40 GENERAL HISTORY OF 



These may be numbered, or otherwise marked, with a 

 writing diamond, and a large collection of them arranged 

 in a very compact case. 



Fossil Infusoria are best preserved in Canada balsam, 

 under thin slips of glass. 



Infusoria, when simply dried, may be relaxed again by 

 moisture, and some of them will bear this operation several 

 times the soft-bodied ones, however, only once. The 

 general colour of Infusoria is retained for a considerable 

 time after they have been dried, but the pigment of the 

 eye is soon lost. It may be well to observe, that when the 

 preserved specimens are intended to illustrate the nutritive 

 system, they should be previously fed with colouring mat- 

 ter ; but for observations on their muscular system this is 

 not advisable. 



SECTION XV. On Infusoria contained in Flints and 

 Semi- Opals. 



It is hardly possible to take up and examine a dozen 

 flints without discovering species of Infusoria inclosed 

 within them. These may be best seen under the micro- 

 scope, when very thin sections are made, like those of 

 fossil woods, teeth, coal, &c. : when these are polished and 

 cemented on glass sliders they are permanent objects. 

 Small splinters of flint, broken off, may be used for 

 investigation by the microscope, but such experiments 

 are attended with very considerable danger to the object- 

 glass of your instrument, by its being brought accidentally 

 into contact with their sharp edges, which oftentimes cut 



