36 



TURBIDITY". 



colourless, — suddenly, without any imaginable 

 cause, become green; and in the course of two days 

 be so opaque that objects could not be discerned 

 an inch from the sides. 



The lens will not detect anything in the fluid in 

 this case ; it requires a very high power of the 

 compound microscope to resolve the cause. With 

 a magnifying power of 560 diameters^ we see an 

 immense number of oval atoms, apparently colour- 

 less (but doubtless, having a very slight tinge of 

 green visible only in the aggregate), and not more 

 than 5-0V0 i^ch in diameter. These I con- 



ceive to be the spores of some green Oscillatoria^ or 

 some kindred plant ; for there is a tendency to the 

 accumulation of the films of such plants in the 

 vessels in which the phenomenon exists. 



Sometimes this evil will continue unchanged for 

 many months, and then clear away as suddenly as 

 it came. At others, it will diminish and promise a 

 return of transparency, then suddenly return, and 

 set in as dark as before. 



Mr. W. A. Lloyd has succeeded in overcoming 

 this difficulty. By drawing off the green water, 

 and putting it into a dark closet, he finds that in 

 two or three weeks the turbidity quite disappears, 

 the water resuming its pristine transparency. The 

 explanation is doubtless as follows : light is neces- 

 sary to the life of plants, or at least the green 

 colouring principle in them cannot be developed 

 without light ; if, then, this be denied, the plants 

 must wane and die. Now the opacity, as I have 

 intimated, consists of the living germs of green 

 plants ; and these on being deprived of light gra- 

 dually die away ; after which the water is quite fit 

 for use again. I have myself instituted experi- 



