The poisonous nature of any lead compound from an industrial point of view is proportional to (1) the size of the ultimate particles of the substance manufactured, and therefore the ease with which such particles are capable of dissemination in the air; and (2) the solubility of the particles in the normal fluids of the body, such as the saliva, pharyngeal and tracheal and bronchial mucus, etc., and the fluids of the stomach and intestine. An instance of the variation in size of the particles of lead compounds used industrially is the difference between ground lead silicate (fritted lead) used in the potteries, and the size of the particles of ordinary white or “raw” lead. By micrometric measurements one of us [K. W. G.[1]] found the average size of the particles of fritt to be ten times that of the white lead particles. Further, direct experiment made with equal masses of the two compounds in such a manner that the rate of settling of the dust arising could be directly compared in a beam of parallel light showed presence of dust in the white lead chamber fifteen minutes after the fritt chamber was entirely clear. It is found as a matter of practice that where dust is especially created, and where it is difficult to remove such dust by exhaust fans, the greatest incidence of lead poisoning occurs. The association of dusty processes and incidence of lead poisoning is discussed in relation to the various trades in Chapters XV. to XVII. Fume and vapour given off from the molten metal or compounds, such as chlorides (tinning), are only a special case of dust. The channels through which lead or its compounds may gain entrance to the animal body are theoretically three in number: 1. Respiratory tract. 2. Gastro-intestinal. 3. Cutaneous. For many years most authorities have held that industrial poisoning by means of compounds of lead takes place directly through the alimentary canal, and that the poison is conveyed to the mouth mainly by unwashed hands, by food contaminated with lead dust, and by lead dust suspended in the air becoming deposited upon the mucous membrane of the mouth and pharynx, and then swallowed. As evidence that lead dust is swallowed, the classical symptom of colic in lead poisoning has been adduced, on the supposition, in the absence of any experimental proof, that the lead swallowed acted as an irritant on the gastro-intestinal canal, thus causing colic, and, on absorption from the canal, setting up other general symptoms. Much of the early treatment of lead poisoning is based upon this assumption, and the administration of sulphuric acid lemonade and the exhibition of sulphate of magnesia and other similar compounds as treatment is further evidence of the view that the poisoning was considered primarily intestinal.