The Animalcules in Water. 119 



example, though by no means the most " difficult" of 

 these test-objects. 



The most remarkable feature of the diatoms may 

 be said to be this, that their valves are made of flint ; 

 the contents are soft, but the flinty part remains, 

 skeleton-like, even after the diatom has been im- 

 mersed in nitric acid, or exposed to the action of fire. 



Mr. Tuffen West, in the article already referred to, 

 says that a good way of readily observing the mark- 

 ings on diatoms is to place them on talc, and expose 

 this to a gentle red-heat. Accordingly one day, 

 happening to observe some roots of duckweed fes- 

 tooned with a quantity of diatoma and gomphonema 

 filaments, and some specimens of navicula moving 

 about near them, I placed them on a little disc of talc 

 (found among the apparatus of an old microscope), and 

 bearing well in mind the vegetable nature of diatoms, 

 and their consequent absence of feeling, carefully laid 

 the talc on a red-hot poker. I removed it when the 

 latter had become cold, and at once mounted the talc, 

 by turning it downwards on a glass slide, and fasten- 

 ing it with paper in my usual way. The effect of the 

 fire was interesting ; the duckweed had turned into a 

 faint heap of ashes, but the numerous diatoms showed 

 in great clearness both of outline and markings. 



Dr. Carpenter remarks that the flinty covering of 

 the diatoms may rather be described as consolidated 

 with silex (flint) than altogether composed of it. He 

 states that Professor Bailey contrived to dissolve the 

 silex with hydro-fluoric acid, and there remained a 

 delicate membrane, bearing all the markings of the 



