Marine Animal and Vegetable Life. 157 



On filtration of water through sand which had been taken from 

 erne of the tanks in the Aquarium, it was found that 94 per cent, of the 

 free, and 18 per cent, of the albuminoid, ammonia had been removed. 

 This great purification was due to the thin layer of algse and diatoms 

 with which the grains of sand were covered, for on heating some-of 

 this sand to 70 C., and subsequently washing it to remove the vege- 

 table debris, the ammonia in the water filtered through it was con- 

 siderably increased in amount. Also on filtering water continuously 

 for some weeks through a layer of sand 16 cm. deep, which contained 

 small quantities of vegetable matter, a half the free and an eighth 

 of the albuminoid ammonia were at first removed. After a week, 

 however, 58 per cent, of the free and 21 per cent, of the albuminoid 

 ammonia were absorbed, and after a fortnight respectively 96 per cent, 

 and 51 per cent. This marked increase of purification was due to the 

 deposition in the sand of the small quantities of algaa and diatoms in 

 suspension in the water, and to their subsequent multiplication. 



This vegetable filter was found to act most efficiently with the 

 water flowing through at its maximum rate. This, in the present 

 experiment, was at 1 litre in three minutes through a superficial 

 area of sand of 177 sq. cm. When, by clamping the exit tube, the 

 rat-e of flow was diminished to a litre in fifty minutes or more, there 

 was no longer any purification, but the amount of free ammonia in 

 the water was increased some threefold, whilst the albuminoid 

 ammonia remained practically unaltered. Also, on discontinuing 

 the current of water through the filter for twenty-four hours and 

 then renewing it, its purifying efficiency was greatly diminished in 

 respect of the albuminoid ammonia, only 9 per cent, of this being 

 now absorbed. In the course of a few days, however, the purifying 

 power began to increase again. The filter was then covered up so 

 as to totally exclude the light, and thereby kill off the chlorophyll- 

 containing organisms. About 90 per cent, of the free ammonia was 

 still absorbed, but never more than 20 per cent, of the albuminoid 

 ammonia, even after twenty-five days more of continuous filtration. 

 This purification must have been due to bacterial agency. Such a 

 " bacterial " sand filter reacts to changes in the rate of flow of the 

 water quite differently to the chlorophyll-containing one. Thus the 

 purification remained practically as great on diminishing the rate of 

 flow to 1 litre in two hours fifty minutes, and the water flowing 

 through immediately after the current had been entirely stopped for 

 ten hours had 93 per cent, of its free ammonia removed. 



In another experiment, sand which at first contained no vegetable 

 matter at all was found to have practically no purifying power. 

 On allowing a current of water to flow through it continuously, 

 however, this gradually developed and increased in amount as in 

 the above experiment. 



