HARDENING THE TISSUES. 183 



to the dyes than does that of cover-slips. In tissues, 

 too, there are other substances beside the bacteria which 

 become stained, and these, unless robbed in whole or in 

 part of their color, may so mask the stained bacteria as' 

 to render them difficult, if not impossible, of detection. 

 Tissues must, therefore, always be subjected to some 

 degree of decolorization, and this must be accomplished 

 without depriving the bacteria of their color. 



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 effiarts 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 

 lai'ger than one cubic centimetre — are to be placed, as 

 fresh as possible, in absolute alcohol. They should rest 

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

 taining the alcohol, in order that they may be elevated 

 and surrounded by the portion 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. A^ hen 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 firm 



1 The details of the method of decolorizatjou will be described iu the 

 gectiOD on the technique of staining, 



