BACTERIA. 51 
—is found also to be made upof cells of protoplasm but without the 
coating of cellulose. So that a Bacterium is to be looked upon 
as a single isolated cell, and may therefore be considered to be 
an organism or living being reduced to its simplest expression. 
It may be asked whether Bacteria are animals or plants. 
Strictly speaking this is one of the cases in which “you pay your 
money and take your choice :” they agree in some respects with 
animals, in others with plants, so that no absolute definition of 
the Animal and Vegetable kingdoms can be framed which shall 
include Bacteria in the one and exclude them from the other. 
It is rather as a matter of convenience that, being allied in many 
ways to the moulds and fungi, they are usually looked upon as the 
lowest kind of plants. 
Then as to the mode of life of Bacteria. Hay-infusion con- 
sists of a solution of the soluble constituents of hay; some proteids 
or albuminous substances, sugar, various mineral salts, etc.; in 
animal infusions there is a larger proportion of proteids. These 
substances as it were soak into the bodies of the Bacteria and are 
there ,converted into fresh living protoplasm, which serves to 
make good the waste of substance occasioned by the active move- 
ments of the creature, for in Bacteria as in everything else, work 
is accompanied by waste, and for every movement a small 
portion of substance is used up—oxidized or burnt in fact 
—and has to be replaced if life is to go on. 
Thusa Bacterium may besaid to live in a nutritive atmosphere: 
feeding to it is no more exertion than breathing to us, and goes 
on as constantly. In fact one may say that a Bacterium eats, 
aeim<s, breathes and -excretes. at the same moment and .con- 
tinuously. 
Bacteria do not need free oxygen ; they live and thrive best 
at a temperature of about 85° F.; a temperature of 140° usually 
kills them ; freezing produces a sort of numbing effect, all vital 
actions being suspended, but with the raising of the temperature 
they regain their vitality unimpaired. They grow quite as well 
in darkness as in light, in fact strong sunlight kills them: this is 
the reason why sunlight is such a sweetener of rooms and ought 
to be letin as much as possible (especially in bedrooms) even at 
the risk of fading the carpet. 
So far I have said nothing about the multiplication of Hees 
which it is evident must go on very rapidly, since microscopical 
examination shows that the gradually increasing cloudiness of a 
putrifying fluid is due to increase in the number of Bacteria. 
Accurate observation has shown how this increase is effected— 
it is best observed in Bacillus. A Bacterium elongates to about 
double its original length, and then divides into two across the 
middle. The two Bacteria thus formed wriggle away from one 
another, and as they do so draw out into a fine thread a little 
bridge of protoplasm which still connects them, When this 
thread is double the length of a cilium it snaps in the middle and 
thus each of the Bacteria comes to be provided with the ordinary 
two cilia. 
