21 



Another characteristic is their rapidity of growth, 

 which is most extraordinary. Cohn, the German naturahst, 

 found that a certain bacterium could double itself in two 

 hours, and calculated that a single specimen, if un- 

 hampered, would become 4,772 millions in 3 days, and 

 that under a week its progen}' might fill the ocean. Fortu- 

 nately this somewhat allegorical idea can scarcely be 

 realized, for the " rolling sea," from the antiseptic pro- 

 perties of the Iodine of its waters, and of the ozone of its 

 surface, even from its very restlessness, is by no means 

 a good " culture medium." Many other things moreover 

 hinder their indefinite reproduction. In water, monads and 

 other small creatures feed upon them ; in confined places 

 they overcrowd and destroy one another ; in the air they 

 become dry and shrivelled up if long exposed. 



A few words about their uses and functions in the 

 economy of nature. They appear to exist to split up com- 

 plex dead organic compounds, which would otherwise be 

 useless, into more simple substances, such as ammonia, 

 nitrates, and various gases, so as to fit them to be the 

 food of plants. 



They thus act as the principal scavengers of the earth's 

 surface, as well as chief manure producers, and the world 

 could not very well get on without them ; yet, whilst 

 playing this beneficent role, they are unfortunately gifted 

 with the power of doing much evil, and are the causes of 

 most noxious smells and of dire diseases. 



The Blastymocetes, or Yeasts, which have been most 

 carefully investigated by Pasteur, preside over the fermenta- 

 tions. One produces that of sugar into alcohol, the most 

 abused and lauded of liquids ; so that without them we 

 should have neither beer, nor wine, nor grog, — nor drunken- 

 ness. Another causes alcohol to further ferment into 

 acetic acid or vinegar, so that to them we owe our 

 pickles, and the flavour of our salads ; whilst others pro- 

 duce Lactic and Butyric fermentations and the like, which 

 cause abominable compounds to appear in our milk, beers, 



