34 TURBIDITY. 



very long handle proceeding from the side — a sort 

 of hearth-brush in miniature, fitted with very close 

 and stiff bristles — will rub off the greenness. It 

 may be used about once in a month, or oftener in 

 summer. On the stones of the bottom, the cement 

 and rock-work, and even on that side of the tank 

 which, being next to the window, is not used for 

 observation, I would recommend that the green 

 growth be not interfered with, but that the marine 

 plants be allowed to grow undisturbed. A crop of 

 self-sown weeds in the tank is far more valuable 

 than such as have been introduced on loose stones. 

 And even from very early age the green growth is 

 found to throw off a copious supply of oxygen 

 bubbles. Care, too, must be taken not to molest 

 or annoy the animals needlessly, as also to leave 

 undisturbed any masses of spawn that may have 

 been deposited on the glass. 



Turbidity. — Occasionally the water in a tank, 

 which has hitherto been quite translucent, becomes 

 all on a sudden so turbid as completely to hide the 

 contents from view, except such as are close to the 

 glass. This turbidity may arise from either of two 

 causes. If it is of a grey or whitish hue, forming 

 clouds here and there, which disperse and form 

 again elsewhere, the microscope will show that it is 

 composed of an innumerable multitude of animal- 

 cules belonging to the Class Infusoria. Their 

 presence is not an evil, but rather a means whereby 

 an already existing evil may be remedied. Their 

 sudden increase to such an extent as to be thus 

 appreciable to the senses, is symptomatic of organic 

 matters in the tank in a state of decomposition. If 

 we allow a minute worm that has died to lie at the 

 bottom of the Tank, we shall see in a day or two, 



