66 



NATURAL HISTORY 



light : w4iether this preference is the choice of the ani- 

 malcules^ or influenced by an ascending current in the 

 water, I have not ascertained. Baker makes no men- 

 tion of the fact.) When exposed at the surface of the 

 water for a short time, the colour of the animalcules is 

 changed from a bright green to a sky-blue ; but they are 

 sometimes so nuQierous as to cover the mud at the bottom 

 of the pond for several inches in thickness ; the masses 

 then appear almost black. When in small companies, 

 they are nearly of the same specific gravity as the water, 

 and therefore when placed at any depth appear station- 

 ary. A small quantity of this matter having been put 

 into a jar of water, one part went down to the bottom, 

 whilst the other continued floating upon the top. All 

 things remained for some time in this condition, until at 

 last each swarm of animalcules grew weary of its situa- 

 tion, and had a mind to change its quarters. Both ar- 

 mies, in short, began their march at the same time, 

 against the side of the jar, and as one proceeded up- 

 wards and the other downwards, after some hours they 

 were nearly meeting about the midway, as the route each 

 of them appeared to take, soon after they began to move, 

 made it very easy to foresee. The desire of knowing in 

 what manner they would behave on this occasion, en- 

 gaged the observer to watch them with a careful eye ; 

 and as they approached still nearer, he beheld to his 

 great surprise the army that was marching upwards open 

 to the riglit and left, and leave a convenient space for the 

 army Avliicli was marching downwards to pass between 



