62 THE ABSORPTION SPECTRA OF SOLUTIONS. 



and quite persistent band at X 4680. This is about the most persistent band 

 in the uranous spectrum, although in the lower strip it is only about 40 

 Angstrom units in width, while the red band is about ten times as wide. The 



' o 



bands XX 4400, 5000, and 5250 are very much alike, being about 100 Angstrom 

 units in width, very weak and diffuse. The band X 5000 is apparently double. 

 The red band is quite strong, running from X 6050 to X 6400. It is accompanied 

 by a very weak band at X 6600. This band is extremely weak, and probably 

 corresponds to the water band in the same region. The addition of hydrogen 

 peroxide simply causes each one of these bands to decrease in intensity, while 

 the many] bands increase in intensity. 



A POSSIBLE METHOD OF MEASURING THE STRENGTHS OF ACIDS. 



Several methods have been employed for measuring the relative strengths 

 of acids. Among these are : The power of acids to divide a base between them, 

 as determined by thermo-chemical and volume-chemical methods; their power 

 to invert cane-sugar; to saponify an ester; to convert acetamide into am- 

 monium acetate; their conductivity, etc. All of these methods measure the 

 relative concentrations of the hydrogen ions in the solutions of the various 

 acids, the degree of dissociation determining, of course, the relative strengths 

 of acids. All acids at infinite dilution, or when completely dissociated, are, 

 then, of the same strength. In more concentrated solutions the strengths 

 of acids vary directly as their dissociation. Our work on absorption spectra 

 has shown that the positions of the uranyl and the uranous bands for different 

 uranium salts are often quite different. We could add just enough of the acid 

 in question to transform completely the original salt into the salt of the acid 

 in question, and then express the concentration of the acids and salts in gram 

 molecules per liter, or by any other convenient method. 



Some spectrograms of this kind have been made indicating the relatively 

 great strength of hydrochloric acid. As yet, however, the subject has only 

 been touched upon. It is important that an accurate quantitative method for 

 measuring the intensity and wave-length of the bands be first devised. There 

 will be one obstacle that may cause some difficulty in this method, and that 

 is the presence of the acid (above, the acetic acid) displaced from the colored 

 salt by the addition of the acid to be tested. 



Experiments of this kind should be carried out with various uranyl, 

 uranous, and neodymium salts, etc., in different solvents, to find whether the 

 relative strengths of acids are the same for the various colored salts, for the 

 different solvents, for different concentrations, for different temperatures, 

 for salts when in the presence of colorless salts, etc. Spectrophotography 

 of this kind only includes a study of the spectra as they are changed from that 

 of one neutral salt to that of another. Such a method, however, could hardly 

 be made as accurate a measure of the strength of acids as the conductivity 

 method, and not nearly so general. 



ARE THE IONS FACTORS IN THE ABSORPTION OF LIGHT? 



It is one of the remarkable facts brought out in this investigation, that 

 ions as such do not seem to play any role in the absorption of light. If the 

 ions were the absorbers of light in solution, then the absorption spectrum of 

 a solution would vary with the dilution of the solution, since the dissociation 

 of a solution varies with the dilution. 



