The effects of the processes of cookery on vegetable substances, though usually very slight and simple, are in some instances both striking and unexpected. For example, some sorts of vegetables are extremely acrid and even poisonous in their crude state, and altogether unfit for human food; yet, by simply boiling them in water, they become bland, sweet, and wholesome. Several species of _arum_ (cuckoo-pint), which are very acrid, and would be dangerous to use raw, become quite palatable pot-herbs when boiled. Their acrimony must reside in a very volatile principle, which, during the boiling, makes its escape, or is chemically altered; but the nature of this principle has not yet been accurately investigated by chemists. A more familiar example than this is found in onions, leeks, and garlick, whose acrimony and strong odour can be almost destroyed, or rather driven off by a sufficiently long application of heat, either directly, or through the medium of water. Many other instances could be given, but we shall content ourselves with one more. Every body knows that potatoes, in a raw state, are nauseous and unpalatable. It is not, perhaps, so generally known that the potatoe, (_solanum tuberosum_,) belongs to the night-shade genus of plants, which are all more or less poisonous. If potatoes were used raw, in any quantity, they would be deleterious to man; nor does it disprove this that cattle eat them with impunity, as sheep and goats eat plants much more strongly poisonous to man, such as hemlockdropwort, [_oenanthe crocata_;] and waterhemlock, [_phelandrium aquaticum_]. By boiling or roasting, however, all the unpalatable and all the unwholesome qualities of the potatoe are changed, and it becomes farinaceous, wholesome, digestible, and highly nutritious. Yet, although this change is remarkable, and could scarcely have been anticipated, very little is lost and nothing is added to the potatoe by either roasting or boiling, yet its immediate constituent parts have evidently suffered a very great chemical alteration, chiefly, in consequence it should seem, from the farinaceous substance being acted on by water. Vegetables, when used as food, are most commonly boiled, and seldom baked or roasted. Salads, indeed, are eaten raw, without any application of heat. The chemical action of heat on pot-herbs, on esculent roots, and leguminous seeds, does not appear to be confined to the mere softening of their fibres, or to the solution or coagulation of some of their juices and component parts; for we have just now seen that their flavour, and other sensible qualities, as well as their texture, suffer a remarkable chemical change, which greatly improves their alimentary properties.