EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 37 



twenty-four hours. The specimen is laid upon a block of wood 

 or, better, the compressed vegetable fibre called vulcanite, and 

 surroimded by thick collodion, and then placed in 70 per cent, 

 alcohol. The collodion makes a firm mass, surrounding and 

 permeating the tissue, and permits very thin sections to be cut. 

 The soluble cotton sold by dealers in photographers' suppHes 

 serves as well as the expensive preparation known as celloidin. 

 To make collodion, dissolve it in equal parts of alcohol and 

 ether. Soluble cotton is also called pyroxylin, and is a kind of 

 gun-cotton. 



Imbedding in Paraffin. — {a) Pieces of tissue 2 to 3 mm. 

 thick which have already been fixed in alcohol or formaldehyde 

 are to be placed ia absolute alcohol for twenty-four hours. 



(b) In pure xylol one to three hours. 



(c) In a saturated solution of paraffin in xylol one to three 

 hours. 



{d) In melted parafiin having a melting-point of 50° C, 

 which requires the use of a water-bath or oven, one to three 

 hours. The xylol must be entirely driven off, and the tissue 

 thoroughly infiltrated. 



(e) Change to fresh paraffin for one hour. 



(/) Finally, place the tissue in a small dish or paper box 

 and pour the melted paraffin about it. Harden as quickly as 

 possible with running water. It is important to fix the piece of 

 tissue in a suitable position, if the position is of importance, 

 before pouring in the melted paraffin. 



Sections of exquisite thhmess may now be cut. The knife 

 need not be wet. Paraffin imbedding is especially desirable 

 when serial sections are to be made. 



In order to mount the sections, proceed as foUows: 



(a) Place the sections on water in a porcelain capsule. 

 Warm slightly, when the sections wiU flatten nicely. Smear 

 the surface of a slide with a very thin layer of Mayer's glycerin- 

 albumen mixture. Dip the slide under the sections ; lift them; 



