312 PRACTICAL COURSE IN BOTANY 
Among others, the germs of pneumonia, diphtheria, and 
tuberculosis are often found in the mouth, nose, and sputum 
of perfectly healthy persons. There are also a number 
of bacteria that are regular inhabitants of the mouth, some 
of which are the cause of decayed teeth and foul breath. 
One form of bacterium, concerned in the production of in- 
flammation and abscesses (Staphylococcus) is so constantly 
present on the human epidermis that one authority has 
declared it impossible to sterilize the skin so thoroughly 
as to free it entirely of this microbe. It is ordinarily not 
harmful unless it comes in contact with open wounds and 
abrasions. 
353. The economic importance of bacteria. —It is hard 
to say whether these organisms concern us most on account 
of the damages attributable to them on the one hand, or 
the benefits we owe them on the other. If they were all 
as harmful as the pathogenic kinds, life would hardly be 
possible on the globe, while without their presence life 
as we know it would have ceased to be possible long ago. 
They are nature’s great army of scavengers, the sole agents 
of decomposition, without which dead organic matter would 
be subject only to the slow changes by which the rocks 
and mineral matter of the earth’s crust are disintegrated, 
and the undecomposed bodies of the vast procession of 
plants and animals that have existed since life first began 
on our globe would long ago have cumbered its surface to such 
an extent as to render impossible the continued develop- 
ment of life such as we know. 
354. Sterilization is the process of ridding a substance 
of living microérganisms. To do this effectively, the pro- 
cess must be repeated several times at intervals, so as 
to give any spores that may have survived previous applica- 
tions time to pass into the vegetative state, when their 
power of resistance is diminished and they are more easily 
destroyed. The incubation period, as the time required 
for the germination of the spores is called, is different for 
