and some suggested Uses of the Finest Threads. 497 



It is probable that a body hung by a fibre of quartz and 

 vibrating in a perfect vacuum would remain twisting back- 

 wards and forwards for a far longer time than a similar body 

 hung by a glass thread, also that the most perfect balance- 

 spring for a watch would be one of quartz. I have a piece of 

 quartz drawn out to a narrow neck which just cannot hold up 

 its head ; this keeps on nodding in all directions for so long a 

 time, even in the air, as to make it evident that the material 

 has very unusual properties. 



3. Uses. 

 As torsion-threads these fibres of quartz would seem to be 

 more perfect in their elasticity than any known ; they are as 

 strong as steel, and can be made of any reasonable length 

 perfectly uniform in diameter, and, as already explained, 

 exceedingly fine. The tail ends of those that become invisible 

 must have a moment of torsion 100 million times less than 

 ordinary spun glass ; and though it is impossible to manipulate 

 with those, there is no difficulty with threads less than y- o oo 

 inch in diameter. 



I have made a spiral spring of glass of about 30 turns 

 which weighs about one milligram ; this, examined by a 

 microscope, would show a change in weight of a thing hung 

 by it of one 10 millionth of a gram. Since this has been 

 annealed its elastic fatigue is that of annealed glass, and 

 therefore very small. I have succeeded in doing the same 

 thing with a quartz fibre, but the difficulties of manipulation 

 are very great in consequence of the rottenness of annealed 

 quartz. The glass spring can be pulled out straight, and 

 returns perfectly to its proper form. 



Since these fibres can be made finer than any cobweb, it is 

 possible that they may be preferable to spider-lines in eye- 

 pieces of instruments ; they would in any case be permanent, 

 and not droop in certain kinds of weather. 



Those who have experienced the trouble which the shifting 

 zero of a thermometer gives, might hope for a thermometer 

 made of quartz. When made, it would probably be more 

 perfect in this respect than a glass thermometer, but the 

 operation of making w T ould be difficult. 



These very fine fibres are convenient for supporting small 

 things of which the specific gravity is required, for they weigh 

 nothing, and the line of contact with the surface of the water 

 is so small, that they interfere but little with the proper swing 

 of the balance. 



It seemed possible that a diffraction-grating made of fibres 

 side by side in contact with one another would produce 



