282 GENERAL DIRECTIONS. 



the like, a pewter spoon bent up to a right angle, 

 with the shaft tied to a slender stick, is very useful. 

 You can, if jou please, make a more elegant affair of 

 it. Two or three simple sticks or rods, some of them 

 widened, spade-like, at the end, are also useful for 

 pushing the specimens to any required point. And 

 one or two small nets, made by stretching a bit of 

 lace or muslin over a ring of wire, fastened to a rod, 

 will serve to catch and lift out such animals as you 

 wish to transfer, for examination, or any other pur- 

 pose, to another vessel. As a general rule, however, 

 they should be disturbed as little as possible, and 

 never handled. 



Artificial Aeration. — Although living and 

 healthy plants will educe and throw off, under the 

 influence of light, oxygen, in sufficient quantity to 

 maintain in health a given number of animals, yet 

 the artificial admixture of atmospheric air with the 

 water may be employed as a valuable auxiliary. 

 I have used it with marked benefit; often having 

 revived animals thereby, which, from the exhaustion 

 of the water, were apparently in a dying state. Its 

 utility as a means of maintaining the purity of the 

 water is still more obvious ; since it is by the fre- 

 quent and successive presentation of the particles of 

 water to the air, that the animal excretions which 

 they hold in suspension become chemically changed, 

 and deprived of their putrescent qualities. This is 

 what takes place in nature. By the perpetual dashing 

 of the waves against the shore, and especially against 

 the ragged rocks, an immense quantity of air becomes 

 entangled, in the form of minute bubbles, which by 



