It is of the greatest consequence to ascertain the extent of the influence of infection, for the means of avoiding and preventing it will very much depend upon this. It is now known, that infection extends itself to a very small distance. There are, indeed, some morbid poisons, such as that of the bite of a mad dog, and that of the venereal disease, which require actual contact to make them take effect. Others are more volatile, and seem to he inhaled by the breath, or absorbed by the skin, but these do not extend far. That of the plague[64] does not reach above a few yards, and that of the small-pox and of fevers is probably equally limited. This discovery is very valuable, by ascertaining the limits of danger; for when a person imagines he runs the same risk when at a considerable distance from the seat of disease, as if he were in contact with the person affected, he will be apt to expose himself wantonly and unnecessarily to the infection. It seems to be owing to the ignorance of the extent of its influence, that the plague has in general been so fatal; for in consequence of the opinion that the whole surrounding atmosphere was affected, it was vainly attempted to purify it by large fires in the open air, or by [65]firing off artillery, instead of trusting to the separation of the sick so as to avoid their near approach, and to the confinement of those in health to their own houses, which are all the precautions necessary to prevent its progress. CHAP. II. Of ALIMENT. SECT. I. Of SOLID FOOD. The most unnatural circumstance in a sea life is the food which men use, and the disease most peculiar to it is one which is owing chiefly to the nature of the aliment; for though other causes conspire in aggravating the scurvy, the depraved state of the INGESTA is the main and fundamental cause of it.