28 GENERAL HISTORY OP 



those aljove mentioned, pour a little water from the vessel 

 containing them into a watch glass, and place it upon a 

 piece of cardboard, rendered half black and half white. 

 The white ground will make the dark specimens apparent, 

 and vice versa ; thus, the required specimens may be taken 

 out singly with one of the tubes, and placed in the aquatic 

 live-box for observation. The observer will derive much 

 assistance in this operation from the use of the pocket- 

 magnifier before mentioned, or from a watchmaker's eye- 

 glass. 



When the Infusoria are extremely minute, they usually 

 congregate at the edge of the water over the white portion 

 of the cardboard, and may be removed from thence with 

 the point of a quill, or of a small wedge-shaped pencil. 

 If a quantity of the Chara, or other aquatic plants, be put 

 into a glass jar with the Infusoria, in the course of a few 

 days, more or less depending upon the temperature of the 

 season, the surface will be covered with a thin pellicle, 

 formed by the decomposition and extrication of gas, 

 causing the small detached pieces of vegetable matter to float 

 upon the water, and with them the Infusoria. Let a small 

 portion of this film be taken from the surface, by means of 

 the feeding pin, described in the Microscopic Cabinet, 

 p. 235, and examined under the microscope, and you will 

 hardly fail of being highly gratified. Among the most 

 interesting genera collected from the surface of these infu- 

 sions, in the manner just stated, are those belonging to the 

 families Arcellina and Astasitea. After the film has 

 remained some days upon the water, many of the above- 

 mentioned genera disappear, and are succeeded by those of 

 the family Vibrionia, especially the Bacterium. These, 



