STAINING METHODS. 27 
as above described, but the writer has for a number of years made 
his mounts of bacteria upon the glass slide, and believes that this 
method has some advantages for every-day work. The thin glass 
covers required when a preparation is to be examined with an im- 
mersion objective of high power, are easily broken and often dropped 
from the fingers or forceps. When the material to be examined is 
spread and dried directly upon the glass slide, the operation is at- 
tended with less difficulty and fewer accidents and the results are 
quite as good. In this case the slide is held in the fingers during the 
various steps in the operation of distributing, drying, and staining, 
while the thin glass cover must be held in delicate forceps. 
Contact Preparations.—When a dry and clean cover glass is 
brought in contact with a colony or surface culture we may often 
obtain a very pretty preparation, showing the bacteria in a single 
layer, and preserving the arrangement, as regards growth, which 
characterizes the species. Similar preparations may sometimes be 
obtained from the surface of liquid cultures, when the bacteria grow 
upon the surface as a thin film. The cover glass is to be gently 
brought into contact with this surface growth, which adheres to it 
and is dried and stained by the usual methods. 
Staining of the dried film is quickly effected by using an aqueous 
solution of one of the aniline colors above mentioned. For general 
use the writer prefers a solution of fuchsin, on account of the prompt- 
ness of its staining action, and because, in preparations for permanent 
preservation, it is not as likely to fade as methylene blue or gentian 
violet. It is also a better color than blue or violet in case a photo- 
micrograph is to be made from the preparation. 
It is best to keep on hand saturated alcoholic solutions of the 
staining agents named, and to make an aqueous solution whenever 
required by the addition of a few drops to a little water in a watch 
glass or test tube ; for the aqueous solutions do not keep well on ac- 
count of the precipitation of the dye as a fine powder, which ren- 
ders the solution opaque. The addition of ten per cent of alcohol 
to the aqueous solution will, however, prevent this precipitation ; 
but, as a rule, freshly prepared solutions are the best. These should 
be filtered before use. We may place a few drops of the filtered 
solution upon the dried film on the slide or cover glass, or the thin 
cover may be floated upon a little of the solution in a watch glass. 
In some cases it is best to use heat to expedite the staining, and this 
may be done by holding the slide or the watch glass over the flame 
of an alcohol lamp until steam commences to be given off. If the 
heating is carried too far the preparation is likely to be spoiled by 
the precipitation of the staining agent. As.a rule, heating will not 
be necessary, and when an aqueous solution of fuchsin (one part to 
