In such case, the rubber is still in a highly porous condition, and it might be advanced that the heat of the smoke may help to maintain that condition. Thus the contained liquid might evaporate so quickly as to leave behind the dissolved substances in the minute cellular structure of the rubber. In other words, instead of the internal moisture exuding slowly to the surface in liquid form, it may leave the rubber, even in the first stages, in an evaporated condition, just as it does in the subsequent stages of drying. Thus no dissolved protein matter would be brought to the surface of the sheet and be deposited there. (_b_) The other theory also demands the first assumption propounded in the preceding theory, but subsequently perhaps is less feasible as it assumes a chemical action of which we have no definite knowledge. The idea is that as the rubber is in a porous condition, and is placed quickly in an atmosphere of smoke, the heat may maintain that condition to such a degree, that some constituents of the smoke may enter the rubber and cause the precipitation _in situ_ of the protein matter held in solution by the contained water or other liquid. The contained liquid would be water which has in solution possibly a very slight trace of the coagulant employed, of sugars, of protein matter, and of inorganic salts. Of these the substances which would evaporate would be probably the water and the coagulant in most cases. If a salt had been used as a coagulant, the dissolved trace would be deposited within the rubber in this case, whereas if a rich latex had been employed or a thicker sheet made from more dilute latex, some of the salt would be brought to the surface and there deposited together with the protein matter. This has actually been experienced in practice, and it has been possible to remove minute crystals from the edges of the rubber so prepared. It will be evident that in order for either theory to contain an element of probability, the rubber must be soft (porous) when placed in the smoke-house, and must also be fairly thin. It is observed in all cases where the method has been successfully employed that both these conditions are generally fulfilled--at all events the rubber is fairly thin. When thicker sheets are made, either from rich latex or from a deeper layer of comparatively dilute latex, the method is not uniformly successful. OTHER VIEWS ON "RUST" CAUSATION.--Later experimental work on "rust" formation by Hellendoorn[22] leads to the observation that "rustiness" is caused, not actually by the deposition of original serum-substances, but by the decomposition thereof, under the action of aerobic micro-organisms.