162 BACTERIOLOGY. 



Tissues must, therefore, always be subjected to some 

 degree of decolorization, and this must be accomplished 

 without depriving the bacteria of their color. 



The details of the method of decolorization will be 

 described in the section on the technique of staining. 



Another point to be remembered in staining tissues 

 is that they cannot be heated and retain their structure 

 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 to lower temperatures. 



Hardening the Tissues. — The bits of tissue — not 

 greater than one cubic centimetre — 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 containing the alcohol, in order that it may be ele- 

 vated 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, about half 

 a cubic centimetre, 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 pre- 

 vent drying of the tissue. When the mucilage is haixl 

 the cork with the piece of tissue upon it may be left in 

 alcohol over night, and on the following day the sec- 

 tions may be cut. 



Section-cutting. — This is accomplished by the use 

 of an instrument known as a microtome. In Fig. 36 



