This defect has caused more trouble during recent years than any other. It is not proposed here to argue the question as to whether the presence of this film, which appears when some rubbers are stretched, is detrimental to the physical qualities of the product on vulcanisation. With the mere statement of opinion that it could do no apparent harm, we may pass to the aspect of the case as it affects the buyer and consumer. If one were to judge by the attention drawn to the appearance of smoke sheet-rubber after it has been stretched and allowed to retract, one would imagine the defect to be of comparatively sudden and recent incidence. This is not so. The peculiarity must have existed for years, and perhaps became more marked as so many estates abandoned the former common practice of allowing varying quantities of water to be placed in the collecting cups. As the substances which cause the defect to be visible are partially soluble in water, it would follow that when working with the very dilute latices which were characteristic of the earlier years of the plantation industry, the remaining liquid in the pan after coagulation would contain an appreciable quantity of soluble substances which would otherwise have been retained in the coagulum. Conversely, the richer the latex, the greater the percentage of protein matter retained in the coagulum. In the case of very rich latex, it must be within the knowledge of every manager that the quantity of remaining liquid in the pans would be almost nil. We may assume that the greater part of these soluble proteins would be enclosed in the structure of the rubber, but as the fresh coagulum must retain a quantity of liquid amounting to from 60 to 70 per cent. by weight (we are now referring to rich latices), it follows that some of the soluble protein matter must be removed when the coagulum is placed under pressure. Even after the pressure is released more of the contained liquid will exude from the surface of the rubber; and from experience it is easy to imagine that this exudation, becoming progressively feebler, will continue until the rubber begins to dry. Then, with the evaporation of the surface moisture, the protein matter, either in original form or as a degradation product, remains on the surface of the rubber as a thin, solid film or crust. As drying continues, the interior moisture escaping through the pores of the rubber evaporates, leaving behind the substances hitherto held in solution. Should, however, the sheet be thick and/or the temperature of drying low, the rubber may dry first on the outside, forming a thin skin of dry rubber, which delays further drying indefinitely.