VARIATION IN PHYSICAL PROPERTIES.--A very large number of tests on vulcanised specimens of plantation rubber have been carried out. The rubber was almost invariably mixed with 7 to 10 per cent. of sulphur, and no other ingredient, and vulcanised to give the maximal breaking load. Unfortunately, this determination is subject to a very appreciable experimental error, so that a large number of determinations are necessary to give a reliable figure. It is quite impracticable to make a large number of determinations in routine testing, on account of the labour involved. It is usual to make five, or possibly ten, determinations, although some investigators have been content with two. It is generally conceded that any exceptionally low figures should be ignored, as probably caused by some flaw or irregularity in the test piece. On the other hand, a study of actual determinations shows an occasional excessively high figure, and it is questioned whether this also should be left out of account. Others ignore all except the highest figure, and take this to represent the true breaking strain. As a consequence, the figures published by different workers show considerable variation. De Vries has analysed a large number of the figures obtained in systematic examination of estate samples, and has constructed curves to illustrate the results.[47] It is open to question how far the variations shown are attributable to experimental error. The figures show, however, that the variation in breaking strain is relatively small, and not very different for crepe and sheet rubber. In our opinion, undue importance should not be attached to very high or exceptionally high figures for breaking strain, which are occasionally met with. Provided the figure does not fall much below the average, the sample may be regarded as satisfactory. It is very seldom that any sample of first latex estate rubber does not show satisfactory figures. [47] "Estate Rubber," p. 466. THE RATE OF CURE OR RATE OF VULCANISATION is subject to more exact measurement, whether this be based on the physical or the chemical properties of the rubber. If the testing machine be provided, as is usual, with an autographic attachment, the position of the curves traced on the recording paper gives a measurement of the rate of cure. These load-stretch curves, to which reference has already been made, take up a definite position in accordance with the physical properties; it is only the length of the curve, or the point where it terminates (which gives the breaking strain and elongation at break), which is largely fortuitous.