62 
K. KOCH. 
material must be removed again. This can be done in various ways. In the 
method originally used by me, of coloring with alkaline methyline-blue solution, 
I had found that the blue coloring of the constituent parts of the tissue could be 
driven out by treating with a solution of vesuvian. The same can be carried out 
in the preparations which are colored according to Ehrlich’s method. When 
these preparations are rinsed otf in water and then put into a concentrated watery 
solution of vesuvian, moved back and forth in the same, and finally put into 
alcohol, one succeeds in almost completely drawing out the dark blue coloiing. 
The preparations, nevertheless, lose their color more quickly and completely by 
Ehrlich’s method of treating them with nitric acid. That this can be done by 
other aniline coloring materials, as for example the above mentioned vesuvian, 
I have mentioned only for the reason that by many the effect of nitric acid has 
been erroneously held for something specific, but this is not the case, since other 
acids work similarly. 
For taking the color out of the preparations, nitric acid, which has been diluted 
with two parts of water, is commonly used. So strong a concentration of the 
acid is nevertheless not absolutely necessary, and of late I use acid diluted with 
from three to four parts of water. Perhaps one can go even farther in the dilu¬ 
tion. One should take care, however, that the nitric acid is free from nitrous 
acid. 
When I spoke of the uncoloring of preparations by means of nitric acid, I fol¬ 
lowed the description which Ehrlich gave of his process. By the treatment of 
the covering-glass preparations with nitric acid, this term is exact, when the 
preparations are not intensely colored ; after a stronger coloring, which gives decid¬ 
edly better and more reliable results, the nitric acid after a few minutes fails to 
take all color from the dyed section, and section preparations, which, as it has 
already been carefully shown, must be colored a longer time and very intensely, 
always keep, after the nitric acid treatment, quite a dark coloring. The ex¬ 
pression “uncolor” is not to be understood literally. The failure of bacilli-color- 
ing appears in most cases to have had its foundation in this very thing. The 
experimenters thought that the preparations after treatment with nitric acid must 
be wholly colorless, and in order to reach this, partly colored too little the prepa¬ 
rations, and partly left them too long in the acid. 
When section preparations have lain in the solution twelve hours, and are then 
put into nitric acid, they lose their black-blue color in a few seconds and take 
a greenish-blue appearance. If they are then put into distilled water, the tone 
of the color changes directly. It becomes again noticeably darker and changes 
into blue, with a bit of violet. The nitric acid, therefore, has left a coloring 
matter in the preparation, which is insoluble in water, and in connection with 
water takes a darker tone. That this remainder of coloring matter is not easily 
soluble, even in nitric acid, can be easily shown. If the section be again dipped 
into the acid, their color will again become greenish-blue, but not paler than in 
the first treatment with the acid, and if washed again with water, they will again 
take the former dark coloring. I conclude from this that a longer remaining of 
the preparations in the acid is of no value for their further uncoloring, and leave 
them therefore, only a few seconds, at the highest half a minute in the same. On 
the contrary, I have found that the coloring matter in the preparations remain- 
