This idiosyncrasy may even be acquired. One of my relations, who was for many years violently affected by very small quantities of the richer kinds of fish, used at a previous period to eat them, and can now again do so, with impunity. Many people have acquired a similar idiosyncrasy with respect to eggs; instances of the same kind will be afterwards mentioned in respect to shell-fish, particularly muscles; indeed there are probably few articles of food in regard to which such idiosyncrasies may not in a few rare instances be met with, if we except the grains and common kinds of butcher-meat. I may add, that from facts which have come under my notice, I have sometimes suspected that a similar idiosyncrasy may be acquired in a slight degree, and for a short time only, in regard even to some kinds of butcher-meat, especially the flesh of young animals and pork. On this subject some illustrations will be found at the close of the chapter on diseased and decayed animal matter. It does not appear well ascertained, that the effect of idiosyncrasy is ever to impair materially the energy of poisons, except in the instances of mercury, alcohol, and opium. On the contrary, the tendency of _habit_ when it does affect their energy, is, with a few exceptions, to lessen it. By the force of habit a person may take without immediate harm such enormous quantities of some poisons as would infallibly kill an unpractised person or himself when he began. There have been opium-eaters in this country who took for days together ten or even seventeen ounces of laudanum daily. The influence of habit has been ascertained precisely in the case of a few common poisons only. On the whole, it would appear that more change is effected by habit in the action of the organic than in that of the inorganic poisons; and that of the former, those which act on the brain and nervous system, and produce _narcotism_, are altered in the most eminent degree. The best examples of the influence of habit are opium and vinous spirits. The action of such poisons is not always, however, entirely thrown away; they still produce some immediate effect; and farther, by being frequently taken, they may slowly bring on certain disease, or engender a predisposition to disease. A very singular exception to this rule prevails in the instance of tobacco; which, under the influence of habit, may be smoked daily to a considerable amount, and, so far as yet appears, without any cumulative effect on the constitution, like that of opium-eating or drinking spirits.