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interesting. Andrade and Tsien hav_e phenomena in single metal crystals ₀f 11 potassium, and Tsien and Chow molybdenum,    ^aVe stp^

The whole of this | spring | explanati ■ elastic properties of long molecules w^of % challenged by Mark, who looks at the prohi ^be^ quite a different way. He has studied^ nian**¹ called “ piastics ” and resins, such as the^?' styrenes.| He has also studied soft rubber, and poi t' out that when these materials are stretched, the w molecules tend to straighten out, and to align themselves in the direction of the stretch,² When held stretched, the molecules may or may not be able to rearrangę tliemselves; but as soon released, molecular motion sets in and a morę probable extent of curvature, which implies a shortening of the materiał, is attained at a ratę depending on the temperaturę and the viscosity. There are so many morę complexions possible in which molecules are not straight than those in which they are that the re-establishment of a morę probable coiled-up condition, and consequent elastic recoyery, is a direct result of the natural tendency for entropy to rise.

The process is very similar to the diffusion of a gas. If a gas is held at a volume V under pressure P at a temperaturę T, and the volume of the con-taining vessel is suddenly doubled, the gas will

and polystyrene

| Monostyrene is <f>. CH = CH_S) -CH .C.CH.C. CH—etc.

| H_a | H_a g jf — C₉H₅)

I The tremendous stretching power of rubber was illustrated in the rubber section of the Paris Exhibition (1937) by showing that an unstrained strand of rubber reaching from Paris to Amsterdam could be stretched to reach from Paris to Moscow without rupture. Fortunately this has no political significance i

into the double volume in approximate HHH| with the simple gas law

_ftCCOra<*    py _ R-p

1*1 i may be said to be due to the extremely Iow obability that all the gas molecules will remain P^r°₀ne half of the vessel. An exactly analogous law has been derived for the elastic behaviour of rubber. Sa=XT,

where S is the stress, | the strain and X a complex parameter. In rubber the links connecting the atomie groupings are much morę solid, or at least have a higher viscosity in one dimension than in the other two. The so-called “solid linkages” have viscosities of the order of io¹⁰ poises, compared with io⁷ for the other two dimensions. Many other materials, both artificial and natural (e.g., muscle), have what is known as a “ rubber structure.'’ Morss is unable to determine the crystal structure of stretched rubber completely by X-rays, but con-cludes that carbon atoms in the molecular chain are not coplanar.

Whether Mark’s explanation is accepted or not, it is striking that isotherms plotted on the basis of this eąuation give surprisingly good straight lines up to 300-400 per cent. deformations. How to reconcile the Mark and Astbury ideas is not yet elear.

It has been shown that soft rubber contracts when heated and that, if stretched when cold, it will retain some of that extension until further applica-tion of heat causes it to contract again. This effect is even morę marked with raw than with vulcanised rubber.

Muscle shows similar effeets. In molecular structure it consists of two interpenetrating systems, a netted non-relaxing system, and a system of free