aliments, contain it. It is eliminated from the system in the urine as sulphates, partly formed by the action of the oxygen of arterial blood on the sulphur of the metamorphosed tissues.

7. Iron. This element is found invariably in the blood-corpuscles, although according to Scherer it is neither essential to hematosin nor necessary to the colour of the blood. Its quantity is about two parts in the thousand, varying in individuals of different temperaments and in different states of health. It is a constituent also of hair. Many articles of food contain iron, as yelk of egg, milk, mustard, potatoes, peas, &c.; but having been found in the gastric juice of the dog and in the chyle, Dr. Pereira regards this as an explanation of how it gets into the blood. Excepting the hair, no account is given of an outlet or organ of waste for this metal.

8. Chlorine is a constituent of blood, and of various secretions and excretions as a compound of sodium; and of the gastric juice as a compound of hydrogen. It is indispensable to health, and is constantly passing out of the system with the secretions, hence it requires to be constantly renewed, and forms an essential part of food in every period of life, being a constituent of milk and both of the yelk and white of egg.

9. Sodium is an essential constituent of blood, of the solids, of the secretions, and especially of bile. It is furnished by most animal foods, but not by all the vegetables usually employed;-is for the most part taken with the food as a condiment, and received into the system in the form of hydro-chlorate of soda, passing out of it again by the urine in the same form, and also in flesh-eating animals, as sulphate and phosphate

of soda.

10. Calcium. This is a component part of all animals, and particularly of their shells, crusts, and bones; also of most of the solids, and of blood, occurring in the form of subphosphate of lime. It is introduced with the food as a constituent of white and yelk of egg, of cereal grains and onions as a subphosphate, of rhuburb as an oxalate, of grapes as a tartrate, &c. Gum and unrefined sugar also yield calcium. Most animal foods contain it, and the water we drink contains its sulphate and bicarbonate. Yet the system appears occasionally to suffer from a deficient supply of calcium.

11. Magnesium occurs, combined with oxygen and phosphoric acid, and often with ammonia, in minute quantities in various animal structures, as blood, teeth, bones, and nervous matter. It is a constituent of both animal and vegetable food, as of cereal grains, potatoes, eggs, and flesh.

12. Potassium. A similar statement may be made of this element; it is required for the blood, and for the formation both of the solids and secretions, being a component part of most plants employed as food which grow inland. Of grapes and potatoes for instance. It is also contained in minute portions in common salt.

13. Fluorine. It may be remarked here that the statement by Berzelius, that fluorine is a constituent of bones and teeth is confirmed by Marchand. Dr. G. Owen Rees affirms, on the contrary, with Fourcroy and Vauquelin, that no such substance exists in the animal body. It has never been detected in plants, and the source of it, as an element of food, is unknown.

The above enumeration of the elements of matter required for the nutrition of the human body, must be taken conventionally, for it is well known that others have been detected. Copper in milk, sweat, and in

the intestines, and manganese constantly in the hair, as also in biliary and urinary concretions, and in the opaque lens, for instance. The account given of their origin is tolerably satisfactory, and were it not that chemical research has developed facts not easily explained, on the assumption that food is the sole source of the material fabric, it would be quite so. The first remark that suggests itself, on carefully considering this part of the subject, refers to the question, whether the vital force is capable of forming and decomposing what are regarded as the elements of matter. Dr. Prout threw out a hint, as early as 1822, that such a circumstance may happen, from considering the quantity of earthy matter contained in the skeleton of the chick. More recently he has stated his belief in the possession of this power by living systems under certain extraordinary circumstances. But with all deference to so high an authority, it is difficult to think otherwise than that, if calcium be actually formed in the chick, it must be in accordance with some general law of nature, and not an extraordinary or incidental event. If azote or carbon be decomposed by the vital force, such decomposition must occur constantly to answer certain ends in the animal economy. All the recent discoveries in animal chemistry tend to explain the origin ab externo, not only of the elements but even of many of the compounds detected in the animal tissues, and no new fact has been developed to confirm Dr. Prout's notion. Dr. Pereira is accordingly warranted in assuming that the calcium found in the skeleton of the chick is derived from one or more of the constituent parts of the egg, and that the elements detected by analysis in the various parts and products of animals have an external origin.

