And he shall look on the plague,.... That which is taken or suspected to be one, being pointed unto by the owner of the house:and, behold, [if] the plague [be] in the walls of the house; for there it chiefly was, if not solely; and from hence Gersom infers that it must be a walled house, and that it must have four walls, neither more nor fewer; and with this agrees the Misnah , according to which it must be four square; the signs of which were, when it appeared,with hollow strakes, greenish or reddish, which in sight [are] lower than the wall: these signs agree with the other signs before given of leprosy in men and garments; the first, the hollow strakes, which are explained by being lower in appearance than the wall, a sort of corrosion or eating into it, which made cavities in it, answer to the plague being deeper than the skin of the flesh in men; and the colours greenish or reddish, or exceeding green or red, as Gersom, are the same with those of the leprosy in clothes; and some such like appearances are in saltpetre walls, or in walls eaten by saline and nitrous particles; and also by sulphureous, oily, and arsenical ones, as Scheuchzer observes , and are not only tending to ruin, but unhealthful, as if they had rather been eaten by a canker or spreading ulcer; who also speaks of a fossil, called in the German language "steingalla", that is, the gall of stones, by which they are easily eaten into, because of the vitriolic salt of the fire stone, which for the most part goes along with that mineral, which is dissolved by the moist air.
Though this leprosy, in the walls of a house, seems not to have risen from any natural causes, but was from the immediate hand of God; and there have been strange diseases, which have produced uncommon effects on houses, and other things: in the times of Narses is said to be a great plague, especially in the province of Liguria, and on a sudden appeared certain marks and prints on houses, doors, vessels, and clothes, which, if they attempted to wash off, appeared more and more .
Misn. Nagaim, c. 12. sect. 1, 2. Physica Sacra, vol. 3. p. 330, 331. Warnefrid de Gest. Longobard. l. 2. apud Scheuchzer.