We learn by the " London Expositor " that Dr. Lyon Playfair recently delivered a lecture before the Royal Institution on the above interesting subject, an abstract of which we shall present with comments. One class of substances taken as food supply the fuel for the system, and other substances supply the materials that form the flesh and the bones. A man absorbs about 700 lbs. of oxygen in a year, to support the combustion of 350 lbs. of carbon equal to 1050 lbs. of CO.2. The consumption of the carbon in the system (fat is principally composed of it) is so great that it would be all exhausted in about three days,, if it were not re-supplied by food. As the temperature of the body is always the same under every climate, those who live in the colder regions of the earth require food which contains a larger amount of carbon, than those who live in warmer climates ; this is for the purpose of promoting a greater amount of combustion—a larger fire as it were—consequently a man living in the arctic regions will inhale more than double the amount ot oxygen of a man living in the tropics, in one year. Fire and warm clothing, however, diminish the necessity of eating so much food in the cold regions, hence farmers who keep their cattle warmly housed in winter, are enabled to maintain them in better condition and with less food than those who have not warm and well sheltered stables. Substances used for food, which contain a great amount of heat, power, are oils, tat meat, sugar, and rice. Flesh giving substances are .those which contain a great amount of nitro-R gen, such as lean meat, peas, cheese, &c. As different kinds of solid food produce different effects in the nutriment of the body, it is requisite that in a well-regulated dietary, that the proportions ot the flesh giving and heat producing foods should be properly adjusted, taking into consideration, age, employment, and climate. Dr. Playfair alluded to a very prevalent opinion respecting changes in the human body, viz., that the entire substance ot it changes in seven years. He could not conceive on what foundation this opinion rested, for judging from chemical decompositions, it might be assumed that an entire change takes place in the human body in forty days, rather than seven years, and some parts of the human body changed much faster than this. In reference to the much-disputed question of the relative values of animal and vegetable food, he observed that there could be no difference between them, chemically speaking, for all animals derive their nutriment from vegetahle matter, either eaten directly or after it has formed part of the organism of an herbiverous animal. He believed, however, that there was much truth in the observation, that the character of a nation depends on the lood of a people; hence, he said, " we may attribute the passion for honor and glory in the French, and the excitable temperament of the Irish to the vegetable diet, whilst the sound sense of the Englishman may be attributed to his beef and beer." The practical conclusion he arrived at was, that the regimen of roast beef and beer should be given to the Irish, in order to bring them up to the character ot the English, and to do this a potato diet should be discountenanced. The character of the Irish, according to Dr. Playfair, must have entirely changed, within 200 years, for the potato is no older in Ireland ; and good sense is dependant on beef and beer. These are certainly very funny conclusions, but neither a desire ror military glory, an excitable temperament, nor sound sense, are dependant upon the general food used by any people ; all history is an evidence of the truth of what we assert, or else we must be forced to the conclusion that the rise, progress, and downfall of all the great nations of antiquity were the results of the food taken at different periods in the history of those nations ; which is absurd. A writer in the Granite Farmer positively asserts that the " effects of poison ivy can be cured by eating a lew of the small green leaves of the pine."