STAINING OF BACTEKIA IN TISSUES. 141 



in the same way that one heats cover-slips. The best 

 results are not obtained in efforts to hasten the staining 

 by subjection to high temperatures, but rather by longer 

 exposures at lower temperatures. 



HARDENING THE TISSUES. The bits of tissue not 

 greater than 1 cm. cube are to be placed, as fresh as 

 possible, in absolute alcohol. The bit of tissue should 

 rest upon a pad of cotton or filter-paper in the bottle con- 

 taining the alcohol, in order that it may be elevated and 

 surrounded by the part of the alcohol which is specifically 

 the lightest, and consequently contains least water. The 

 alcohol abstracts water from the tissue, and, as the 

 dehydration proceeds, the tissue becomes accordingly 

 more and more dense. When of about the consistency 

 of fresh solid rubber, or preferably not quite so dense, it 

 is ready to cut. A small portion of about 0.5 cm. cube 

 should be cemented to a bit of cork with ordinary 

 mucilage, and allowed to remain in the open air for a 

 minute or two for the mucilage to harden. Alcohol should 

 be dropped upon it occasionally, to prevent drying of 

 the tissue. When the mucilage is hard, the cork with 

 the piece of tissue upon it may be left in alcohol over 

 night, and on the following day it may be cut. 



SECTION-CUTTING. This is accomplished by the use 

 of an instrument known as a microtome (Fig. 22). It is 

 an apparatus provided with a clamp for holding the cork 

 upon which the tissue is cemented and also a sliding 

 clamp which carries a knife. The tissue is clamped 

 horizontally, and the knife is caused to slide across its 

 upper surface, also in the horizontal direction. Beneath 

 the clamp for holding the tissue is a milled disc, by 

 means of which a screw is caused to revolve, and in 

 revolving raises or lowers the clamp holding the tissue, 



