Either physical or chemical methods may, therefore, be used for determining the rate of cure of ordinary sheet or crepe rubber, but great care must be taken when interpreting the results obtained with rubber prepared in an unusual manner. The rate of cure may be expressed in terms of the time taken to vulcanise the rubber at a constant temperature (in our case 138° C.), so as to give an elongation of 850 per cent. at a load of 130 kilos, or to give a coefficient of five units. The higher the figure so obtained, the slower curing the rubber. To express the results more directly as rate of cure, we have adopted the plan of taking an average crepe rubber, calling the rate of cure 100 units, and expressing the rate of cure of other samples in these terms. Thus, a sample which gave a coefficient of four only, in the time taken by the standard to give a coefficient of five, would have a rate of cure four-fifths of the standard, that is, 80; or if a sample takes only two hours to give an elongation of 850 per cent., whereas the standard takes three hours, the rate of cure of the sample will be 3/2 of standard or 150.[50] [50] _Journal Soc. Chem. Ind._, 1918, p. 280. As stated, the coefficient is approximately directly proportional to the time of cure; it is also independent of the proportion of sulphur, if in fair excess, and in the presence of inert ingredients. It is also independent of the amount of mastication given to the original raw rubber, however great. On the other hand, the position of the load-stretch curve is variously modified by these factors--in some respects, therefore, the coefficient is a more reliable index. However, the coefficient is influenced by accelerators, so that here also great care must be exercised when interpreting results. For the purpose of detecting variations in rate of cure, it is best to choose a mixing which is particularly sensitive. In the first place, there must be an ample excess of sulphur; and in the second place, no ingredient should be added which will complicate the load-stretch curves, and no accelerators should be present which may possibly tend to obscure the vulcanising properties of the rubber itself. It has been found, therefore, that the best mixing to use consists of rubber with an excess of sulphur--say, in the proportion 9:1 without other ingredients. The rate of cure of a specimen of plantation rubber is attributed to the presence of certain natural vulcanising catalysts, because it is found that carefully purified raw rubber (that is, with the resinous and nitrogenous constituents removed) vulcanises very slowly or hardly at all, but that on replacing the extracted matter the rate of vulcanising is restored. The natural catalysts contained in the extracted matter are influenced to a varying degree by some of the common ingredients of manufactured rubber articles. This applies particularly to litharge (oxide of lead), to which reference has already been made. Thus, acetone