724 A MANUAL OF BACTERIOLOGY 



water deeply for at least half an hour ; (3) chlorine gas or iodine 

 tablets, 1 in both cases the taste of the agent being destroyed j 

 by the addition of sodium sulphite ; (4) copper and copper 

 sulphate. Sufficient metal is dissolved from bright copper in 

 twenty-four hours to destroy 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 is one of the simplest and : 

 most efficient agents, and is now being largely employed. The 

 chlorine may be derived from cylinders of the compressed gas (the 

 simplest method), or from bleaching powder, or from sodium 

 hypochlorite obtained by the electrolysis of sea-water. Moor 

 and Hewlett 2 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 dis- 

 appears 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. 767). 



ICE AND ICE-CREAMS may be examined by methods similar to 

 those used for water, the material being first melted at a low 

 temperature. Some of the fluid should also be centrifuged and 

 the deposit examined microscopically for gross contamination. 



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

 sewage-polluted layings (see p. 423). The following method may 

 be employed for their examination (after Houston) : 



The outsides 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 con- 

 , tained 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 



1 Nesfield, Jaurn. Prev. Med., vol. xiii, 1905, p. 623. 



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



