The Relations between Marine Auimal and Vegetable Life. 355 



On the left side of this table are given the results obtained 

 when the water was allowed to flow through at its maximum rate, 

 whilst in those given on the right side, the oiitflow tube was clamped 

 in varying degrees. Froni this table it will be seen that during the 

 first week of the experinient only about half of the free ammouia 

 was removed from the water. At the end of the second week, 

 however, as much as 96^ was removed. The values for the organic 

 ammonia removed show a most striking inerease in amount from 

 day to day, only \1% being removed on the first day of the experiment, 

 and as much as ^\% on the fifteenth. On the 22"*^ day of the ex- 

 periment the eurrent of water was by inadvertence stopped from 

 running through the sand for twenty-four hours. This had a very 

 harmful effect on the purifying capacity of the sand as far as the 

 organic ammonia was concerned, but this capacity began to return 

 somewhat after a few days. Also the maximum rate at which the 

 water would flow through the sand was diminished some threefold. 

 This was due to the thin layer of diatoms and alga.', which was 

 gradually being formed on the top and in the interstices of the sand 

 by deposition from the eurrent of water, being partially killed and 

 transformed into a more cobesive and homogeneous layer. Thus on 

 the 26*^ day of the experiment, a number of small holes were made 

 through this vegetable crust on the top of the sand by means of a 

 needle, whereby the rate of flow of the water was increased three- 

 fold for the next few days. As these small holes became filled up 

 however, the rate of flow gradually became almost as slow as before. 

 Accordingly on the 36''' day of the experiment, after collecting and 

 analysing some of the outflowing water, the crust on the top of the 

 sand, consisting of a layer about 2 mm. thick, was scraped away, 

 and the water now ran through at the rate of 1 litre in 2 min., instead 

 of 1 litre in 972iniu-; moreover, as may be seen from the figures in 

 the right half of the table, the water when running through at this 

 increased rate, and without the influence of the vegetable crust on 

 the top of the sand, was if anything more purified than before. Thus 

 though 8^ less of the free ammonia disappeared, yet \1% more of 

 the organic ammonia did so. It seems somewhat extraordinary that 

 the removal of the vegetable crust on the top of the sand, which 

 was imagined to be the chief cause of the purification, should bave 

 produced a favourable rather than an unfavourable efi'ect. The 

 continued purifying efficiency of the sand must bave been partly due 

 to the layer of diatoms and algse which could be seen collecting 



