The Prophet ordered his followers to kill serpents and scorpions if they intruded at prayers; but on other occasions, he seems to have required first to admonish them to depart, and then, if they remained, to kill them. The Doctors, however, differ in opinion whether _all_ kinds of snakes or serpents should be admonished first; or whether _any_ should; for the Prophet, say they, took a covenant of the Jinn [probably after the above-mentioned command], that they should not enter the houses of the faithful: therefore, it is argued, if they enter, they break their covenant, and it becomes lawful to kill them without previous admonishment. Yet it is related that 'Áisheh, the Prophet's wife, having killed a serpent in her chamber, was alarmed by a dream, and, fearing that it might have been a Muslim Jinnee, as it did not enter her chamber when she was undressed, gave in alms, as an expiation, twelve thousand dirhems (about £300), the price of the blood of a Muslim.[76] The Jinn are said to appear to mankind most commonly in the shapes of serpents, dogs, cats, or human beings. In the last case, they are sometimes of the stature of men, and sometimes of a size enormously gigantic. If good, they are generally resplendently handsome: if evil, horribly hideous. They become invisible at pleasure (by a rapid extension or rarefaction of the particles which compose them), or suddenly disappear in the earth or air, or through a solid wall. Many Muslims in the present day profess to have seen and held intercourse with them. The Zóba'ah, which is a whirlwind that raises the sand or dust in the form of a pillar of prodigious height, often seen sweeping across the deserts and fields, is believed to be caused by the flight of an evil Jinnee. To defend themselves from a Jinnee thus "riding in the whirlwind," the Arabs often exclaim, "Iron! Iron!" (Ḥadeed! Ḥadeed!), or, "Iron! thou unlucky!" (Ḥadeed! yá mashoom!), as the Jinn are supposed to have a great dread of that metal: or they exclaim, "God is most great!" (Alláhu akbar!).[77] A similar superstition prevails with respect to the water-spout at sea, as the reader may have discovered from the first instance of the description of a Jinnee in the present work, which occasions this note to be here inserted.