MATERIALS. 315 



seen with a i /6 inch. Examine the bacteria and sketch. In this way 

 make a microscopic examination of all of the cultures isolated and puri- 

 fied, and compare with Figs. 7 and 9. If it is desired to preserve the speci- 

 men place a drop of Canada balsam on the bacteria after drying, and 

 then cover with a cover-glass. 



Motility. To determine the motility of bacteria transfer a small quan- 

 tity from an agar slant to a bouillon tube, and allow to grow for 24 hours. 

 Place a drop of the 24-hour-old bouillon culture on a slide and put upon it 

 a cover-glass. Examine this with a i /6 inch objective and with the dia- 

 phragm nearly closed. The best light for the purpose is artificial light (elec- 

 tric) placed near the microscope and reflected through by the plain surface 

 of the mirror. It will be very difficult at first to see the bacteria, but with 

 careful focusing they will appear as transparent dots or rods. Examine 

 carefully to determine whether they are stationary or motile, calling 

 only those motile that move back and forth across the stage and not those 

 that simply dance back and forth without locomotion (the Brownian 

 motion). It is sometimes desirable to keep the specimen under observa- 

 tion for some time in which case a hanging-drop method may be used. 

 A concave slide is to be used and a ring of vaseline painted around the 

 depression. The drop containing the living bacteria is placed in the 

 middle of a large cover-glass and inverted over the concavity of the slide. 

 By pressing it firmly into the vaseline ring it will be sealed so as to prevent 

 evaporation and may be kept under observation for hours. 



No. 9. Bacteria in the Mouth. With a clean knife scrape a little of 

 the material attached to the teeth and spread it in a very thin layer over a 

 slide. Dry, fix and stain, and with a microscope note the large numbers 

 of bacteria present. Sketch the varieties seen. 



No 10. Gram Stain. Prepare the following: 



Anilin Oil Gentian Violet. 



Saturated alcoholic solution of gentian violet, 6 c.c. 

 Absolute alcohol, 5 c.c. 



Anilin water,* 50 c.c. 



Grams lodin Solution. 



lodin, i gm. 



Potassium iodid, 2 gm. 



Distilled water, 300 c.c. 



Spread and fix on a slide, a little of one of the cultures of bacteria, 

 and stain for one and one-half minutes in the gentian violet solution. Pour 

 off stain, without washing, and place in the iodine solution for one and one- 

 half minutes. Apply 95 per cent, alcohol until the drippings do not stain 

 white filter-paper. This will take about three minutes and the specimen 

 will be largely decolorized. Wash in water and study with microscope to 



*Made by adding 2 to 3 c.c. anilin oil to 50 c.c. of water and shaking thoroughly, with subse- 

 quent filtering. 



