Leviticus 11:37-38

And if any part of their carcase fall upon any sowing seed which is to be sown, it shall be clean.

If any part of their carcass fall upon any sowing seed ... it shall be clean - because the impurity of the contact would be absorbed by the earth. But the case was very different if the seed had been immersed in water contaminated by a carcass-for the grains, being moistened by the water, would imbibe the uncleanness, the taint of which would be transmitted to the future produce. These regulations must have often caused annoyance by suddenly requiring the exclusion of people from society as well as the ordinances of religion. Nevertheless they were extremely useful and salutary, especially as enforcing attention to cleanliness.

This is a matter of essential importance in the East, where venomous reptiles often creep into houses, and are found lurking in boxes, vessels, or holes in the wall; and the carcass of one of them, or a dead mouse, mole, lizard, or other unclean animal, might be inadvertently touched by the hand, or fall on clothes, skin bottles, or any article of common domestic use. By connecting, therefore, the touch of such creatures with ceremonial defilement, which required immediately to be removed, an effectual means was taken to prevent the bad effects of venom and all unclean or noxious matter.

Of the importance of these regulations, and their absolute necessity to prevent accidents from poisoning, Michaelis gives the following proofs: 'Of the poisoning of liquors by toads creeping into casks we often read; and Hasselquist relates an instance where the poison of a gecko in a cheese had nearly proved fatal. Mice and rats likewise sometimes poison meat that is uncovered, by means of the poison laid for themselves being vomited upon it. I remember the case of a brewing of beer which, to all the people of a town who had drank it, occasioned most violent agonies, and in regard to which, although it was most peremptorily denied by the magistrates and the brewers, there appeared perfectly good reason for believing that arsenic had in this manner gotten among the malt.'