14 TEXT-BOOK OF BACTERIOLOGY. 



The extraordinary capacity of resistance to external influences 

 must be regarded as the most remarkable property of the spore. 

 This is no doubt chiefly owing- to the stout, firm membrane which 

 surrounds it, and which possesses an almost unlimited power of re- 

 sistance. The continued or temporary influence of dryness and 

 moisture, heat and cold, is well borne by the spore, as are also many 

 chemical agencies which it withstands without damage and with- 

 out injury, though they destroy all other life. One must, in fact, 

 regard them as the most enduring organic formations of our world. 



The impunity with which spores can bear high temperature is 

 practically very important. While we can easily succeed in de- 

 stroying sporeless bacteria, we find great difficulty in destroying 

 those capable of sporulation. A dry heat of not less than 140 C. 

 destroys all life in the spores with certainty after an exposure of 

 several hours, and a boiling heat also requires several minutes to 

 produce the same result. 



It is true this does not apply equally to all spores. The resist- 

 ance of the permanent forms is very different in the different species, 

 and is subject to considerable variations even in the same species. 

 We are acquainted with bacteria which yield to slight injurious in- 

 fluences, and others which are almost miraculously unimpression- 

 able. 



Globig has observed a particular sort of potato bacillus the 

 spores of which he had to expose for more than four hours to the 

 action of a jet of steam of 100 C. before he could finally destroy 

 them. 



Fortunately such cases are great exceptions, and in general the 

 figures given will be found correct. It was a long time before we 

 arrived at these facts by experiment and utilized them, thus avoid- 

 ing the mistakes which formerly made our knowledge of the bac- 

 teria unsafe in a high degree. 



IV. CONDITIONS NECESSARY FOR THE GROWTH OF BACTERIA. 



The bacteria originate only in germs of their own species. Easily 

 comprehensible and natural as this assertion may seem to us at the 

 present day, it has taken an immense amount of time and trouble 

 to establish it as a universally-acknowledged fact. 



It is not very long ago since the spontaneous generation of the 

 bacteria was seriously defended, and this view was not fairly aban- 

 doned till Pasteur made his victorious campaign against generatio 

 aequivoca. 



It is true he found the ground well prepared. Already, in the 



