314 LABORATORY WORK. 
always the case and therefore the culture must be purified. With the tip 
of a platinum needle remove a minute quantity of the growth on the agar 
and place it in a 9 c.c. water blank. Thoroughly shake and transfer two 
platinum loopfuls of the water to a melted agar tube. Shake gently so 
as to mix, but not to produce bubbles, and then transfer two loopfuls of 
this agar to a second agar tube, mixing as before. Pour the contents of 
each agar tube into a Petri dish, harden and incubate as usual. If the cul- 
ture was pure the colonies should be all alike. Pick out one of them with 
the platinum needle, inoculate it upon another agar slant and label it a 
pure culture from milk or whatever may have been its source. In this 
condition it may be set aside and preserved for a long time. As long as the 
agar remains moist the bacteria will usually be alive. 
In the above manner isolate and purify a considerable number of cul- 
tures of bacteria from the plates made in Nos. 4-6, and keep these in a cool, 
aark place for use in various experiments given below. 
No. 8. Microscopic Study of Bacteria. Prepare one or both of the 
following staining solutions: 
Methylene Blue. 
Saturated alcoholic solution of methylene blue, 1 5 cm. 
Potassium hydrate (i: 10,000),* 50 c.c. 
Fuchsin Solution. 
Saturated alcoholic solution of fuchsin, 5 c.c. 
Five per cent, solution of carbolic acid, 45 c.c. 
In the middle of a clean microscopic slide place a drop of water (steril- 
ized). With a platinum needle remove a very small quantity of the bac- 
teria growth from the surface of one of the slant cultures prepared in No. 7 
and place in the drop of water. Stir this drop with the needle, to distribute 
the bacteria and then (a) spread it over the slide. Allow it to dry in the air, 
and then pass the slide three times slowly through a gas flame. The pur- 
pose of this is to (b) fix the bacteria firmly to the slide so that they will 
not be washed away. It is necessary not to use too much heat. This may 
also be done by leaving the slide for a few minutes on a water-bath. After 
fixing, cover the bacteria completely with several drops of one of the stain- 
ing solutions and allow to (c) stain for several minutes. The length of time 
necessary for this varies with conditions, one to five minutes being usually 
sufficient. Wash off the stain in running water and then dry the slide by 
gentle heat. Place a drop of immersion oil upon the stained bacteria and 
place the slide under the microscope. Use a i /is inch immersion, lowering 
the objective into the immersion oil and focusing very carefully. If the 
microscope has an Abbe condenser or a diaphragm it is best to have this 
widely open. The bacteria are so minute that it is hardly possible to study 
them with a lower magnifying power than a 1/12, although they can be 
* To make this solution add i c.c. of a 10 per cent. KOH solution to 99 c.c. of water, and then 
add 5 c.c. of this to 45 c.c. of water. 
