338 LABORATORY WORK 
In a note-book make a brief record of the kind of colony from which it was obtained. 
Allow the inoculated tube to grow, either in the incubating oven or in the room, until 
a growth appears on the surface of the agar. This is an agar slant. 
Purification —If the colony thus isolated has grown from a single bacterium this 
growth on the agar will be a pure culture. But this is not always the case and there- 
fore 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 9g 
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 con- 
tents of each agar tube into a Petri dish, harden and incubate 
as usual. If the culture 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 what- 
ever may have been its source. In this condition it may be set 
aside and preserved for along time. As long as the agar remains 
moist the bacteria will usually be alive. 
In the above manner isolate and purify a considerable num- 
ber of cultures of bacteria from the plates made in Nos. 4-6, and 
keep these in a cool dark 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, 15 cm. 
Potassium hydrate (1: 10,000),* 50 C.C. 
Fuchsin Solution 
Saturated alcoholic solution of fuchsin, 5 cc. 
Five per cent. solution of carbolic acid, 45 C.C. 
Fic. 61.— 
Platinum In the middle of a clean microscopic ‘slide place a drop of 
meedle and water (sterilized). With a platinum needle remove a very small 
Platinum loop. quantity of the bacterial 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 purpose 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 or a steam radiator. After fixing, cover the bacteria completely with 
*To make this solution add 1c.c of a ro per cent. KOH solution to 99 c.c. of water, and 
then add 5 c.c. of this to 45 cc. of water. 
