418 H. M. Vernon 



which are suspended in the water, and to a lesser extent deposited 

 on the sand of our sea shores. Owing to the immense quantities in 

 which they oecur, and the rapidity with which they can act, they 

 probably exert a very much greater purifying effect than the larger 

 algse, which can grow only in shallow waters in the neighbourhood 

 of land. 



These experimental results also have a hearing on the purification 

 of fresh waters. Probably in their case the action of bacteria is in 

 comparison not so great as for sea-water. Thus it is only seldom 

 that fresh water occurs in lakes at such depths as to be in absolute 

 darkness, and water which has filtered through moderate depths of 

 soil seems to be thereby rendered almost if not quite bacteria free. 

 On the other band, alg?e grow abundantly in almost all rivers, but 

 })robably eveu in them the minute alg?e and Diatomacese are the 

 more powerful ageucy. Thus in all but the foulest streams there is 

 a layer of vegetable growth deposited on the stones and grains of 

 sand in the bed of the river, which is continually exerting its purifying 

 action. The rise and fall of the level of the water in rivers causes 

 a continuous circulation of water backwards and forwards through 

 this vegetable layer on the river bed, whereby the purifying action 

 is greatly increased. 



This purifying action of alga?, diatoms and bacteria is becoming 

 more and more recognised every day. Thus Pettenkoper ^ considers 

 that the self-purification of river-water depends more on vegetable 

 life than on any other agency. Low 2 also thinks that algte play a 

 considerable part, and he mentions several instances in which fresh 

 water algse have been proved to thrive and increase in weight in 

 water containing putrefactive and other organic bodies. To me it 

 seems that vegetable life is practically the only agency in the 

 purification of water. True, the solid matter suspended in the water 

 is removed by mechanical deposition and filtration, but practically 

 nothing but vegetable life can get rid of the dissolved impurities. 

 Aeration and mechanical disturbance of the water can do little or 

 nothing directly. Sunlight indeed kills some of the bacteria, but 

 these would die of themselves without any such agency, once the 

 water became purified through the vegetable life it contained. 



As regards the methods employed in this research, it may be 



1 Arch. Hyg. 12. Bd. pag. 269. 

 ■^ Arch. Hyg. 12. Bd. pag. 261. 



