Theory of Staining 45 



Adsorption, on the other hand, furnishes a very satisfactory 

 explanation even of much differential staining, and many once 

 claimed that all staining phenomena might be accounted for in this 

 way. Adsorption is the property possessed by a solid body of 

 attracting to itself minute particles of matter from a surrounding 

 fluid; these particles may be compounds suspended in the fluid, 

 or they may be ions incapable of existing independently except in 

 solution. The principle of selective adsorption is well known to 

 physical chemists, whereby certain ions may be adsorbed by 

 certain substances much more readily than by others. Equally 

 well known is the fact that the rate of adsorption of any ion is 

 strongly influenced by the presence of other ions in the solution, 

 and that an especially profound influence is exerted by the re- 

 action of the solution, in other words by the concentration of free 

 hydrogen or hydroxyl ions in the fluid; the influence of the latter 

 upon the adsorption of basic ions being exactly the opposite from 

 its effect upon the adsorption of acid ions. Such principles as 

 these, well established in physical chemistry, may offer an explan- 

 ation of such phenomena as the differential staining of different 

 cellular elements, action of mordants, variations in rate of staining 

 with changes in the salt content of the staining solution, and in- 

 fluence of H-ion concentration upon the color assumed by tissue 

 when exposed to the action of both acid and basic dves. Bavliss 

 (1906) developed this particular phase of the physical theory into 

 what he calls an "electrical theory of staining." See also Parks 

 and Bartlett (1935a and b). 



Some of those who held in general to the physical theory of 

 staining admitted that these simple phj^sical phenomena alone can- 

 not explain everything, as for example, instances in which a dye 

 penetrates different cell elements equally readily, but can be easily 

 extracted from some of them while scarcely at all from others. It 

 was assumed, therefore, that the dyes penetrated the cells by mere 

 absorption and diffusion, but were in some cases precipitated there 

 by acids or bases, or other chemical reagents present, thus pre- 

 venting their extraction by simple solvents. Such a theory admits 

 the possibility of chemical action without assuming an actual chem- 

 ical union between the dye and the tissue. 



This precipitation theory would furnish a satisfactory ex- 

 planation of the action of mordants. It is well known that certain 

 tissues, that stain feebly if at all with certain dyes, "take" these 

 same dyes deeply if previously treated with the proper chemical. 

 Such a mordant, already present in the tissue, might well cause 

 the precipitation of the dye inside the cell walls. On the other 

 hand the action of mordants may be equally readily explained if it 

 is assumed that staining takes place by means of adsorption or as 

 a process of solution. In the first place, all mordants contain 

 ions that are known to have a decided influence upon the rate of ad- 



