1898.] the Absorption Bands of Didymiam Salts, etc. 43 



In the case of the nitrate, there is a more marked absorption 

 of a like kind by the dilute solutions, but it is not widened so 

 much by the stronger solutions. 



Throughout the series the absorption bands which are common 

 to the chloride and nitrate are always, except in the most dilute 

 solutions, a little stronger for the chloride than for the nitrate, the 

 difference being more pronounced for stronger solutions. This 

 may be accounted for by supposing a larger proportion of the 

 chloride to be ionized than of the nitrate. 



When the absorption produced by a thickness of three inches 

 of a solution of the chloride is compared with that produced by a 

 thickness of six inches of solution of half the strength, it appears 

 that the bands in the blue, supposed to be due to the metallic 

 ions, are stronger with the thickness of six inches of the weaker 

 solution, whereas the effect of the broad diffuse absorption, affect- 

 ing chiefly the most refrangible part of the spectrum, and supposed 

 to be due to the un-ionized salt, is more marked with the smaller 

 thickness of stronger solution. These effects are not very evident 

 when the salt is nitrate ; but that may be because the absorption 

 ascribed to the un-ionized nitrate is both stronger and more sharply 

 denned than that of the un-ionized chloride, so that shades of 

 variation cannot be so easily observed when the nitrate is used. 

 As far as the chloride is concerned, these observations point to a 

 relative increase in the proportion of the salt ionized as the 

 solution is diluted. 



Generally these observations tend to confirm the theory of 

 Ostwald, but much more extended observations are needed, and 

 some with other salts are already in progress. The effects of 

 temperature will be taken in hand as soon as possible. 



It is difficult to see how the ionization theory of solution can 

 be tested spectroscopically in any other way, at least until we 

 have learnt what the spectroscopic characters of the ions are. It 

 is not possible, with our present knowledge, to predict the proper- 

 ties of ions, except so far as to say that, since the ions differ in 

 their intrinsic energies from the elements of the salts ionized, it is 

 almost certain that they will differ in their properties too. 



Ostwald speaks of their being statically charged with elec- 

 tricity as a cause capable of changing their chemical and other 

 properties ; but statical charges seem to be affections of the field 

 rather than of the charged bodies, and the intrinsic properties of 

 charged substances are not found experimentally to be different 

 from those of the same substances uncharged. On the other 

 hand, what are called allotropic variations of a substance differ 

 from one another in intrinsic energy, and the changes of energy 

 which occur when the same element enters into the formation of 

 two salts of different types, as for example ferrous and ferric 



