H. C. Pearson has pointed out that the natives of some parts of the Amazon districts are accustomed to mix sulphur with the latex of Hevea before employing it for waterproofing. True combination between the rubber and the sulphur, however, apparently only occurs at a high temperature. We have already distinguished in the last chapter between hot and cold vulcanisation. Under hot vulcanisation again two processes may be separately considered. Of these, the more important is the dry process originally patented by Goodyear. In this process the proper amount of sulphur is incorporated with the rubber in the mixing rollers before the articles are made up, and vulcanisation is effected by heating either directly in steam under pressure or in steam-jacketed chambers, or in hydraulic presses heated by steam in various ways.
Hancock's wet process, on the other hand, is chiefly of historical interest In this process articles made of sheet rubber, without any previous admixture of sulphur, are immersed in molten sulphur for a certain time and at a certain temperature.
Finally, in the cold process, patented by Parkes in 1848, the articles are immersed in a solution of chloride of sulphur, in carbon bisulphide, or in benzene. This process is exceedingly rapid, and can only be applied to articles of very thin sheet