1892.] on the Surface-Film of Water, dec. 541 



soapy water, is even more contractile. We have already seen that 

 the surface-film clings with considerable tenacity to any solid body 

 introduced into it, and that its hold increases with the length of 

 the line of contact. It is for this reason that fine meshes offer so 

 great a resistance to the passage of the surface-film. Air can pass 

 through the meshes with perfect ease; water also, if not at the 

 surface, can pass through readily enough, but the surface-film in 

 contact with air will only pass through with difficulty, and if there is 

 water behind it, the water may thus be restrained from passing 

 through the meshes. 



(6) Muslin bag hung in front of the lantern. Water poured into 

 the bag (a large spoonful) does not flow out ; but when the muslin 

 beneath the water is rubbed with a rod, it becomes wetted, the 

 surface-film passes to the outside of the bag, and the water trickles 

 through. 



There are many plants which take advantage of this property of 

 the surface-film of water, viz. that it will not penetrate small spaces, 

 in order to keep themselves dry. You must have observed how the 

 hairy grasses repel water. The surface-film is unable to pass into 

 the fine space between the hairs, and accordingly the water above the 

 surface-film is kept from contact with the leaf. This simple artifice 

 is often employed by plants which float at the surface of water. Here 

 it is important that they should keep dry, not only for the purpose of 

 respiration, but for another reason too. They commonly have great 

 power of righting themselves when accidentally submerged, and this 

 self-righting property depends upon the fact that the under surface of 

 each leaf is always wet, while the upper surface is incapable of being 

 wetted. The microscopic hairs which thickly cover the upper surface 

 are sufficient to exclude the water. A leaf of Pistia is now sub- 

 merged, and shown as an opaque object in the lantern. You see by 

 the gleaming of its surface that it is overspread by a continuous flat 

 bubble of air, which looks like quicksilver beneath the water. I will 

 next invert a leaf of Pistia by means of a rotating lever. It is now 

 brought up beneath the surface of the water in an inverted position, 

 and you see that, notwithstanding its buoyancy, it is unable to free 

 itself and rise to the surface, because of the air-bubble, which adheres 

 both to the leaf and to the disk at the end of the lever, and ties both 

 together. Complete separation of the leaf from the disk would 

 involve the division of the air-bubble into two smaller bubbles, one 

 adhering to the leaf and the other to the disk. In this operation the 

 surface-film would necessarily be extended directly in opposition to 

 its natural tendency to contract. Several other water-plants exhibit 

 the same properties as Pistia. I will mention two of the water-ferns 

 — Salvinia and Azolla. Salvinia is found floating on still water in 

 the warmer parts of Europe, as well as in other quarters of the globe. 

 The leaves are attached on opposite sides of a horizontal stem. Long 

 hairy roots (or what look like roots, and really answer the same 

 purpose) hang down into the water. Salvinia has in a remarkable 



