478 SCIENCE PROGRESS 



found that it is also valid in the case of tough jellies when 

 maintained at constant torsion. With the second of Boltzmann's 

 formulae — that which claims to give the value of the torsion at 

 any time under the action of a constant couple, and the recovery 

 after release — the experimental evidence is not so completely 

 in agreement. In some substances the recovery curve is not 

 of exactly the same form as that representing the increase of 

 deformation resulting from a constant stress. Thus, in the 

 case of the stretching and recovery of lead wires, although the 

 recovery may be approximately represented by an equation of 



the type 



6 = a + b log t, 



the stretching cannot be so represented. This increase in 

 length is partly due to a purely viscous flow from which there 

 can be no recovery, and hence cannot correspond exactly to the 

 latter. On the other hand, there are substances in which no 

 viscous flow occurs, and for these the deformation and recovery 

 curves are identical in shape, and can be fitted to the same 

 equation. Mr. Phillips has observed that for indiarubber and 

 certain metal wires, including gold and copper, this is the case, 

 and that the curves of stretching and recovery are capable of 

 being denoted by the equation 



L — a + b log /. 



There is, however, a further exception. The same observer has 

 found that this formula is not valid in the cases of iron and 

 steel wires. But it is not surprising that certain exceptions 

 should occur in common with other physical laws. 



There are several points of interest in connection with the 

 first law, viz. that the stress necessary to maintain constant 

 the strain in a suddenly deformed body may be represented by 



D = a - b log t. 



It is apparent that if this formula holds good until / has the 

 value given by 



log t - J 



D, the stress, has then a zero value, i.e. within a finite time the 

 stress in the body has entirely disappeared. Or again, suppose 

 that the body is perfectly elastic up to a certain limit, and let the 



