chemical action is that the tissue never removes the dye completely 

 from solution, even tho very dilute; whereas ordinary chemical 

 reactions tend to continue until one of the components of the re- 

 action is exhausted. Such facts as these, to the exponents of the 

 physical theory, are enough to refute the possibility of chemical 

 action. 



It has, indeed, been pointed out that all of the ordinary dyeing 

 or staining phenomena can be explained on a physical basis. It is 

 evident, to be sure, that the action of the dyes is not confined to the 

 surface of the material colored; but as the substances stained are 

 always more or less porous, absorption of the dye after passing thru 

 cell membranes by osmotic action accounts for the penetration. To 

 account for the selective action of different dyes upon different 

 parts of the cell it is possible to use the principle of absorption as an 

 explanation. Adsorption is the property possessed by a solid body 

 of attracting to itself by purely physical means from a surrounding 

 solution certain compounds or ions present in that solution. Hav- 

 ing once entered the tissue it is possible that the dyes remain there 

 in a state of solid solution, similar to that in which gold is retained 

 in ruby glass. This possibility, in the opinion of those who hold the 

 physical theory, is the more likely because a dye causes the tissue 

 to become the same color as the dye shows in solution, but not 

 necessarily the same as it shows in its solid form. Dry fuchsin, for 

 example, is green; its solution, however, is red, and so are tissues 

 stained by it, no matter how completely they may be dried. 



Some of those who hold in general to the physical theory of stain- 

 ing admit that these simple physical phenomena alone cannot ex- 

 plain everything, as for example, instances in which a dye pene- 

 trates different cell elements equally readily, but can be easily ex- 

 tracted from some of them while scarcely at all from others. It is 

 assumed, therefore, that the dyes penetrate the cells by mere ab- 

 sorption and diffusion, but are in some cases precipitated there by 

 acids or bases, or other chemical reagents present, thus preventing 

 their extraction by simple solvents. Such a theory admits the 

 possibility of chemical action without assuming an actual chemical 

 union between the dye and the tissue. 



In this connection the action of mordants is interesting. Some 

 tissues do not stain directly with certain dyes, or if they do the 

 color is very feeble. If, however, they are treated previously or 

 simultaneously with certain chemicals, the dyes "take." 



It is possible that this phenomenon may be due to chemical 

 affinities of the mordant for the tissue on the one hand and the dye 

 on the other; but the special value of iron and aluminium salts as 

 mordants makes it seem quite probable that their action may be 

 actually to precipitate the dye in the tissue. To the bacteriologist 

 the behavior of different bacteria to the Gram stain immediately 

 suggests itself. In this technic (see p. 68) one of the methyl violet 

 dyes is allowed to act on the bacteria for a definite length of time, 



100 



