical and chemical conditions are connected with given spectroscopic 

 changes, then a study of the absorption spectra of crystals should be of aid 

 in giving us an insight into the physical and chemical conditions within 

 crystals. An example of this kind of study might be found in the different 

 varieties of ice. Tammann 1 has shown that there are four distinct varieties, 

 ice i and iv being lighter than water, and ice n and in being denser. 

 According to Tammann the latter two varieties are composed of the simpler 

 molecules. These different forms of ice are produced at different tempera- 

 tures and pressures. The freezing-point of ice i is 



t -5.53 -10.42 -15.66 



p 675 1141 1597 kg./cm. 2 



Whereas ice in is formed by compressing to 3000 kg./cm. 2 and cooling 

 to 80, ice n is formed by cooling to 80 and then compressing to 2700 

 kg./cm. 2 Ice iv is probably identical with the tetragonal ice observed by 

 E. Nordenskiold 2 or with the regular ice crystals formed from 75 per cent alco- 

 holic solutions. 3 The absorption spectra of colored salts like those of neodym- 

 ium, samarium, or uranium would probably aid in a study of these different 

 crystalline types. 



Another subject that seems to be of interest and promise is the possibility 

 of breaking up absorption spectra into related groups. The work of Wood on 

 sodium vapor has been very successful in this respect, each monochromatic 

 stimulating wave-length exciting a certain resonating system within the atom 

 or molecule, the resultant resonance spectrum including light of the same 

 wave-length as that of the exciting light. 



Among the rare earth elements praseodymium, neodymium, samarium, 

 europium, terbium, dysprosium, and erbium show characteristic absorption 

 spectra in the visible wave-lengths, and also are examples of excellent phos- 

 phorogens. Lanthanum, gadolinium, and yttrium do not possess absorption 

 spectra, and only act as diluents. Following are some tables given by Urbain 

 of the various phosphorescent spectra studied by him : 



1 Zeit. phys. Chem., 72, 609 (1910). 



2 Ann. d. Phys., 114, 612 (1861). 



3 Barendrecht: Zeit, phys. Chem., 20, 240 (1896). 



