408 M. C. Lea— Color Relations of 



from molecules. For these last are often black, and this may 

 be the case even with elements, as for example, iodine in the 

 solid form, platinum black, etc. The absolute difference 

 between atoms and molecules, the entire absence of color rela- 

 tion between them, was mentioned in the first part of this 

 paper ; it must always be one of the most remarkable facts in 

 chemistry. 



The production of a black molecule in no way depends 

 upon the combination of two or more atoms having different 

 relations to light, such that one should extinguish the rays 

 which the other or others allow to pass. This might almost be 

 expected, but is far from being the case. Two colorless atoms 

 may unite to form a black molecule, as in the cases just men- 

 tioned. 



It seems scarcely necessary to say that these remarks relate 

 to inorganic molecules only. For in organic molecules, chem- 

 ical composition has little influence, isomeric combinations 

 exhibit the greatest differences, and their spectra show little 

 relation to those of their elements.* 



With organic compounds, which are formed almost exclu- 

 sively of colorless atoms, it is always the color of the molecule 

 that is important. With inorganic compounds the reverse is 

 the case, we have much less to learn from the color of the 

 molecule ; that of the atom is all important. One and the 

 same molecule may present various colors, absolutely unlike 

 each other. Phosphorus and sulphur may be black, red or 

 yellow. Gold* is yellow by reflected, blue or green by trans- 

 mitted light, and may be red when in a state of fine division. 

 Who shall say in either case which of these colors is character- 

 istic ? With the ion, and therefore with the atom, at any given 

 valency, there is no such ambiguity. Its color, its absorption 

 spectrum, are invariable; these are always highly characteristic. 



III. Acid Indicators ; Theory of their Reactions. 



Acid indicators are such as show a striking change by the 

 addition of an alkaline solution, either from one color to 

 another, or from a colorless to a strongly colored solution. 

 They may be considered as of the nature of weak acids. Their 

 reactions have been explainedf in this way, that the radical has 

 as an ion a color different from that which it has in the undis- 

 sociated compound. 



This explanation is unsatisfactory, for two reasons : 

 1st. It makes the color of an acid indicator in presence of 

 an alkali depend on dissociation. But it can be shown that 



*Ostwald, Lehrbuch, 2d Ger. ed., i, 4T2. 

 flbid., 800. 



