With modern ideas of erection of factories to guard against the introduction of direct sunlight, it was hoped that this defect had practically ceased to exist. In one grade of rubber it would be expected that tackiness would continue to appear. Earth-rubber, often exposed to direct sunlight for a week, would naturally become tacky, and this tackiness cannot be avoided unless the earth-scrap is to be collected more frequently. But in many cases even the higher grades of rubber show signs of tackiness. Experiments have been carried out at various times and in various places to determine the cause of tackiness. For some time the theory of bacterial origin was in favour, but none of the experimental results was convincing. Bacteria may be present in tacky rubber; but, on the other hand, many cases of bacteria in rubber have been observed in which there was no tackiness. Experiments were made by one of us some years ago with a view to testing the bacterial theory by inoculating latex with small pieces of tacky rubber. In opposition to the results which were stated to have been obtained, there was no spread of tackiness. Other investigators have obtained similar results. One writer proposed to explain tackiness as caused by excess of moisture. This perfectly simple explanation unfortunately displays only a profound ignorance of the subject, and does not take into account the fact that tackiness is incident in rubber after dryness has been reached. It need not be pointed out to planters in Malaya that wet sheets of rubber are often exposed to direct sunlight by workers of native holdings, with no resulting harm as long as plenty of moisture is present in the rubber. TACKINESS THE RESULT OF A SLOW PROCESS OF CHANGE.--As stated above, tackiness does not appear until the rubber is dry, and even then it is to be noted that it is possible for tackiness to appear in rubber arriving in London, which showed no indications of tackiness when packed for shipment. TACKINESS CAUSED BY TRACES OF COPPER SALTS.--Spence, as the result of investigations, has pointed out that none of the various theories put forward to account for tackiness--viz., the action of bacteria, premature putrefaction, oxidation, excess of moisture, the action of enzymes, etc.--have any basis in scientific proof, and believes that the cause of tackiness cannot be directly attributed to bacteria. It has been stated that the only known way of causing rubber to become tacky is to expose it to sunlight or heat. While agreeing that in the ordinary way this statement is correct as far as one rules out the employment of chemical substances, it must be pointed out that tackiness of the worst degree may be caused by the presence of traces of copper or copper salts. This point has already been touched upon in a preceding paragraph dealing with the defect of "green streaks" in pale crepe rubber.