TURBIDITY. 287 



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, if we watch 

 it with a powerful lens, that it is encompassed by a 

 little cloud of moving atoms, which are the animal- 

 cules in question, and which are busily engaged in 

 devourmg not only the solid parts, but also the juices 

 and invisibly minute particles 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 decomposing 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 

 carefully 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 vegetable in its 

 origin, and arises from an infinite number of the spores 

 (or seeds) of green Algce dispersed 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 un- 

 sightliness, and because it effectually interferes with 

 our observation of om^ cherished favourites. It is, 

 too, a most inveterate evil ; unlike the former, it is not 



