74 THE ABSORPTION SPECTRA OF SOLUTIONS. 



halogen compounds the largest decrease of resistance is found for hydrogen 

 chloride (17 per cent for i> = 100, p = 3000 and t=l9 C.) and lithium chloride 

 (14 per cent); and then follow potassium chloride (9), sodium chloride (8), 

 potassium bromide (6), sodium bromide (5), potassium iodide (2), and sodium 

 iodide (1). 



SOLVATES AND THE EFFECT OF CHANGE IN TEMPERATURE ON 

 THE RELATIVE INTENSITY OF SOLVATE BANDS. 



The absorption spectra of colored salts, in general, in the visible region 

 are not very characteristic, but the absorption spectra of neodymium, erbium, 

 uranyl, uranous, and a few of the other rare-earth salts show fine characteristic 

 bands, more or less distributed throughout the spectrum. There are, however, 

 even among these spectra, very few cases where the absorption spectra of the 

 same salt in different solvents, or of different salts of the same metal in the 

 same solvent, are very different. There are, however, a few examples of well- 

 defined "solvent bands," and some of these will now be discussed. 



The most striking examples of the effect of the solvent on absorption 

 spectra are: those of water and ethyl alcohol upon several of the neodymium 

 salts, water and alcohol upon uranous chloride or uranous bromide, and water 

 and glycerol upon several uranyl and uranous salts. The spectra may well 

 be described as consisting of the "water," the "methyl alcohol," the "glycerol," 

 etc., bands of the particular salt. When the salt is dissolved in a mixture of 

 two of the solvents, both sets of solvate bands appear, and if the solvents are 

 mixed in a certain proportion, both sets of bands may be obtained of approxi- 

 mately the same intensity. The intensity of any set of solvate bands will be 

 proportional to the amount of that solvent present. As the amount of the 

 solvent present is changed, the solvate bands, in general, do not shift their 

 position, but only change in intensity, and this change in intensity seems to 

 be the same for all of the bands characteristic of that solvent. Solvent bands, 

 in this respect, differ from the phosphorescent bands described by Lenard 1 

 and Klatt. 



Just as a characteristic arc or spark spectrum has been regarded as 

 indicating the presence of a certain element, just so a characteristic absorption 

 spectrum will be regarded as evidence for the presence of a compound; and 

 these compounds will be considered as being composed of one or more mole- 

 cules or ions of the salt ("aggregate") and one or more molecules of the sol- 

 vent. On account of the definite spectra, we shall speak of the "methyl 

 alcoholate" of uranous chloride, the "hydrate" of neodymium bromide, etc. 



Suppose, for instance, that the number of radicles in an aggregate of neo- 

 dymium salt molecules and ions can vary, then the compound in which absorp- 

 tion takes place might be represented symbolically in the following manner: 



x { \J0 2 }y\ U0 2 C1 2 } 2 { CI } a { CH 3 OH } 

 and 



+ 

 + 



;r';Nd; ? /'{NdCl 3 }2'{Cl}a'{CH30H} 



1 Ann. d. Phys., 31, 642 (1910). 