Another point, of even greater importance than the former, is the question whether these elements are received into the system, to become a part of the vital structures, otherwise than as aliment, and by the digestive organs. Liebig is a high authority on the negative side of this question, which has been most discussed in reference to the origin of the nitrogen. In this country there appears to be a strong opinion among practical chemists and physiologists, that, at all events when the food with which animals are fed, is deficient in nitrogen, this element becomes absorbed in the lungs. Dr. Prout suggests also, in the last edition of his work, that atmospheric air involved during mastication is a source of azote to vegetable feeders, and contributes to the formation of azotized compounds in the alimentary canal. (p. 504.) The principal arguments employed by Liebig, in favour of his view of the case, are briefly these: nitrogen is not created in the system, nor have we any satisfactory evidence that it is absorbed from the atmosphere in the vital process; the food of all animals contains nitrogenized compounds, identical in composition with the principal constituents of the blood and organized tissues, and therefore, substances devoid of this element are not required for the production of these constituents; the quantity of the nitrogenized principles of food consumed is amply sufficient for the nutrition and waste of the body; and the non-nitrogenized foods alone are incapable of supporting animal life. On the other side, Dr. Pereira advances a long argument, of which the following are the principal items. Many physiologists believe, from the results of experiments, that nitrogen is absorbed in the lungs we have proof that non-nitrogenized compounds

really do take part in the transformations of the animal tissues, and of the possibility of their conversion into the nitrogenized constituents of an animal body, in the case of benzoic acid appearing as hippuric acid in the urine in animals that feed on vegetable food, Dr. Prout detected no albumen in the chyme until after it had passed the pylorus, and sugar is not found in the blood, from which it may be inferred that in the duodenum, the saccharine is converted into the albuminous principle: diet even of fibrin, or of any nitrogenized principle exclusively, is incapable of supporting life. Dr. Pereira further remarks, that if nitrogenized aliment is derived from plants ready formed, the fact that fibrin, albumen, and gelatin, taken together or separately, are incapable of supporting animal life, while gluten from wheat or maize is alone sufficient to satisfy complete and prolonged nutrition, as stated by the commissioners of the French academy in 1842, remains unexplained. Neither is the necessity very obvious of more complex organs of digestion for the herbivora than for the carnivora. A hint is also given that nitrogen may be derived from the ammonia of the atmosphere, but this is unsupported by any proof except the analogy of plants, which, according to Liebig, assimilate nitrogen from that source.

We cannot undertake to settle this important question; but the remark here suggests itself, that so long as it remains undetermined, one class of physiologists believing in the formation of fibrin, for instance, from the nitrogen of the atmosphere, another class denying that nitrogen gas is ever assimilated, and affirming that nitrogenized compounds are received into the system only in a state of ready formation, it will be impossible to settle certain points in reference to food and diet. If Liebig's opinion is not established conclusively to every mind, neither is it invalidated by Dr. Pereira's reasoning. This can only be done by experiments on a wider scale. On some points there has certainly been a misapprehension of Liebig's meaning. Thus, by the expression "no nitrogen is absorbed from the atmosphere," taken with the context, this chemist manifestly means that no nitrogen from the atmosphere is employed in forming the azotized compounds of the blood, nor in what Dr. Prout terms secondary assimilation. It may also be here remarked, that protein, the basis of these compounds, is newly discovered, and its qualities by no means completely ascertained. This base, or complex radicle, has to be searched for in the chymous mass, and it is highly desirable that some of our practical chemists should inform us in what state it exists in the "incipient albumen," met with in the alimentary canal, as described by Dr. Prout. Although there are other channels by which the elements of the human body may be received into the system, as during the absorption of water by the skin, there is perhaps, after all, little room to doubt that, under ordinary circumstances, they are mainly if not altogether derived from the ingesta. Dr. Pereira has done a good service in making the chemical elements of food a distinct consideration in dietetics, and in future all treatises on this branch of science must be considered imperfect in which it is omitted. Already is it manifest that health depends upon a due and proportionate supply of these elements, considered apart from the organic combinations into which they are formed, and that an excess or deficiency can no longer be separated from our notions of the causation of disease. Facts before us sanction the opinion, that regarding them in their ele

