196 
NOTES AND MEMOEANDA. 
tubes introduced by Saloinonscn, wliicli rendered tlieir tliorougli isola- 
tion in putrifying blood possible ; also the experiments of Frisch on 
the spread of putrifying organisms in tissues and the inflammatory 
appearances caused by inoculation of the cornea. 
The principal difficulties which arisC' in investigating Bacteria 
are, as Dr. Koch considers, connected with their small size, their 
movement, the simjdicity of their form, and their want of colour or 
power of strongly refracting light. But an equally great hindrance 
has been the want hitherto of a process for preserving the Bacteria in 
their natural shape and position, and producing undistorted repre- 
sentations of them. To obviate these difficulties the author adopted a 
process which consists in drying on the covering glass a very thin 
film of the fluid containing the Bacteria, in order to fix the latter in a 
jdane. This film is afterwards treated with staining fluids and 
again moistened, so as to bring back the Bacteria to their natural 
forms and make them more plainly visible. The preparation is 
then enclosed in a preserving fluid, and finally photographed, to pro- 
duce representations true to nature. 
The separate parts of this process are conducted as follows : — 
First, the drying. A drop of the fluid containing the Bacteria is 
spread out in as thin a film as possible on a covering glass, so that 
Bacteria, blood-corpuscles, &c., do not overlap, but are separated by a 
space more or less great. Generally the preparation is ready after a 
minute or two for further manipulation. Albuminous fluids, and 
especially blood, are left somewhat longer to dry — if possible, a few 
hours. Covering glasses thus prepared may lie for months, and the 
dried Bacteria will be unchanged ; they must, of course, be carefully 
protected from dust. The objection which might be raised against 
drying them thus, viz. that their form must thereby be considerably 
altered is, as experience has taught the author, unfounded ; for he ob- 
served, to his astonishment, that the Bacteria did not shrink together 
into shapeless masses ; but, like rigid bodies surrounded by a slimy 
sheath, adhered to the glass by this sheath, and dried without visibly 
altering their shape, particularly as to length and breadth. 
The second part of the process consists in moistening and staining 
the dried film. For the moistening a solution of acetate of potash 
(one part in two parts of distilled water) may be used with good 
results ; a swelling without separation from the glass being brought 
about, and at the same time the Bacteria resume perfectly their 
original form, only appearing rather paler and more transparent than 
before. Since the Bacteria which have swelled up again in this fluid 
do not change any further, it is specially adapted to preserve the 
preparation, which may be forthwith cemented. 
The Bacteria are often too pale for examining and photographing, 
and must be made more distinct by being stained. For this purpose 
the aniline colouring matters appear to the author to be the most 
suitable. The Bacteria take, in fact, the aniline stain so quickly and 
completely that these colours may be used as reagents, to distinguish 
Bacteria from crystalline and amorphous precipitates, as also from 
the finest fat-globules and other minute bodies. Moreover, the 
