Section B. (Chemistry and Mineralogy), Literary and Philosophical Society's Meeting Hall, George-street.

Section C. (Geology and Physical Geography), Athenæum Lecture Theatre, George-street.

Section D. (Zoology and Botany), Council Room of the Natural History Society's Museum, Peter-street.

Section E. (Medical Science), News Room of the Mechanics' Institution, Cooper-street.

Section F. (Statistical Science), Refreshment Room of the Assembly Rooms, Mosley-street.

Section G. (Mathematical Science), Lecture Theatre of the Mechanics' Institution, Cooper-street.

Each of the sectional meeting rooms has an adjoining room, in which the respective committees of sections meet every morning at ten. The following paper occupied Section B. on Thursday :—

Dr. Playfair said that Professor Liebig had been requested, some few years ago, to apply himself to the consideration of questions in vegetable and animal physiology. The professor's first letter had been read at the meeting of the association at Glasgow, in the year 1840. The second letter he was about to bring before their notice. And in his third letter, the professor intended to apply the principles of organic chemistry to diet and dietetics; and under this head would be comprised the nutriveness of particular vegetables in the fattening of cattle. This third report, it was at first supposed, would occupy Professor Liebig for two years; but he (Dr. Playfair) hoped he should have the pleasure to bring it before the association at their next annual meeting. It must be borne in mind that he had prepared his abstract for a mixed and general audience like the present, and which comprised, he was happy to observe, a number of ladies. The first part of Professor Liebig's report consisted of the examination of the processes employed in the nutrition and reproduction of the various parts of the animal economy. In vegetables, as well as in animals, we recognise the existence of a force in a state of rest. It is the primary cause of gowth or increase in mass of the body, in which it resides. By the action of external influences, such as by pressure of air and moisture, its condition of static equilibrium is disturbed; and, entering into a state of motion or activity, it occupies itself in the production of forms. This force has received the appellation of vital force, or vitality. Vitality, though residing equally in the animal and vegetable kingdoms, produces its effects by widely different instruments. Plants subsist entirely upon manures belonging to inorganic nature. Atmospheric air, the source whence they derive their nutriment, is considered to be a mineral by the most distinguished mineralogists. All substances, before they can form food for plants, must be resolved into organic matter. But animals, on

the other hand, require highly-organised atoms for nutriment. They can only subsist upon parts of an organism. They possess within them a vegetative life, as plants do, by means of which they increase

in size, without consciousness on their part; but they are distinguished from vegetables, by their faculties of locomotion and sensation-faculties acting through a nervous apparatus. The true vegetative life of animals is in no way dependent upon this apparatus, for it proceeds when the means of voluntary motion and sensation are destroyed: and the most energetic volition is incapable of exerting any influence on the contractions of the heart, on the motion of the intestines, or on the processes of secretion. All parts of the animal body are produced from the fluid circulating within its organism, by virtue of vitality, which resides in every organ. A destruction of the animal body is constantly proceeding. Every motion, every manifestation of force, is the result of the transformation of the structure, or of its substance. Every conception, every mental affection, is followed by changes in the chemical nature of the secreted fluids. Every thought, every sensation, is accompanied by a change in the composition of the substance of the brain. It is to supply the waste thus produced, that food is necessary. Food is either applied in the increase of the mass of a structure (i. e. in nutrition), or it is applied in the replacement of a structure wasted (i. e. in reproduction). The primary condition for the existence of life is the reception and assimilation of food. But there is another condition equally important-the continual absorption of oxygen from the atmosphere. All vital activity results from the mutual action of the oxygen of the atmosphere, and the elements of the food. All changes in matter proceeding in the body are essentially chemical, although they are not unfrequently increased or diminished in intensity by the vital force. The influence of poisons, and remedial agents on the animal economy proves, that the chemical combinations and decompositions proceed-ing therein, and which manifest themselves in the phenomena of vitality, may be influenced by bodies having a well-defined chemical action. Vitality is the ruling agent by which the chemical powers are made to subserve its purposes; but the acting forces are chemical. It is from this view, and no other, that we ought to view vitality.

