QUARTZ FIBERS. 321 



a beaded web that I iiave made myself by simply stroking 

 a quartz fiber 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 fiber, 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 

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

 at the heat even of an electric arc, that makes these fibers 

 possible. The only diflerence between quartz in the oxy- 

 hydrogen 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 perfection of form and color that we are familiar with in (I 



the soap bubble. 



An invaluable property of quartz is its power of insulating 

 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 fibers be made of extreme fineness, but they are won- 

 derfully uniform in diameter. So uniform are they that they perfectly 

 stand an optical test so severe that irregularities invisible in any mi- 

 croscope 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 colors. These colors are only produced when 

 the fibers are sufficiently fine. If you take one of these webs and exam- 

 ine it in the sunlight, you will find that the colors are variegated, and 

 the efiect consequently is one of great beauty. 



A quartz fiber of about the same size shows colors in the same way, 

 but the tint is perfectly uniform on the fiber. If the color of the fiber 

 is examined with a prism, the spectrum is fouiul to consist of alternate 

 bright and dark bands. Upon the screen are photographs taken by 

 Mr. Briscoe, a student in the laboratory of South Kensington, of the 

 spectra of some of these fibers at different angles of incidence. It will 

 be seen that coarse fibers have more bands than fine, and that tlie num- 

 ber increases with the anglesof incidence of the light. There are jiecu- 

 liarities in the march of the bands as the angle increases which 1 can 

 not describe now. I may only say that they appear to move not uni- 

 formly but in waves, presenting very much the appearance of a cater- 

 pillar walking. 



H. Mis. 129 21 



