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The minute divisions of the leaves of subaquatic plants, as mentioned in the note on Trapa, and of the gills of fish, seem to serve another purpose besides that of increasing their surface, which has not, I believe, been attended to, and that is to facilitate the separation of the air, which is mechanically mixed or chemically dissolved in water by their points or edges; this appears on immersing a dry hairy leaf in water fresh from a pump; innumerable globules like quicksilver appear on almost every point; for the extremities of these points attract the particles of water less forcibly than those particles attract each other; hence the contained air, whose elasticity was but just balanced by the attractive power of the surrounding particles of water to each other, finds at the point of each fibre a place where the resistance to its expansion is less; and in consequence it there expands, and becomes a bubble of air.
As the material thus necessary to life seems to abound more in air than in water, the subaquatic leaves of this plant, and of sisymbrium, coenanthe, ranunculus aquatilis, water crowfoot, and some others, are cut into fine divisions to increase the surface; whilst those above water are undivided.
I became absorbed in a sort of subaquatic mirage, in which I seemed to distinguish reflections beneath the surface. My eyes refocused with a jerk, and I realized that something had unconsciously been perceived by my rods and cones, and short-circuited to my duller brain.
I will only say here that some spiders use these peculiar glands to form light webs by whose aid, though wingless, they float balloon-wise through the air; that others employ them to line the sides of their underground tunnels, and to make the basis of their marvellously ingenious earthen trap-doors; that yet others have learnt how to adapt these same organs to a subaquatic existence, and to fill cocoons with air, like miniature diving bells; while others, again, have taught themselves to construct webs thick enough to catch and hold even creatures so superior to themselves in the scale of being as humming-birds and sunbirds.
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