extraction of raw rubber to remove resinous matter has but little effect on the vulcanising properties of a mixture of rubber and sulphur. But if litharge be a constituent, it is found that acetone-extracted rubber will hardly vulcanise at all. From this, it follows that a rubber giving a low acetone extract may be found to vulcanise exceptionally slowly in a mixing containing litharge, whereas it shows no such defect when compounded with sulphur only.[51] Litharge is used to a very large extent, as it has a balancing effect in a rubber compound--that is to say, it allows of appreciable variation in vulcanising conditions, without corresponding alteration in the state of cure.[52] [51] _Journal Soc. Chem. Ind._, 1916, p. 874. [52] _Ibid._, 1915, p. 524. INFLUENCE OF VARIOUS FACTORS IN RAW RUBBER PREPARATION ON THE "RATE OF CURE," OR "RATE OF VULCANISATION."--As the capacity of a rubber for vulcanisation depends on the presence of small quantities of accessory substances in the serum which act as catalysts, the rate of vulcanisation (or curing) will depend on the nature and quantity of such substances present in the rubber. A very small quantity of these substances has a considerable influence on rate of vulcanising, and as the substances are difficult to isolate and identify, our knowledge of their formation and chemical nature is not as definite as is desirable. Substances have been isolated having the characteristics of "simpler bases." Bodies of this class are formed by putrefaction of organic matter, and can be separated in much larger quantity from coagulated latex, which has been allowed to putrefy before working up than from such which has been worked up without giving time for an appreciable amount of putrefaction to take place. Further, rubber from putrefied coagulum vulcanised much faster than that ordinarily prepared, so that we are justified in connecting the putrefaction bases with the rate of vulcanisation. Moreover, it has been shown that any treatment of the latex or coagulum which inhibits the development of putrefactive organisms also prevents the rubber vulcanising as fast as would otherwise have been the case.[53] Also, the crude bases isolated from fast vulcanising rubber have the power of increasing the rate of vulcanisation when added to ordinary slow vulcanising rubber.[54] [53] Eaton and Co-workers: See Bulletin No. 27, F.M.S. Department of Agriculture. [54] _Journal Soc. Chem. Ind._, 1917, p. 365.