MICROSCOPIC OBJECTS. 79 



matter in damp garden walks, and the hair-like weeds in 

 ditches, are famihar examples of the freshwater Algse. 

 The sea- weeds wliich so abundantly cover the rocky 

 shores of our coast are marine Algse. Of the latter the 

 bays in the South of Devon are rich in specimens, as the 

 labours of Mrs. Griffiths of Torquay testify. For the 

 microscope the smaller kinds of a bright scarlet colour 

 are the most valuable, when mounted between slips of 

 glass in Canada balsam. 



The genera Chara and Nitella, which exhibit the cel- 

 lular circulation so distinctly, and the disjoined, or Dia- 

 tomese, (considered as animalcules by some naturalists,) 

 belong to the freshwater Algae. 



As articles of food the Algae are almost valueless to 

 man; but their use in manufacture is considerable. 

 Some produice glue ; others bariUa ; while the new ele- 

 ments iodine and bromine are obtained from them. 



The sea-weeds often have Zoophytes adhering to them : 

 they are then splendid opaque objects, the pearly white 

 dry polypidoms of the Zoophytes being seen in strong 

 reHef upon the red Algae. 



Ashes. — The elementary organs of some plants are 

 composed of sihca, lime, &c. These substances may be 

 detected in the ashes of the plant after it has been burnt. 

 The most favourable example for showing the form in 

 which silica occurs in plants, is the husk of the oat of 

 wheat. If a husk of oat be examined under the micro- 

 scope, (being previously mounted in water or Canada bal- 

 sam between plates of glass,) a series of bright parallel co- 



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