372 EXPERIMENTS WITH PLANTS 



tracted by eating raw oysters, which are frequently 

 grown in water contaminated by sewage. 



Fig. 211 shows the distribution of cholera cases in 

 the Hamburg epidemic of 1892. It will be noticed that, 

 in the words of Professor Koch, "cholera went right up 

 to the boundary of Altona and there stopped. In one 

 street, which for a long way forms the boundary, there 

 was cholera on the Hamburg side, whereas the Altona 

 side was free from it." This is equally true when the 

 boundary runs diagonally through a block. The ex- 

 planation lay in the fact that while both cities used the 

 same water supply (which received raw sewage from 

 several towns) , Altona filtered the water carefully, while 

 Hamburg did not. 



The filtration of water is a very simple process. It 

 is passed through several feet of sand (sometimes 

 mixed with charcoal) and a film soon forms on the sur- 

 face of the sand which catches the bacteria. Such a 

 film may be artificially produced, if necessary, by add- 

 ing chemicals (e. g. alum) to the water without injur- 

 ing it for drinking purposes. Every town or city water 

 supply should be filtered; the cost is slight and the 

 benefits very great. The small filters sold for house- 

 hold use are for the most part highly injurious and 

 serve only as breeding places for bacteria, owing to the 

 fact that they cannot be properly cleaned. 



Let us now examine some milk. Fill three vials 

 half full of nutrient gelatin, plug, sterilize, and set 



