228 MICRO-ORGANISMS IN WATER 



water exhibited at the end of a much shorter time a. 

 marked decline in their numbers, but after some months 

 the decrease becomes more and more slow, until at the 

 end of from ten to twelve years the number of bacteria 

 is equal to, or is only a half or a third of the num- 

 ber present at the time when the sample was taken. 

 Miquel summarises his results by observing that a rapid 

 but transitory power of multiplication characterises the 

 bacteria in pure spring- waters, whilst in impure waters, 

 or in waters rich in microbes, the multiplication is sloic 

 and persistent. 



Miquel has pursued this subject of bacterial multi- 

 plication further, and has found that after a water has 

 supported the multiplication of a particular species of 

 micro-organism, the latter, on being reintroduced into 

 the same water, will not only not again multiply, but 

 in many cases will actually suffer rapid destruction. 

 He compares the phenomenon to that of a zymotic 

 disease, and the immunity which generally follows as a 

 consequence. Thus the water which has been " afflicted ' 

 with a ' plague ' of a particular microbe acquires im- 

 munity towards further attacks of the same organism. 

 This immunity he ascribes to the generation by the 

 bacteria of soluble and toxic products which inhibit 

 their further growth and multiplication. It is the ab- 

 sence of such toxic products in pure spring-waters that 

 permits of the astonishingly rapid and extensive mul- 

 tiplication of the few bacteria which they initially ex- 

 hibit, and causes these waters to present such a marked 

 contrast in this respect to more contaminated surface 

 I waters. These toxic products are destroyed on boiling, 

 for a water which will not support any further bacterial 

 multiplication acquires this property after boiling (for 

 experimental confirmation of this statement, see p. 229). 

 According to Miquel, however, these products, at any 



