1887.] MICROSCOPICAL JOURNAL. 191 
ployed these casual observations methodically in bacteria investigation. He 
spread out upon a cover-glass in a very thin layer a drop of fluid containing 
bacteria, so that the individual elements were brought very nearly in the same 
plane. This thin layer was then fused by simply drying in the air. In order 
to eliminate the slight alteration produced by this it is necessary afterward to 
cause again a swelling up of thebacteria. If the layer which has been dried in 
the air remains too long in the water or glycerin used for this purpose it is en- 
tirely dissolved, instead of only partially swelling. ° 
If the cover-glass with the dried layer is laid in absolute alcohol, or a one- 
half per cent. solution of chromic acid, the layer is rendered insoluble in 
water and glycerin and no longer swells up. But if the layer which has been 
made insoluble is put into potassium acetate it swells up sufficiently without 
being entirely dissolved, and all forms seem to be in a natural condition. 
The solutions of the analin dye have the same action and cause the same 
swelling without removing the layer, and at the same time stain the bacteria. 
In the use of this method in the investigation of the blood, Ehrlich found 
that the rapid drying prevented coagulation of the cell-albumens and retained 
the natural staining capacity of the elements. Only the hemoglobin was 
extracted by aqueous and glycerin solutions of the dyes. But if the prepa- 
rations were kept for a few hours at a temperature of 115°-120° C., the ele- 
ments of the blood, without any important alteration and without the appear- 
ance of artificial products, retained their elective affinities for dyes. In 
following up these observations Koch (1881) discovered that in place of the 
fixation by alcohol the application of heat for only a few minutes answered 
‘the same purpose. 
A drop of the fluid containing bacteria, either undiluted or after the addi- 
tion of a drop of distilled water (according to the amount of its morphological 
elements), is spread out in a thin layer upon the cover-glass by means of the 
point of a scalpel or platinum wire, and the excess of fluid soaked up with 
filter-paper ; or a drop on one cover-glass, and a second is applied to this, 
which through its pressure spreads out the drop in an even layer. If, then, 
the two cover-glasses are drawn apart with pincettes we have two similar 
preparations. The cover-glasses, protected from dust, are allowed to remain 
until completely dry, or they can be dried in a dry oven somewhat more 
rapidly. The drying can also be hastened by holding the cover-glass with 
the prepared side upward high above the gas flame, and moving it to and fro 
to prevent the direct action of the flame. 
Upon the dried preparation a drop of the staining solution can then be placed 
to stain the elements, but only in case the fluid is free from albumen and the 
staining follows quickly, since by the prolonged action of the staining solu- 
tion the layer is completely loosened. If the dried layer consisted of an 
albuminous substance, such as blood, tissue fluids, or sputa, on the addition 
of the staining solution precipitation occurs. 
On this account it is especially necessary that the preparation, after drying 
in the air, should be more securely fixed by heating. For this purpose the 
cover-glass may be placed in a drying-box, or upon a copper plate. The 
copper plate is laid upon a tripod, and one end is heated bya gas flame, so 
that the different portions, at different portions, at different distances from the 
flame have varying degreesof temperature. A few minutes exposure to a tem- 
perature of 125° C., or ten or twenty minutes at 110° C., is sufficient to thor- 
oughly dry bacteria preparations. This may be done more conveniently, 
and in some cases also with certainty, according to Koch-LoefHlers, if the 
cover-glasses with the dried layer are drawn rather rapidly three times through 
a gas or spirit flame. 
The reason for heating in exactly this manner, according to Koch (1884), 
