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 tothe number of their principal constituents, are indispensableto the accomplishment of the laws of life.”

A distinction may be established between animals andvegetables, moreover, by a consideration of the effects ofdifferent reagents applied to their tissue. This distinction is,however, less satisfactory than that derived from a knowledgeof their chemical composition. “ Vegetable membranes,”M. Peyen remarks, “which are well aggregated, are notsensibly alterable in the presence of a host of reagents, suchas iodine, chlorine, the alkalies, and acids diluted, tannin,many neutral salts, alcohol, and creosote, which colour,attack, dissolve, or strongly contract the membranes ofanimals, but the distinction which is founded upon theirelementary composition is still more certain.”

The same agent, iodine, seems to detect the presence of theproduct starch, and of the element azote, by imparting tostarch globules a blue tint, and to those substances contain-ing azote a yellow coloration.

A few remarks upon the subject of circulation may herebe introduced. If we except the order Cliaracece, to bedescribed in its proper place, but few traces of a circulationin the freshwater Conferva exist. There is probably amotion of fluid in the tubular structure which connects thelight green granules of the Alga, and in the rays whichproceed from the central organ in Zygnema; there is also,doubtless, an action of endosmosis and exosmosis carried onbetween the different contents of the cells, and between theseand the water in which the Conferva dwell.

The uses of the freshwater Conferva may be regarded asfourfold; two of these uses pertain to the animal creation ingeneral, the other two to man in particular.

The first and most obvious use to notice is, the abundantsupply of delicate and nutritious food which they furnish tomyriads of the inhabitants of our fresh waters. It is scarcely