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NA TURE 



\yuly II, 1889 



stable form. It can no more exist than can a pencil stand 

 on its point. It immediately breaks up into a series of 

 spheres. This is well illustrated in that very ancient 

 experiment of shooting threads of resin electrically. 

 When the resin is hot, the liquid cylinders which are 

 projected in all directions break up into spheres, as you 

 see now upon the screen. As the resin cools, they begin 

 to develop tails ; and when it is cool enough, i.e. 

 sufficiently viscous, the tails thicken and the beads become 

 less, and at last uniform threads are the result. The 

 series of photographs show this well. 



There is a far more perfect illustration which we have 

 only to go into the garden to find. There we may see in 

 abundance what is now upon the screen — the webs of those 

 beautiful geometrical spiders. The radial threads are 

 smooth like the one you saw a few minutes ago, but the 

 threads that go round and round are beaded. The spider 

 draws tliese webs slowly, and at the same time pours upon 

 them a liquid, and still further to obtain the effect of 

 launching a liquid cylinder in space he, or rather she, 

 pulls it out like the string of a bow, and lets it go with a 

 jerk. The liquid cyhnder cannot exist, and the result is 

 what you now see upon the screen (Fig. 8). A more perfect 



Fig. 8. 



Fig. 



illustration of the regular breaking up of a liquid cylinder 

 it would be impossible to find. The beads are, as Flateau 

 showed they ought to be, alternately large and small, 

 and their regularity is marvellous. Sometimes two still 

 smaller beads are developed, as may be seen in the second 

 photograph, thus completely agreeing with the results 

 of Plateau's investigations. 



I have heard it maintained that the spider goes round 

 her web and places these beads there afterwards. But 

 since a web with about 360,000 beads is completed in an 

 hour — that is, at the rate of about 100 a second — this does 

 not seem likely. That what I have said is true, is made 

 more probable by the photograph of a beaded web that 

 I have made myself by simply stroking a quartz fibre with 

 a straw wetted with castor oil (Fig. 9). It is rather larger 

 than a spider line ; but I have made beaded threads, using 

 a fine fibre, quite indistinguishable from a real spider web, 

 and they have the further similarity that they are just as 

 good for catching flies. 



Now, going back to the melted quartz, it is evident that if 

 it ever became perfectly liquid it could not exist as a fibre 

 for an instant. It is the extreme viscosity of quartz, at the 



heat even of an electric arc, that makes these fibres possible. 

 The only difference between quartz in the oxyhydrogen jet 

 and quartz in the arc is that in the first you make threads 

 and in the second are blown bubbles. I have in my hand 

 some microscopic bubbles of quartz showing all the per- 

 fection of form and colour that we are familiar with in 

 the soap bubble. 



An invaluable property of quartz is its power of in- 

 sulating perfectly, even in an atmosphere saturated with 

 water. The gold leaves now diverging were charged 

 some time before the lecture, and hardly show any change, 

 yet the insulator is a rod of quartz only three-quarters of 

 an inch long, and the air is kept moist by a dish of water. 

 The quartz may even be dipped in the water and replaced 

 with the water upon it without any difference in the 

 insulation being observed. 



Not only can fibres be made of extreme fineness, but 

 they are wonderfully uniform in diameter. So uniform 

 are they that they perfectly stand an optical test so severe 

 that irregularities invisible in any microscope would 

 immediately be made apparent. Everyone must have 

 noticed when the sun is shining upon a border of 

 flowers and shrubs how the lines which spiders use as 

 railways to travel from place to place glisten with brilliant 

 colours. These colours are only produced when the fibres 

 are sufficiently fine. If you take one of these webs 

 and examine it in the sunlight, you will find that the 

 colours are variegated, and the eff"ect consequently is one 

 of great beauty. 



A quartz fibre of about the same size shows colours in 

 the same way, but the tint is perfectly uniform on the 

 fibre. If the colour of the fibre is examined with a prism, 

 the spectrum is found to consist of alternate bright and 

 dark bands. Upon the screen are photographs taken 

 by Mr. Briscoe, a student in the laboratory at South 

 Kensington, of the spectra of some of these fibres at 

 different angles of incidence. It will be seen that coarse 

 fibres have more bands than fine, and that the number 

 increases with the angle of incidence of the light. There 

 are peculiarities in the march of the bands as the angle 

 increases which I cannot describe now. I may only say 

 that they appear to move not uniformly but in waves, 

 presenting very much the appearance of a caterpillar 

 walking. 



So uniform are the quartz fibres that the spectrum from 

 end to end consists of parallel bands. Occasionally a 

 fibre is found which presents a slight irregularity here 

 and there. A spider line is so irregular that these 

 bands are hardly observable ; but as the photograph 

 on the screen shows, it is possible to trace them running 

 up and down the spectrum when you know what to 

 look for. 



To show that these longitudinal bands are due to the 

 irregularities, I have drawn a taper piece of quartz by 

 hand, in which the two edges make with one another an 

 almost imperceptible angle, and the spectrum of this 

 shows the gradual change of diameter by the very steep 

 angle at which the bands run up the spectrum. 



Into the theory of the development of these bands I 

 am unable to enter : that is a subject on which your Pro- 

 fessor of Natural Philosophy is best able to speak. Perhaps 

 I may venture to express the hope, as the experimental 

 investigation of this subject is now rendered possible, that 

 he may be induced to carry out a research for which he 

 is so eminently fitted. 



Though this is a subject which is altogether beyond 

 me, I have been able to use the results in a prac- 

 tical way. When it is required to place into an in- 

 strument a fibre of any particular size, all that has to 

 be done is to hold the frame of fibres towards a bright and 

 distant light, and look at them through a low-angled 

 prism. The banded spectra are then visible, and it is 

 the work of a moment to pick out one with the number of 

 bands that has been found to be given by a fibre of the 



