6 THe MIcroscopg. 
stuff; but if the writer may here state his views, the change is 
molecular and not atomic and so cannot be regarded as chem- 
ical, or else molecular differences are simply made manifest by 
the ingestion of thedye. A discussion of this subject, however at- 
tractive it may be, would be out of place in an elementary article 
such as the present number, but it may be well to state that it 
is almost certain that color depends upon the molecular consti- 
tution of a substance and not upon its chemical nature. The 
photographic differences which are developed by staining, may 
also be adduced astevidence for this belief, because the actinic 
powers of the various structures are quite changed by the 
process, so that they shown in different intensities in the photo- 
_ graph. It cannot be denied that in some instances at least, 
there is a change greater than that due to a simple impregna 
tion by the stain. Thus many dyes, notably the aniline series, 
tint a section different shades or even colors inits various parts, 
and the only reasonable explanation of this is that different 
chemical compounds are formed in these several parts or that 
molecular changes in different degrees are induced by the stain- 
ing medium. 
This subject will again be considered in a future article while 
treating of the anilines, the “ Inversion of Color,” ete. 
METHODS AND FORMULA. 
It will be found in almost all cases that the alcoholic dyes 
give the best results. They are quicker and more certain in 
their action, the liability of maceration and structural degen- 
eration is lessened or quite removed even though the immersion 
is unavoidably prolonged, and there is no danger of a precipi- 
tate in the alcohol hardened specimens such as usually occurs 
when aqueous solutions are employed. As it is now customary 
to stain in mass when possible, the alcoholic media are indis- 
pensable as their penetration and preservative power is so much 
better, the deeper portions of the object being acted upon in the 
same degree as the surface. 
The grade of alcohol used in making these solutions may be 
determined by the following principles: The stronger the alcohol 
the greater its preservative action and penetration, but the more 
*A,E. Jenkins, Methods of Work, No. 2. 
