So far the evidence which has been brought before you, that chronic irritation has a causal connection with cancer, has been of a circumstantial nature: it has often enough been found in what we may call suspicious circumstances, but that does not prove that by itself it can directly cause the disease. If a man is seen hanging about the place where a burglary has been committed, it does not prove that he participated in it. He may be a burglar, or he may be what lawyers call an accessory before the fact, and before we can feel reasonably sure that he is a guilty party we must, unless we can actually see him committing the crime, find that whenever he is present, and he has a chance, a burglary takes place. Now in scientific investigation we can do what in ordinary life is not possible; we can take our burglar, arrange a set of suitable circumstances and see what happens and with what degree of regularity thefts occur. In the last four or five years something like this has been done on a large scale with cancer, and a large body of evidence is accumulating which suggests that, given suitable circumstances, chronic irritation will produce cancer with a fair degree of regularity, at least in some places. If it will do so in some places there is no reason to doubt that, under circumstances which for the moment we do not quite understand, it will do so in all the places where cancer is found. That this is so has not yet been completely proved, but I think there is a good deal of evidence along this line. It has been known for a great number of years that certain skin cancers are constantly found in people whose occupations necessitate their skin being in contact with certain chemical irritants. For instance, the workers in shale oil are often afflicted with cancer of the skin. In the spinning industry, when reaching over to deal with the machinery, a place on the worker’s leg is always rubbing up against an oily spindle. This process goes on for years at the same spot, and these people are found frequently to get cancer, beginning at the irritated place. Some aniline dyes are excreted in the urine, and growths of the bladder are very frequent in aniline workers. In India, some native tribes carry little metal boxes containing charcoal next to their skin in order to warm themselves, and the warmed spot frequently becomes the seat of a malignant ulcer. Further, in chimney-sweeps, whose skin is always more or less impregnated with carbon, we find that cancer frequently develops in those places where the soot is difficult to wash completely away and often is not cleaned off for years at a time. Finally, we have the well-known examples of skin cancer among X-ray workers, and mouth-cancer in those who chew betel nut.