286 GENERAL DIEECTIONS. 



a nail-brusli with a 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 gTOW 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 com- 

 posed of an innumerable multitude of animalcules 

 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 in- 

 crease to such an extent as to be thus appreciable to 

 the senses, is symptomatic of organic matters in the 



