INTRODUCTION. 41 



Memoir already referred to, lays down the following pro- 

 position with regard to nitrogen. 



" In the two kingdoms, the bodies which admit azote to 

 the number of their principal constituents, are indispensable 

 to the accomplishment of the laws of life." 



A distinction may be established between animals and 

 vegetables, moreover, by a consideration of the effects of 

 different reagents applied to their tissue. This distinction is, 

 however, less satisfactory than that derived from a knowledge 

 of their chemical composition. " Vegetable membranes," 

 M. Peyen remarks, "which are well aggregated, are not 

 sensibly alterable in the presence of a host of reagents, such 

 as iodine, chlorine, the alkalies, and acids diluted, tannin, 

 many neutral salts, alcohol, and creosote, which colour, 

 attack, dissolve, or strongly contract the membranes of 

 animals, but the distinction which is founded upon their 

 elementary composition is still more certain." 



The same agent, iodine, seems to detect the presence of the 

 product starch, and of the element azote, by imparting to 

 starch globules a blue tint, and to those substances contain- 

 ing azote a yellow coloration. 



A few remarks upon the subject of cu-culation may here 

 be introduced. If we except the order Characece, to be 

 described in its proper place, but few traces of a circulation 

 in the freshwater ConfervcB exist. There is probably a 

 motion of fluid in the tubular structure which connects the 

 light green granules of the Algce, and in the vajs, which 

 proceed from the central organ in Zygnema ; there is also, 

 doubtless, an action of endosmosis and exosmosis carried on 

 between the different contents of the cells, and between these 

 and the water in w^hich the Confervce dwell. 



The uses of the freshw\ater Confervcs may be regarded as 

 fourfold ; two of these uses pertain to the animal creation in 

 general, the other two to man in particular. 



The first and most obvious use to notice is, the abundant 

 supply of delicate and nutritious food which they furnish to 

 myriads of the inhabitants of our fresh waters. It is scarcely 



