600 A MANUAL OF BACTERIOLOGY 



typhoid and cholera. Copper sulphate 1 in 100,000 or less is 

 similarly germicidal, and in still smaller quantities (1 in 1,000,000) 

 destroys algae, and has been used for the purification of reservoirs 

 overgrown with algae. On the large (also small) scale, chlorine 

 derived from hypochlorites is one of the simplest and most efficient 

 agents. Moor and Hewlett * showed that 0-25 part of chlorine 

 (equivalent to about 0-75 part of good chloride of lime) per million 

 parts of chalk water is sufficient to kill B. coli in half an hour. 

 The taste disappears quickly in bright sunlight and on standing, 

 or may be removed by an addition of sodium sulphite. If the 

 water is organically polluted, more chlorine must be used. 



Ozone produced by high-tension electric discharge is also employed 

 on the large scale for the sterilisation of water-supplies, e.g. at 

 Chartres (see also p. 637). 



EXAMINATION OF SHELL-FISH. Shell-fish may come from sewage- 

 polluted layings (see p. 362). The following method may be 

 employed for their examination (after Houston) : 



The outside of the shells are cleansed by thorough scrubbing and 

 rinsing in tap-water, and a final rinse in sterile water. The fish 

 after cleansing are laid on a sterile towel. The operator then 

 cleanses his hands and opens the shells aseptically with a sterile 

 oyster-knife, care being taken to avoid loss of their contained liquor. 

 The liquor as each fish is opened is poured into a sterile litre cylinder, 

 and the fish is cut up with sterile scissors and added to the liquor 

 in the cylinder. Ten fish should be treated, the volume of fish -f- 

 liquor noted, and sterile water is then added to make up to 1 litre ; 

 100 c.c. liquid therefore corresponds to one fish. In addition, 

 four dilutions of the liquid are prepared 1 in 10, 1 in 100, 1 in 

 1000, and 1 in 10,000. With the liquid and dilutions gelatin and 

 agar plate cultivations are prepared for the enumerations of the 

 organisms present. Cultures are also made in litmus lactose bile- 

 salt peptone water and in milk for the enumeration and isolation 

 of B. coli and B. Welchii respectively, taking 100 c.c., 10 c.c., and 

 1 c.c. of the liquid, and 1 c.c. of each of the four dilutions ; in this 

 way the contents of the fish, ranging from one fish to TTnr l___ o f 

 a fish, are examined. The process and principles involved corre- 

 spond to those described for water. Houston has suggested for 

 oysters as a lenient standard less than 1000, and as a stringent 

 standard less than 100, B. coli per oyster. Even ten B. coli per 

 fish should be viewed with suspicion, for Hewlett and others have 

 shown that oysters from pure layings contain no B. coli. 



Watercress, etc., may be examined in a similar manner, 100 grm. 

 1 Rep. Med. Off. Loc. Gov. Board for 1909-10, p. 559. 



