200 BACTERIOLOGICAL DIAGNOSIS. 
5. Wash thoroughly in tap water, continuing the 
washing until the sections have a decidedly blue tinge. 
The hematoxylin compounds are very much like litmus, 
being red in presence of acids and blue in presence of 
alkalies; the sections are to be coloured blue, and the 
necessary alkali is contained in the tap-water. It will 
hasten the process to rinse them in a very dilute solu- 
tion of ammonia. 
6. Stain in watery eosin for a minute or so. This is 
the counterstain. The hzematin will stain all nuclei blue, 
but will scarcely tinge anything else; the eosin is added 
to stain other structures a pale pink and thus make 
them more visible. It stains almost instantaneously. 
7. Wash off the eosin under the tap. 
The sections are now stained. But they are opaque 
and not in a suitable condition to be examined under 
the microscope, and are to be rendered transparent by 
being mounted in balsam. Now this cannot be done in 
the same way as was used in the mounting of films, for 
the drying would cause the sections to shrivel and 
obscure their structure. The water is to be removed, 
it is true, but by the use of absolute alcohol; at least 
two lots should be used, and the slide rocked. from time 
totime. Then the alcohol (which will not mix with 
balsam) is to be removed by the use of xylol, balsam 
added, and the section covered with a cover-glass. 
The remaining steps are therefore :— 
8. Absolute alcohol, two lots (to dehydrate). 
g. Xylol, two lots (to render the section permeable to 
balsam). 
io. Balsam and a cover-glass. 
The last three steps are practically the same as the 
first three, but in the reversed order, and similar pheno- 
mena are seen. The section is opaque whilst wetted 
