46 Biological Stains 



sorption, and their action may be thus accounted for; they also 

 are nearly always substances that decrease the solubility of any 

 dye and thus increase the readiness with which a dye may enter 

 into solution within the tissue to be stained, if one accepts the 

 solution theory. In other words, the action of mordants can be 

 accounted for by any of the theories of staining. 



It is, moreover, frequently pointed out that there is no sharp 

 distinction between chemistry and physics, and in such delicate 

 reactions as those involved in staining, w^e may be well in the 

 borderland between the two branches of science, where it is im- 

 possible to say that a given phenomenon is purely physical or 

 purely chemical. There are, however, certain chemical principles 

 distinctly different from the physical ones just mentioned, that 

 may well enter into the phenomenon of staining; and it is these 

 that were considered most important by the exponents of the chem- 

 ical theory. 



The Chemical Theory. It was claimed on behalf of the chemical 

 theory that just because physical forces alone can explain the 

 facts one is not justified in assuming that chemical unions do not 

 take place when the opportunity for them is present. It is agreed 

 that some parts of the cell are acid in reaction, others alkaline; 

 and it is a well-known chemical principle that the former w^ould 

 tend to combine with the cations in solutions with which they 

 come in contact, the latter with the anions. Now inasmuch as in 

 certain dyes the color exists in the cation (basic dyes) and in 

 others in the anion (acid dyes), it is natural to expect chemical 

 combinations to take place between dye and tissue, depending 

 upon the reaction of the latter. Arguments for the physical 

 theory which exclude chemical action must furnish strong proof 

 that no chemical union occurs; and those who favored the chemical 

 theory claimed that such proof is Licking. That the stained tissue 

 does not present any characteristics to the eye not possessed by 

 either tissue or dye before staining does not prove that no new 

 substance has been formed, nor is this claim refuted by the fact 

 that suflSciently long action of solvents removes the color. Alcohol 

 and even water are not absolutely inert chemically and may with- 

 draw the dye by chemical instead of physical action; the very 

 length of time necessary to remove the color completely (some- 

 times so long as to allow bacterial decomposition of the tissue) 

 indicates that a rather strong union between dye and tissue has 

 taken place. As to the fact that a tissue which has a strong affinity 

 for some particular dye never withdraws that dye completely from 

 a very dilute solution, those who favored the chemical theory pointed 

 out instances where chemical reactions are known to take place 

 and yet to stop before either component is exhausted. The failure 

 to exhaust the dye completely in a surrounding solution may 

 merely mean that an equilibrium has been reached; chemical 



