288 GENERAL DIRECTIONS. 



self-curative, at least not certainly so, and it cannot 

 "be foreseen. I have had a large vessel that had been 

 in full occupation for a year and a half, — during the 

 whole of which time it had remained brilliantly 

 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 anj^thing in the fluid in 

 this case ; it requires a veiy high power of the com- 

 pound microscope to resolve the cause. With a mag- 

 nifying power of 560 diameters, we see an immense 

 number of oval atoms, apparently colourless (but, 

 doubtless, having a very slight tinge of green visible 

 only in the aggregate), and not more than 5-0^170 th of 

 an inch in diameter. These I conceive to be the 

 spores of a 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 gi*een water, and put- 

 tine it into a dark closet, he finds that in two or three 

 weeks the turbidity quite disappears, the water re- 

 suming its pristine transparency. The explanation 

 is doubtless as follows : light is necessary to the life 

 of plants, or at least the green colouring principle in 



