216 MR. RITCHIE ON THE ELASTICITY OF THREADS OF GLASS, 
If the vitreous molecules be held together by the attraction of their poles or 
points of greatest affinity, it is obvious that these points will be displaced by 
torsion along the whole line of Communication. The points of greatest 
attraction thus displaced, will therefore endeavour to regain their former 
state of stable equilibrium, and the thread will of course untwist itself till 
the needle returns to its former position. It would be curious to ascertain 
if a thread of glass, twisted as much as it can safely bear, and kept in that 
position for several months or years, would return exactly to its former posi- 
tion, or whether the atoms might not in course of time take up a new state of 
stable equilibrium. 
3. The number of times a thread of glass may be twisted without breaking, 
will of course depend on its length and diameter. It is almost incredible the 
number of times a thread of a substance so brittle as glass may be twisted, 
before the points of greatest attraction of the vitreous molecules be actually 
removed beyond the sphere of attraction, or in other words, before the thread 
he broken. I have succeeded in drawing threads of glass of such extreme 
tenuity, that one of them, not more than a foot long, may be twisted nearly a 
hundred times without breaking. Hence it is obvious, that if a thread could 
be drawn so fine as to consist of a single line of vitreous molecules, torsion 
could have no tendency to displace the points of greatest attraction, and this 
elementary thread might be twisted for ever without breaking. In that case 
the compound molecules of glass would only turn round their points of 
greatest attraction, like bodies revolving on a pivot. 
4. It is difficult to prove by direct experiment some of the laws of torsion 
established by Coulomb, as belonging to metallic wires, on account of the dif- 
ficulty of procuring threads of glass of a uniform diameter throughout their 
whole length. It is difficult, for example, to prove by experiments, that the 
force of torsion of a glass thread is directly as its length, and inversely as the 
fourth power of its diameter. Fortunately, however, the only property which 
we are to employ in the construction of the following instruments, can be 
proved by direct experiment. This property is, within certain limits, common 
to all elastic threads, namely, “ that the force of torsion, or that force with 
which a thread tends to untwist itself, is directly proportional to the number 
of degrees through which it has been twisted*.” 
* Biot, Traite de Physique, tom. i. p. 486. 
