1889.] on Quartz Fibres. 553 



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 j^i'obable by the photograph of a 



beaded web that I have made myself by simjily strok- ^^^' ^' ^^^- ^• 



ing 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 be- ^ 

 tween quartz in the oxyhydrogen jet, and quartz in the ? 

 arc, is that in the first you make threads, and in the t 

 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 insulating j)erfectly, 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 

 dij^ped 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. Every one 

 must have noticed, when the sun is shining upon a border of flowers 

 and shrubs, how the lines which the spiders use as railways to travel 

 upon from place to place glisten with brilliant colours. These colours 

 are only j)roduced 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 effect consequently is one of 

 great beauty. 



The 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 