According to Lavoisier, an adult man takes into his system, every year, 837 lb. of oxygen, and yet he does not increase in weight. What, then, becomes of the enormous quantity of oxygen introduced in the course of the year into the human system? The carbon and hydrogen of certain parts of the body have entered into combination with the oxygen introduced through the lungs and through the skin, and have been given out in the form of carbonic acid, and the vapour of water. At every moment, with every expiration, parts of the body are thus removed, and are emitted into the atmosphere. No part of the oxygen inspired is again expired as such, Now it is found that an adult inspires 32 oz. of oxygen daily. This will convert the carbon of 24 lb. of blood into carbonic acid. He must, therefore, take as much nutriment as will supply this daily loss; and, in fact, it is found that he does so; for the

ture.

average amount of carbon in the daily food of an adult man, taking moderate exercise, is 14 oz., which require 37 oz. of oxygen for their conversion into carbonic acid. But it is obvious, as the inspired oxygen can be removed only by its conversion into carbonic acid and water, that the amount of food necessary for the support of the animal body must be in direct ratio to the quantity of oxygen taken into the system. Thus a child, in whom the organs of respiration are naturally in a state of great activity, requires food more frequently and in greater proportions to its bulk than an adult, and is also less patient of hunger. A bird, deprived of food, dies on the third day; whilst a serpent, which inspires a mere trace of oxygen, can live without food for three months. The capacity of the chest in an animal is a constant quantity. We, therefore, inspire the same volume of air, whether at the pole or the equator. But the weight of the air, and consequently of the oxygen, varies with the temperaThus an adult man takes into the system daily 46,000 cubic inches of oxygen, which, if the temperature be 77°, weigh 32 ounces; but, when the temperature sinks down to the freezing point (32°), it will weigh 35 ounces. Thus an adult in our climate in winter may inhale 35 ounces of oxygen; in Sicily he would inspire only 28 ounces, and if in Sweden 36 ounces. Hence we inspire more carbon in cold weather, when the barometer is high, than we do in warm weather; and we must consume more or less carbon in our food in the same proportion. In our own climate, the difference between summer and winter, in the carpon expired, and therefore necessary for food, is as much as an eighth. Even when we consume equal weights of food, an infinitely wise Creator has so adjusted it as to meet the exigencies of climate. Thus the fruit on which the ininhabitants of the south delight to feed contains only twelve per cent. of carbon; whilst the bacon and train oil enjoyed by the inhabitants of the Arctic regions contain from sixty-six to eighty per cent. of the same element. Now the mutual action between the elements of food and the oxygen of the air is the source of animal heat. All living creatures, whose existence depends on the absorption of oxygen, possess within themselves a source of heat, independent of the medium in which they exist. This heat, in Professor Liebig's opinion, is wholly due to the combustion of the carbon and hydrogen contained in the food which they consume. Animal heat exists only in those parts of the body through which arterial blood (and with it oxygen in solution) circulates. The carbon and hydrogen of food, in being converted by oxygen into carbonic acid and water, must give out as much heat as if they were burned in the open air. The only difference is, that this heat is spread over unequal spaces of time; but the actual amount is always the same. The temperature of the human body is the same in the torrid as in the frigid zone. But, as the body may be considered in the light of a heated vessel, which cools with an accelerated rapidity the colder the surrounding medium, it is obvious that the fuel

