Susceptibility and immunity to poisoning by lead may be considered, according to the type of lead compound absorbed, further in its relation to age and sex. All compounds of lead are not poisonous in the same degree; the more easily soluble compounds are more poisonous than the less soluble. On the other hand, compounds which appear at first sight unlikely to produce poisoning may do so; for instance, fritted lead or lead silicate, a substance largely used in the potteries as a glaze, and manufactured by fusing together litharge and a silicate, would appear at first sight to be quite an innocuous substance. Owing to its method of preparation, however, it is not a pure compound of lead and silica, but contains lead oxide, metallic lead, etc., entangled in its meshes, and experimentally one of us (K. W. G.) has demonstrated that such a compound may be acted on by the tissues of the body, both when injected subcutaneously and even when inhaled, and so gradually produce definite symptoms of lead poisoning, but at a much slower rate than the more poisonous lead compounds. The fineness of division in which the compound of lead exists is another factor affecting its poisonous nature; the more finely divided particles find their way into the lung more easily than the coarser particles. Various subsidiary matters may also determine the susceptibility in a given individual, and of these a certain number require mention, as they probably act as definite predisposing factors. Age and sex may be regarded as predisposing factors to lead poisoning, and certain diseases also. =Age.=--Young persons are regarded as more liable to lead poisoning than adults, although it is difficult to obtain definite figures on the point, the duration of employment acting as a disturbing factor in estimating the susceptibility of young persons. They may have worked in a lead works for a year or more without showing any signs of poisoning, but develop them later in adult life, although it is very likely that absorption had taken place during the earlier period. In the Report of the Departmental Committee on the Use of Lead in the Potteries (Appendix XII.), the attack rate for the period 1899 to 1909 for young persons is 19·3 per 1,000, and for adults 18·8 for the same period, but the figures upon which these attack rates are based are too small to build any conclusion. The general clinical conclusions of appointed surgeons and certifying surgeons in the various lead factories would be, we believe, that the susceptibility of young persons is at least twice that of adults, and there is some ground for supposing that the tissues of an adult when growth has ceased more readily adapt themselves to deal with the absorption and elimination of poisonous doses of lead than do the tissues of a young person.