BACTERIA OCCURRING IN SEWAGE 8i 



.ater, sulphides are formed, probably by the reduction of the 

 sulphates in the sea water, and the seaweed blackens from the 

 formation of ferrous sulphide, which disengages sulphuretted 

 hydrogen by the action of the fatty acids. Two species of 

 bacteria seem specially concerned. 



Letts and others consider the occurrence of this alga in 

 quantity to be associated with sewage pollution — first, on 

 account of its nitrogen being in excess of that recorded in any 

 other seaweed ; secondly, because by cultivation experiments 

 it was found that its power of absorbing nitrogen was remark- 

 ably high (thus in one experiment it absorbed in seventeen 

 hours the whole of the free ammonia from a polluted sea water 

 containing 0*05 part per 100,000 ; nitrates were also rapidly 

 absorbed, but not albuminoid matters ; the plant remained 

 healthy) ; thirdly, it grows most abundantly where sewage is 

 discharged. He remarks that, *' while thus acting as scavenger, 

 it may itself give rise to a very extensive nuisance," and that 

 in this case biological effluents rich in nitrates and non-putre- 

 factive also supply the nitrogen for the growth of the Ulva 

 I and other plants. 

 I Many varying odours occur on shores unpolluted by sewage, 

 and can arise from either animal or vegetable sources. In 

 some parts the deposits of birds evolve an ammoniacal and 

 mngent odour. The smell of seaweeds is frequently strong 

 jven in the fresh state, and when thrown up on shore and 

 lecaying they may be very offensive. Dead marine animals 

 re often drifted in quantities into hollows in the coast ; and 

 irge numbers of jelly-fish (medusae), shells, and crabs frequently 

 >utrefy on the sands, and sea-anemones on the rocks. Littoral 

 lowering plants when growing profusely, as in salt marshes, 

 sometimes produce very distinct odours, which may be strong 

 md unpleasant in addition to the smell from masses of decaying 

 vegetation. In this latter connection it must be remembered 

 that brine of the strength of sea water has no antiseptic power. 

 Besides the above occasional causes of local nuisance, there 

 Lre a number of vegetable and animal microscopic organisms 

 ^hich in their growth develop odours of various kinds, while 

 any of the species, when unusually abundant, can after their 

 death render wide areas of water offensively putrid. Apart 

 from these peculiarities, large groups of " macroscopic " life, 

 mcluding diatoms, the smaller algae, the infusoria, and minute 

 crustaceans, act harmlessly as scavengers in waters which may 



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