248 



NATURE 



[July II, 1889 



moves in a manner which shows that it is not influenced 

 by the last deflection alone : the glass remembers what 

 was done to it previously. For this reason spun glass is 

 quite unsuitable as a torsion thread ; it is impossible to 



10 



Scale of loooths of an 



inch for Figs, i to 7. 

 much finer. 



The scale of Figs. 8 and 9 is 



Fig. 



Fig. 



say what the twist is at any time, and therefore what is 

 the force developed. 



So great has the difficulty been in finding a fine torsion 

 thread that the attempt has been given up, and in all the 

 most exact instruments silk has been used. The natural 



Fig. 



cocoon fibres, as shown on the screen (Fig. 4), consist of 

 two irregular lines gummed together, each about one two- 

 thousandth of an inch in diameter. These fibres must be 

 separated from one another and washed. Then each 

 component will, according to the experiment of Gray, 



carry nearly 60 grains before breaking, and can be safely 

 loaded with 15 grains. Silk is therefore very strong, carry- 

 ing at the rate of from 10 to 20 tons to the square inch. 

 It is further valuable in that its torsion is far less than 

 that of a fibre of the same size of metal or even of glass, 

 if such could be produced. The torsion of silk, though 

 exceedingly small, is quite sufficient to upset the working 

 of any delicate instrument, because it is never constant. 

 At one time the fibre twists one way, and another time in 

 another, and the evil eff"ect can only be mitigated by using 

 large apparatus in which strong forces are developed. 

 Any attempt that may be made to increase the delicacy 

 of apparatus by reducing their dimensions is at once pre- 

 vented by the relatively great importance of the vagaries 

 of the silk suspension. 



The result, then, is this. The smallness, the length 

 of period, and therefore delicacy, of the instruments at 

 the physicist's disposal have until lately been simply 

 limited by the behaviour of silk. A more perfect sus- 

 pension means still more perfect instruments, and there- 

 fore advance in knowledge. 



It was in this way that some improvements that I was 

 making in an instrument for measuring radiant heat came 



F"ic. 4. 



to a deadlock about two years ago. I would not use 

 silk, and I could not find anything else that would do. 

 Spun glass, even, was far too coarse for my purpose ; it 

 was a thousand times too stiff. 



Thiere is a material invented by Wollaston long ago, 

 which, however, I did not try because it is so easily broken. 

 It is platinum wire which has been drawn in silver, and 

 finally separated by the action of nitric acid. A specimen 

 about the size of a single line of silk is now on the screen, 

 showing the silver coating at one end (Fig. 5). 



As nothing that I knew of could be obtained that 

 would be of use to me, I was driven to the necessity of 

 trying by experiment to find some new material. The 

 result of these experiments was the development of a 

 process of almost ridiculous simplicity which it may be 

 of interest for me to show. 



The apparatus consists of a small cross-bow, and an 

 arrow made of straw with a needle point. To the tail of 

 the arrow is attached a fine rod of quartz which has been 

 melted and drawn out in the oxyhydrogen jet. I have a 

 piece of the same material in my hand, and now after 

 melting their ends and joining them together, an opera- 

 tion which produces a beautiful and dazzling light, all I 



