[185] For quantity in such compounds is measured by fixed limits and rules, and it is not safe either to stop short before one has reached them, nor to advance beyond them. For if too little be applied, it relaxes, and if too much, it strains the natural powers; and each extremity is mischievous, the one from its impotence being capable of producing any effect at all, and the other by reason of its exceeding strength being necessarily hurtful. Again it is very plain with reference to smoothness, and roughness, and thickness, and close compression, or on the other hand leanness and slackness, how very much influence all these differences have in respect of doing good or harm.