LIFE AND ENERGY 37 



stituents in cells are nearly always neutral salts, but in the one set 

 the coloured ion is a complex organic acid combined with an 

 inorganic cation, usually sodium ; in the other set, the cation is the 

 coloured one, and is combined with an inorganic acid, usually 

 hydrochloric or sulphuric. The former set is often called that of 

 the " acidic " dyes, the latter, " basic " dyes, but such names are 

 clearly misleading, in that they suggest that the dyes themselves 

 have the properties of acids or bases. It was supposed at one time 

 that chemical combination occurred between particular constituents 

 of the cell and dyes of a definite chemical composition, so that the 

 staining of some particular structure indicated that it had some 

 particular chemical composition. Although this seems to be the 

 case in some rare instances, further investigation has shown that 

 a great variety of physical conditions also play a part, and that a 

 conclusion of the kind referred to cannot be drawn without other 

 evidence. Some points that are instructive may be mentioned 

 here. The ordinary form of adsorption must play a part, but there 

 are also those phenomena in which electrical forces come into 

 action, and sometimes in a rather complex fashion. Most of the 

 surfaces in cells have negative charges, and in order to see how 

 they behave to various dyes, some experiments with filter paper 

 should be made ; since this has a negative charge in water, the 

 conditions in general can be readily controlled (E., p. 179). The 

 results obtained apply, naturally, with the appropriate change of 

 sign, also to surfaces having a positive charge. 



Let us take pure white paper and stain some pieces of it in 

 crystal violet, a " basic " dye, and others in congo red, an " acidic " 

 dye. The former will rapidly become deeply stained, the latter 

 very faintly. The explanation is, no doubt, that in the first 

 case the ion which stains the paper is the electro-positive one, and 

 is attracted ; in the latter case, the coloured ion is electro-negative, 

 and is repelled by the paper. What staining occurs in the case of 

 congo red is the mechanical adsorption due to direct effect on the 

 surface tension. That this is so is shown by the curious fact that 

 the coloured matter deposited in the case of " basic " dyes is the 

 free base, whereas in the other case it is the neutral salt itself. 

 This is the reason why, in our previous experiment with charcoal 

 (E., p. 1 68), we used acidified alcohol to remove the dye from 

 the surface. Next, add a neutral salt, say sodium chloride, to both 

 the stains, and repeat the above experiments with filter paper. It 

 will be found that congo red stains very deeply, while crystal violet 

 stains less deeply than in the pure state. Why is this ? The 

 negative charge on the paper is neutralised, or changed to a positive 

 one, by adsorption of sodium ions from the solution, so that the 

 attractive and repulsive powers of the paper towards the two 



