STPtUCTUrvE AND SUBSTANCE. 11 



part immersed in water, while the part that is immersed 

 becomes filled with the fluid and assumes a fresh ap- 

 pearance, the part that is not in the water remains dry as 

 before. 



The fronds of Algse are not only variable in form, but 

 also in substance. Some are like masses of jelly, such as the 

 fresh-water Batracliospermum and Draparnaldia ; others are 

 very gelatinous, such as the marine Mesogloia; others are like 

 silk threads, others are so filmy and membranaceous that by 

 children they are called Sea- silk ; others are cartilaginous as 

 gristle and elastic as India-rubber, a quality which I parti- 

 cularly observed in a Micromega that I found in Arran ; 

 some are tough as leather, others firm as wood. The leaves 

 of some are delicate and transparent, wliile those of others 

 are thick and opake ; some are finely veined, others are 

 without nerves ; some of the filamentous kinds have joints 

 and longitudinal siphons ; some are destitute of both. The 

 DiatomacecBj both fresh-water and marine, have an organized 

 covering of flint, which withstands the action of fire, so 

 that they are found abundantly in a fossil state in the sub- 

 stance called BergmeJil, or mountain-meal, extensive strata 

 of which occur on the continent ; and the microscope detects 

 them also in a portion of Bergmehl in my possession, from 



