156 BACTERIOLOGY. 



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 

 containing 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 accord- 

 ingly more and more dense. When of about the con- 

 sistency 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 pre- 

 vent 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 the sections 

 may be cut. 



SECTION-CUTTING. This is accomplished by the use 

 of an instrument known as a microtome. In Fig. 32 

 is seen the form now commonly employed. It is 

 known by the name of the maker, as Schanze's micro- 

 tome. 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 plane. 

 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, so that the tissue may be brought closer to or 