necessary to retain its heat must vary in different climates. Thus less heat is necessary in Palermo, where the temperature of the air is that of the human body, than in the Polar regions, where it is about 90 degrees lower. In the animal body the food is the fuel, and by a proper supply of oxygen we obtain the food given out during its combustion in winter. When we take exercise in a cold atmosphere, we respire a greater amount of oxygen, which implies a more abundant supply of carbon in the food; and by taking this food we form the most efficient protection against the cold. A starving man is soon frozen to death; and every one knows, that the animals of prey of the Arctic regions are far more voracious than those of the torrid zone. Our clothing is merely an equivalent for food; and the more warmly we are clothed, the less food we require. Were we to go destitute of clothes, like certain savage tribes, or if, in hunting or fishing, we were exposed to the same degree of cold as the Somoyides, we could with ease consume 10lb. of flesh, and perhaps a dozen tallow candles into the bargain, as warmly clad travellers have related, with astonishment, of those people. Then could we take the same quantity of brandy or blubber of fish, without bad effects, and learn to appreciate the delicacy of train oil. We thus perceive an explanation of the apparently anomalous habits of different nations. The maccaroni of the Italian, and the train oil of the Greenlander and the Russian, are not adventitious freaks of taste, but necessary articles fitted to administer to their comfort in the climates in which they have been born. The colder the region, the more combustible must the food be. The Englishman in Jamaica perceives with regret the disappearance of his appetite, which, in England, had been a constant recurring source of enjoyment. By the use of aromatics, he creates an artificial appetite, and eats as much food as he did at home. But he thus unfits himself for the climate in which he is placed; for sufficient oxygen does not enter his system to combine with the carbon consumed; and the heat of the climate prevents him taking exercise to increase the number of his respirations. The carbon of the food is therefore forced into other channels, and disease results. England, on the other hand, sends her dyspeptic patients to southern climates. In our own land, their impaired digestive organs are unable to fit the food for that state in which it best unites with the oxygen of the air, which therefore acts on the organs of respiration themselves, thus producing pulmonary complaints. But when they are removed to warmer climates, they absorb less oxygen, and take less food; and the diseased organs of digestion have sufficient power to place the diminished amount of food in equilibrium with the respired oxygen. Just as we would expect from these views, in our own climate, hepatic diseases, or diseases arising from excess of carbon, are more prevalent in summer; and in winter pulmonic diseases, or those arising from excess of oxygen. The professor then went on to disprove the notion, that animal heat is due to nervous influence, and

not to combustion-an error which had its origin in supposing that the combustion proceeds in the blood itself. He also showed that animal heat must not be ascribed to the contraction of the muscles. The professor proceeded to prove, that the heat evolved by the combustion of carbon in the body is sufficient to account for the phenomena of animal heat. He showed that the 14 ounces of carbon which are daily converted into carbonic acid, in an adult, disengage no less than 197-477° of heat; a quantity which would convert 24 lb. of water at the temperature of the body, into vapour. And if we assume that the quantity of water vaporised through the skin and lungs amounts to 3 lb., then we have still 146.380° of heat to sustain the temperature of the body. And when we take into calculation the heat evolved by the hydrogen of the food, and the small specific heat possessed by the organs generally, no doubt could be entertained that the heat evolved in the process of combustion, to which the food is subjected in the body, is amply sufficient to explain the constant temperature of the body. From what has preceded, it is obvious that the amount of carbon consumed in food ought to depend on the climate, density of air, and occupation of the individual. A man will require less carbon when pursuing a sedentary occupation than when he is engaged in active exercise. Professor Liebig, having thus discussed the source of animal heat, proceeds next to consider what are the ingredients in the food, which may properly be considered to be nutritious. Physiologists conceive that the various organs in the body have originally been formed from blood. If this be admitted, it is obvious that those subjects only can be considered as nutritious which are susceptible of being transformed into blood.-The professor then entered upon an examination of the composition of blood, and of the identity in chemical composition of fibrine and albumen. The nutritive process is simplest in the case of the carnivora. This class of animals live on the blood and flesh of the graminivora, whose blood and flesh is identical with their own. In a chemical sense, therefore, a carnivorous animal, in taking food, feeds upon itself, for the nutriment is identical in composition with its own tissues. The professor then inquired from what constituents of vegetables the blood of the graminivorous is produced. The nitrogenized compounds of vegetables forming the food of graminivorous animals are called vegetable fibrine, vegetable albumen, and vegetable casein. Now, analysis has led to the interesting result, that they are exactly of the same composition in 100 parts; and, what is still more extraordinary, they are absolutely identical with the chief constituents of the blood; animal fibrine and animal albuBy identity, be it remembered, we do not imply similarity, but absolute identity, even as far as their inorganic constituents are concerned. These considerations showed the beautiful simplicity of nutrition. In point of fact, vegetables produce, in their inorganism, the blood of all animals. Animal and vegetable life are therefore most closely connected. The professor has still to account for the

men.