The x-ray diagrams of some rubbers give a long spacing, which was first reported by Clark and collaborators. According to them, it is found only in gel rubber, where it has the value of 54 a.u., but it is absent in sol rubber. This spacing is oriented in stretched rubber, the x-ray reflections contracting into two spots in the equatorial plane. This means that the long spacings lie in planes at right angles to the fibre axis. As we found it difficult to account for this result and, furthermore, had indications that this long spacing might depend on the nature of the rubber, we examined a number of specimens of different origin and preparation. We used a camera of about 10-inch radius, reducing the air scattering by inserting an evacuated length of wide tube with thin cellophane windows at both ends. The x-ray source in these experiments was the 50 kw. x-ray generator of this laboratory, working with an input of 30 to 40 kw.

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