This division includes all substances, themselves inert, which are employed to cover exposed surfaces, so as to guard them against irritating agents. it is obvious that they must be themselves wholly un-irritating; as otherwise, they would directly contribute to produce or increase what they are intended to prevent, diminish, or remove. Hence the great practical importance of ascertaining that the substances used are pure, and uninjured by exposure, fraudulent admixture, or careless preparation. The physician is often foiled in his purpose by want of attention to this object.