In the absence of an anti-oxidant the degree of oxidation may vary daily and in different batches of latex on any one day, so that there is always the possibility of a lack of uniformity due to oxidation effects. This would be more evident in air-dried sheets than in smoke-cured rubber, as in the latter case the darkening of the surface would be masked by the colour induced by the smoke-drying process. To obviate this variation anti-oxidants are used on most estates, but the accidental or misinformed abuse of these chemicals may lead to further lack of uniformity. Hence it is necessary to follow carefully the formulæ prescribed by experience. COLOUR OF SMOKED SHEETS.--It may be of interest to note that the effect known as oxidation is attributed to the presence of micro-organisms called enzymes (ferments) in the latex. It can also be produced artificially in various ways--_e.g._, by the use of the crude product of wood-distillation (pyroligneous acid) as a coagulant, or by the addition to the latex of small quantities of a phenol such as carbolic acid. It is thus possible to prepare in sheet form a rubber which has the appearance of having been smoke-cured, although it may never have been in a smoke-house. It will be clear, therefore, that apart from other causes, the colour of the cured sheets may be influenced by oxidation of the fresh coagulation, and by the constituents of the smoke. It follows that the smoke from timbers which are richer than others in certain chemical bodies set free by combustion will produce a rubber darker in colour. There is thus no real connection between colour and period of cure, although in a general sense the longer the interval the darker the colour. Similarly it is now plain that when anti-oxidants are employed in excess the paleness of the rubber is in no degree truly indicative of the period during which the rubber has been smoke-cured.