MICRO-ORGANISMS IN RELATION TO MAN. 99 
formed as to the number of species in the original liquid. 
There are objections, however, to this method, and it is now 
almost entirely superseded by that known as gelatine-plate 
culture. This consists in mixing the diluted liquid with nutri- 
tive material which has been rendered solid by the addition of 
five or six per cent. of gelatine. When being used, slight 
heat is applied to render the mixture fluid, and it is then 
spread out on glass plates, or round the walls of some con- 
venient vessel, and allowed to set. In this way, the cells it 
is desired to cultivate are distributed throughout the gelatine, 
and when this sets, each cell is fixed at the spot where it 
happens to be, and develops there. By means of suitable 
apparatus, the cells in the solid gelatine can be observed 
from the commencement, and their growth followed through- 
out—in this way absolute certainty can be obtained that each 
growth proceeds from one cell, and that it is, therefore, a 
pure culture. 
In addition to this use of “ plate-cultures,” there is another 
which I will mention here, as I shall have occasion to refer 
to it again later. You will readily understand from what 
has been said, that different species of bacteria show great 
differences with regard to the food and the medium which 
they prefer. ‘Ihus, some species flourish best in an acid, 
others in an alkaline medium. Then again different species 
show variations in the shape or colour of the colonies or 
growths they produce in the same medium; thus, some 
growths are white or yellow, whilst others, like the so-called 
bleeding bread, concerning which there was so much super- 
stition in the past, are blood-red. Another distinctive feature 
is the power, or the reverse, to liquefy the gelatine in which 
they grow. Again, some bacteria require the presence of air 
before they can grow, others will not grow in the presence of 
air, All these, and many other characteristics, are carefully 
studied and noted with regard to various known species of 
bacteria ; and then by cultivation under definite conditions, 
