The crackling noise which several salts make when suddenly heated, accompanied by a violent exfoliation of their particles. It has been attributed to the sudden conversion of the water they contain into steam; " but," observes Dr. Ure, "it is the salts which are anhydrous, or contain no water, that decrepitate most violently; those that contain water generally enter into tranquil liquefaction on being heated." Salts decrepitate for the same reason that glass, quartz, and cast iron crack with an explosive force, on being suddenly heated, namely, from the unequal expansion of the laminae which compose them, in consequence of their being imperfect conductors of heat.