lO 



NATURE 



[May 5, 1892 



squeezes part of it out by pressing together the bases of 

 its last pair of legs. The bubble rises, but is detained by 

 some of the threads previously spun across its path. 

 Then the spider returns to the surface to fetch another 

 bubble, and repeats the operation as often as is necessary. 

 Now and then she secures the growing bubble by 

 additional threads, and before long has a bubble nearly 

 as big as a walnut, inclosed within an invisible silken 

 net, which imprisons the air as effectually as a dome of 

 glass would do. The spider takes care to conceal her 

 home from observation, and before long the minute Alga;, 

 growing all the more vigorously because of the air brought 

 to them, effectually conceal the habitation. The mouth 

 of the dome, which is of course beneath, is narrowed to 

 a small circle, and Plateau has observed a cylindrical 

 horizontal tube, seven to eight millimetres in diameter, by 

 which the spider is enabled to enter or leave her home 

 without being observed. The air within is renewed as 

 required, by the visits of the spider to the surface. 



Besides this home, which is the ordinary lurking-place 

 of the spider, another is required at the time when the 

 young are hatched. The new-born spiders are devoid 

 of the velvety covering of hairs, and would drown in a 

 moment if placed in a nursery with a watery floor. The 

 female spider therefore makes a special nest for this 

 particular occasion, which floats on the surface of the 

 water, rising well above it. It is bell-shaped and strongly 

 constructed. The upper part is partitioned off, and con- 

 tains the eggs. Beneath the floor of the nursery the 

 mother takes her station, and watches over the safety of 

 her brood, defending them against the predatory insects 

 which abound in fresh waters. It is interesting to see 

 how the faculty of spinning silk, used by the house-spider 

 for her snares, and at other times for the fluffy cocoon in 

 which the eggs are enveloped, furnishes to the water- 

 spider the materials of her architecture. It is not less 

 interesting to observe the economy of material which 

 results from the use of the tenacious and contractile sur- 

 face-film, in place of a solid wall. 



We will next consider another property of the surface- 

 film, which is turned to account in the daily life of the 

 very commonest of our floating plants, I mean the duck- 

 weed, which overspreads every pond and ditch. A num- 

 ber of the green floating leaves of duckweed are now 

 placed in a shallow dish in the field of the lantern, and I 

 will ask you to observe how they are grouped. They have 

 spontaneously arranged themselves in a very irregular 

 fashion, forming strings and chains which spread hither 

 and thither over the surface of the water. This is not the 

 way in which most floating bodies behave. Let us re- i 

 move the duckweed, and replace it by another dish of j 

 water in which I will put anumberof small disks of cork.' j 

 You will see that the bits of cork are attracted one to | 

 another and crowd together in one place. Let us inquire i 

 why the floating bits of cork are thus attracted towards one j 

 another. If any soHd capable of being wetted by water 

 is partly immersed in water, the liquid rises round it in an 

 ascending capillary curve. If the solid is not wetted by 

 water, the curve will turn downwards. We may get as- 

 cending or descending capillary curves in other ways. 

 If, for instance, I were to lay a sheet of paper upon water, 

 and turn its edges up at certain places, we should get 

 marked ascending curves at these points. The raising of 

 some parts of the surface causes other parts to sink, and 

 may bring about descending curves, or make previously 

 formed descending curves more marked. We shall find 

 it helpful in our experiments to notice one very simple 

 plan of producing a descending capillary curve round the 

 edge of a vessel. If we take a glass of water, and fill it 

 until the water is level with the brim, we naturally speak 

 of the glass as full ; but if we are careful to avoid rude 



' In order to avoid the inconvenience caused by the attraction of the sides 

 of the vessel, the dish should be over-full of water. 



NO. I 175, VOL. 46] 



shaking, we may still add a considerable quantity of water 

 without spilling any. The glass will then become what 

 we may call over-full, and its surface will be bounded by 

 a descending capillary curve. Now, it is of immediate 

 importance to us to observe that like capillary curves, 

 whether ascending or descending, attract one another, 

 and that unlike curves repel one another. The theo- 

 retical explanation of this point is not difficult, but it must 

 not detain us here. To place the fact itself beyond 

 dispute, we will try a little experiment. A circular 

 dish of water is now placed in the field of the lantern, 

 and we will introduce into it a small disk of wood. Both 

 the disk and the side of the vessel are wetted by water, 

 and an ascending capillary curve rises round each. The 

 result is that the two bodies attract one another. Every 

 time the disk is moved away it is powerfully drawn 

 towards the side of the vessel. With a little syringe we 

 will add water to the dish in sufficient quantity to raise 

 the level above the edge of the vessel. You will observe 

 that the wooden disk is now repelled by the edge of the 

 vessel, and floats free in the centre. i3y sucking up a 

 little water, it becomes attracted once more, and so we 

 may go on, causing it to be attracted or repelled, accord- 

 ing as we add or subtract a small quantity of water. But 

 what has all this to do with the duckweed? In order to 

 explain the behaviour of duckweed, I must ask you to 

 examine a careful representation of its form. This com- 

 mon plant has not, to my knowledge, been faithfully 

 represented in any botanical book. You will see that 

 the leaf is of an irregular oval shape, broader at one end 

 than at the other, and that the narrow end is pointed. A 

 raised ridge extends along the length of the leaf, from 

 the point to the middle of the opposite or rounded border. 

 Duckweed almost invariably propagates itself by budding. 

 New leaves are pushed out symmetrically on each side 

 of the point. They grow bigger and bigger, and gradu- 

 ally free themselves. The point upon each leaf marks 

 the place where it was last attached to the parent leaf. 

 Sometimes the budding is so rapid, that, before a fresh 

 pair of leaves have become free, they have already 

 budded out a second pair, which we may call the grand- 

 daughters of the parent leaf. The pointed end of the 

 leaf, and also the opposite end of the ridge, are raised 

 above the general level, and very marked capillary curves 

 ascend from the general water-level to these points. The 

 free edge of every bud is also raised above the general 

 water-level, and a capillary curve ascends to meet it. 

 Hence, when a number of leaves of duckweed are float- 

 ing freely on water, they are powerfully attracted one to 

 another at certain points, while at intervening points they 

 are relatively inert. If you take a floating leaf of duck- 

 weed, and bring near it a clean needle or a pencil-point, 

 or any similar object, provided that it is not greasy, you 

 will see that the leaf is at once attracted towards the 

 point, but it always turns itself so as to bring one of its 

 ascending curves round to the needle or pencil. We all 

 see in the lantern how readily a leaf of duckweed is made 

 to rotate rapidly by causing a needle-point to revolve 

 round it, without ever touching it. Let us now try to 

 imitate the behaviour of the leaves by some rude models. 

 I have here some elliptical paper floats, cut out with a 

 pair of scissors, and having each of the pointed ends a 

 little turned up. We place these one by one on the sur- 

 face of the water, and you see in the lantern how they 

 are attracted to one another, point to point, and how they 

 form, long chains, which have a tendency to break up into 

 stars. It is the existence of such points of attraction on 

 the margin of the leaves which causes the duckweed to 

 form chains and strings, so long as there is any unoccu- 

 pied surface in the pond. A moment's consideration 

 shows how profitable this tendency is to the plant Were 

 the duckweed to crowd together like the floating bits of 

 cork, the pressure towards the centre of any considerable 



