650 BACTERIA IN WATER. 
angular surface and leaving a margin of about one centimetre around 
the edge of the plate. The Koch’s dish in which the gelatin plate is 
placed for the development of colonies should be carefully sterilized 
by heat or by washing it out with a sublimate solution. A circular 
piece of filtering paper, saturated with sublimate solution or distilled 
water, is placed at the bottom of the lower dish to keep the air in a 
moist condition and prevent drying of the gelatin. Usually two or 
three plates made at the same time are placed one above the other on 
glass supports made for this purpose. If many liquefying organisms 
are present it will be necessary to count the colonies before these run 
together—usually on the second day ; but in the absence of liquefy- 
ing colonies it is best to wait until the third, or even the fifth day, as 
the number of visible colonies and the ease of counting them will be 
greater than at an earlier date. The development of afew scattered 
liquefying colonies which threaten to spoil the plate may be arrested 
by taking up the liquefied gelatin from each with a bit of filtering 
paper, and then, by means of a camel’s-hair brush, applying a solu- 
tion of potassium permanganate to the margin of the colony. The 
growth of colonies of mould fungi, which have developed from spores 
from the atmosphere falling upon the plate while it is exposed, can 
be checked by the application of collodion containing bichloride of 
mercury. 
Counting of the colonies is a simple matter when they are few 
in number; when they are numerous it is customary to place the 
plate over a dark background, and to place above it a glass plate 
divided into square centimetres by lines ruled with a diamond. By 
means of a lens of low power the colonies in a certain number of 
squares are counted and the average taken. This multiplied by the 
number of square centimetres in the gelatin-covered surface gives 
approximately the entire number of colonies which have developed 
from the amount of water used in the experiment. 
Instead of using Koch’s original plate method, as above described, 
the shallow, covered glass dishes recommended by Petri may be 
employed. These are from one to one and one-half centimetres high 
and from ten to fifteen centimetres in diameter. The liquefied gel- 
atin is poured into the lower dish and the cover at once placed over 
it. The gelatin does not dry out very soon, but, if necessary, several 
.of these Petri’s dishes may be placed in a larger jar, which serves as 
a moist chamber. 
The roll tubes of Esmarch may also be used, and have the ad- 
vantage that accidental colonies from air-borne germs are excluded. 
The counting of colonies is not quite as easy, but by the use of a 
mounted lens especially designed for the purpose it is attended with 
no great difficulty. The surface of the tube is divided into squares 
