Niddah 55 - Substances
Some substances convey ritual impurity while they are moist, but not after they dry up. These include the emission of a zav (a special white discharge from a male member), his phlegm and saliva, creepy crawly animals (such as rats), an animal carcass, and human semen. Others, however, convey impurity both while moist and also when dry, and this includes the blood of a niddah, and the flesh of a corpse.
Even those substances about which we said that they loose their capacity to convey impurity when dry (such a spit of a zav), if they can be soaked and regain their nature, in such instances they continue to convey impurity even when dry. How long should they be soaked for this test? - In warm water, for a full day.
How are these laws derived? From extra words or letters in the Torah. For example, when the Torah said, "Zav and his emission," or literally "emission of his," the extra words "of his" teach that his emission is compared to him in the degree of impurity that it conveys.
Art: Santiago Rusinol i Prats - Convalescent