SEWAGE OUTFALLS AND DISCHARGE 



323 



cumstances no material risk in consuming the watercress supplied 

 from the majority " of these beds. Objection is taken on 

 topographical, chemical, and bacteriological grounds to certain 

 beds where there was actual gross pollution, " and effort should 

 be made to prevent the consumption of watercress from these 

 beds in their present condition." 



The annual consumption of watercress in London is about 

 1,500 tons, and the 120 beds examined, with areas from less 

 than ^ to 40 acres, comprise all those within 50 miles known to 

 supply the London market. In most cases the waters were 

 examined by the chemist, who gives his own classification 

 according to the albuminoid ammonia as follows : — 



Thirteen of the waters were submitted to the bacteriologist, 

 and those containing coli in i cc. or less are condemned. It 

 will be seen that our Guildford chlorine-treated effluents would 

 have easily passed this test (pp. 189, 190). The two worst 

 samples were effluents from sewage farms, and contained 

 10,000 coli-like microbes per cc, and enteritidis spores in 'i cc 



Dr. Houston concludes that " no ordinary amount of wash- 

 ing could be relied on to rid cress grown in polluted waters of 

 all undesirable microbes." It was of great moment to ascertain 

 whether pathogenic organisms could exist and multiply in the 

 interior of plants, and this was partially studied by analogy 

 with B. coli. The results on the whole were against the 

 hypothesis, as Sir Shirley Murphy notices in his summing up, 

 although Dr. Houston lends some probability to the idea that 

 the leaf and stalk of cress " may foster undesirable organisms." 

 Even if it were true that pathogenic infection of the interior of 

 living plants were occasionally possible, I believe that in the 

 acid washings I have described at p. 193, the acid, being more 

 diffusible than the bacteria, would reach them in sufficient 

 quantity to destroy them, and then be removed in the subsequent 

 washing w^ith water. A point in favour of the success of cleansing 

 is that the B. typhosus is actively motile, and therefore would 



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