TURBIDITY. 35 



if we watcli it with a powerful lens, that it is en- 

 compassed by a little cloud of moving atoms, which 

 are the animalcules in question, and which are 

 busily engaged in devouring not only the solid 

 parts, but also the juices and invisibly minute par- 

 ticles that float off; and thus in a very short time 

 they effectually dispose of the offensive substance. 

 So, in the case of their increase to the extent just 

 supposed, of producing a general turbidity, they 

 will, if left to themselves, soon clear away the de- 

 composing matter if it be not too great, and then 

 themselves gradually disappear, allowing the water 

 to resume its original clearness. As soon, however, 

 as we perceive such an appearance, we should care- 

 fully transfer the principal animals to another vessel, 

 and search for the decomposing bodies, on the 

 removal of which the water will presently be trans- 

 parent and sweet as before. 



But the opacity of the water may be dependent 

 on a totally different cause. If it is of a green 

 colour, rapidly deepening in intensity, it is vege- 

 table in its origin, and arises from an infinite 

 number of the spores (or seeds) of green Algoi dis- 

 persed through the fluid, and held in suspension 

 there. Now, this appears to have no deleterious 

 influence either on the plants or animals, which 

 live and thrive as well as when the water is clear ; 

 but it is annoying because of its unsightliness, and 

 because it effectually interferes with our observation 

 of our cherished favourites. It is, too, a most 

 inveterate evil ; unlike the former, it is not 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 

 d2 



