158 Methods of Observing Bacteria 



of bichlorid of mercury* may also be used and are par- 

 ticularly useful when the bacteria are to be studied in 

 relation to the cells of the tissues. 



Tissues preserved in 95 per cent, alcohol, Miiller's fluid, 

 4 per cent, formaldehyd, and other ordinary solutions 

 rarely show the bacteria well. 



Embedding. The ordinary methods of embedding suffice. 

 The simpler of these are as follows : 



/. Celloidin (Schering). The solutions of celloidin are 

 made in equal parts of absolute alcohol and ether and 

 should have the thickness of oil or molasses. From the 

 hardening reagent (if other than absolute alcohol) pass the 

 blocks of tissue through: 



Ninety-five per cent, alcohol, twelve to twenty- four 

 hours ; 



Absolute alcohol, six to twelve hours; 



Thin celloidin (consistence of oil), twelve to twenty- 

 four hours; 



Thick celloidin (consistence of molasses), six to twelve 

 hours. 



Place upon a block of vulcanite or hard paraffin, allow the 

 ether to evaporate until the block can be overturned without 

 dislodging the specimen; then place in 80 per cent, alcohol 

 until ready to cut. The knife must be kept flooded with 

 alcohol while cutting. 



Celloidin is soluble in absolute alcohol, ether, and oil of 

 cloves, so that the staining of the sections must be accom- 

 plished without the use of these reagents if possible. 



Celloidin sections can be fastened to the slide, if desired, 

 by firmly pressing filter paper upon them and rubbing hard, 

 then allowing a little vapor of ether to run upon them. 



//. Paraffin. Pure paraffin having a melting-point of 

 about 55 C. is used. The hardened blocks of tissue are 

 passed through: 



*Zenker's fluid: 



Bichromate of potassium 2.5 grams 



Sulphate of sodium 1.0 gram 



Bichlorid of mercury 5.0 grams 



Water 100.0 " 



At the time of using add 5 grams of glacial acetic acid. Permit 

 the specimens to remain in the solution for a few hours only, then wash 

 for twenty-four hours in running water and transfer to 80 per cent, 

 alcohol. 



