ASPECTS OF ADSORPTION PHENOMENA 219 



to show distinctly the two dark bands on the red side of the D-line 

 in a certain cell. An equivalent molecular solution of cosin was 

 taken, so that when mixed in equal volume there should be two 

 molecules of methylene-blue to one of eosin. This eosin solution 

 was placed in a cell of equal depth to that containing the methylene- 

 blue and the two cells then placed before the slit of the spectroscope. 

 The combined spectrum of the two dyes was seen. Equal volumes 

 of the two solutions were then mixed, a portion of the mixture at 

 once placed in both cells and these then observed. By taking a double 

 layer it would seem that there would be no change in the spectrum, 

 unless chemical changes had taken place. What one sees is that the 

 band of methylene-blue next the D-line has almost completely 

 disappeared, leaving a taint shade. No change is to be seen in the 

 band in the red, nor in the eosin band at F. The same disappearance 

 of the one band of methylene blue may be observed when any other 

 acid-dye such as anilin-blue or congo-red is taken instead of eosin. 

 On the other hand, it thionin or toluidin-blue is taken instead of 

 methylene-blue, no obvious change is produced in the spectrum until 

 precipitation occurs, when, of course the bands become much fainter. 

 1 believe the explanation of the disappearance of the methylene-blue 

 band is to be found in the fact that it is the first to disappear on mere 

 dilution. Now when an oppositely charged colloid is added to a 

 solution of the blue the precipitate does not fall at once and it might 

 be supposed there was no important change ; but, if the experiment 

 is made in a vessel through which a beam of bright light is passing, 

 the track of the beam is barely visible in the methylene blue, but 

 almost immediately on adding the other dye the beam begins to appear, 

 gradually becoming brighter as the particles increase in size. In point 

 of fact then, methylene blue is really taken out of solution, and the 

 first band to disappear would be the one next the D-line. The other 

 thiazin-dyes, so far as investigated, do not show the same behaviour 

 on dilution, the various bands all disappear or fade in nearly the same 

 proportion. 



These dye-compounds, although almost insoluble in water, are 

 soluble in alcohol, so that it might be supposed that by taking a strong 