mentary point of view, the quantities and proportions in which they are supplied to individuals, races, and generations, are, with other external agencies, intimately concerned in the production of temperaments and varieties of constitution. Truly our knowledge under this head is meagre, but we are in possession of a few strong facts. The satisfactory results of experiments made to determine the quantity of iron in the blood, and the effects which follow an addition or subtraction of this element, are in point. Nor can it be doubted, were our investigations pushed far enough, that equally conclusive evidence would be obtained as respects calcium, potassium, sulphur, &c. At the same time it is equally true that these elements require to be supplied to the animal system in certain forms of combination. We cannot depend upon phosphorus, or sulphur, or any other substance being assimilated to organic principles by the vital force when exhibited in the simple state; although it is highly probable that this may occur. When employed, even as medicines, the form of combination is of the utmost importance, as illustrated in the very obvious effects of certain preparations of the metal just mentioned. This element is assimilated by the blood much more rapidly when the citrate than when the sulphate is employed, a fact which may be explained by the powerful affinities exerted by the acid of the latter. When the citrate becomes absorbed the organic acid is destroyed by combustion, and the iron is readily yielded up, to enter into the structure of the blood-corpuscle. Thus, as between citrate and sulphate of iron, so, in the matter of diet, the supply of any particular element rarely admits of being regarded as a simple proposition. As in the case of nitrogen, extremely useful scales of the relative value of foods, measured by their elementary constitution, may be constructed upon the principle of Boussingault's scale of equivalents, before alluded to, yet these scales can only be regarded as approximations to the real or comparative nutritive qualities of the various articles of food.

II. ALIMENTARY PRINCIPLES. Proceeding to examine the second chapter of the work before us, we find, in the food of animals, that the elements to be employed in the economy are combined together in certain groups of two, three, or more, constituting distinct substances, the properties of which have been separately studied. To distinguish the nutritive from the innutritive parts of our aliment is no new doctrine. Hippocrates was aware that particular parts of food only are destined to take the nature and form of the human body. Many of the older authors regarded the nutritive principle as being always uniform, foods differing from each other in the proportions they contain. Others held that alimentary matters of whatever nature, must be converted into this uniform nutritive principle. Boerhaave, regarding all animal food as originally derived from vegetables, recognized aqueous, saponaceous, oily, and spirituous principles, as separable from the grosser materials; and Cullen reduced the alimentary matter of vegetables to three substances, acid, sugar, and oil. Dr. Fordyce classed vegetable foods accordingly as they contained sugar, farinaceous matter, oil, gum, and several indefinite substances called mucilage; the nutritive principles of animal foods being referred to coagulable mucilage, lymph, and oil; believing that all the animal solids are composed of mucilage and water. It is, however, to Dr. Prout that we are indebted for the first rational account of the alimentary

principles, who referred them to four great classes, the aqueous, the saccharine, the albuminous, and the oleaginous. Dr. Pereira, for reasons in some instances adduced, to which we shall refer, has extended these to twelve classes. As with the elements, we propose to describe them seriatim, reserving our principal commentary till afterwards.

1. Water constitutes three fourths of the weight of the human body. It is less immediately essential to our existence than air, and more so than solid food, being probably the only natural drink after the period of infancy. Many of the purposes fulfilled by water are well known. It is furnished to the system both by aqueous drinks and by many of the solid substances employed as food. The latter contain very variable proportions thus, wheat flour, 14.5, fresh meat about 75, cow's milk about 87, and beef-tea 98.4 per cent. These are instances taken promiscuously from a very useful table, at p. 80, of the proportions of water in different articles of diet. Whether water ever yields up its elements to assist in the formation of the organic tissues of animals is uncertain, but, according to Liebig, the hydrogen of vegetable tissues is derived from it. Dr. Prout also appears to admit that such a power is occasionally exerted by animals, and either water or its elements are intimately concerned, as shown both by Prout and Liebig, in the metamorphoses which other alimentary principles undergo in the living system. Its importance as a solvent, both in primary and secondary assimilation, and in the removal of the effete materials of the body, is equally well established. Many of the processes essential to animal life depend upon the chemical action of water, as the formation of the hydrochloric acid of the gastric juice, and of the soda of bile, from the chloride of sodium of the blood. It may be added that the quantity of water employed by the system amounts to several pounds daily, varying according to external temperature, exercise and rest, the quantity and quality of the materials received into the digestive organs or into the blood, and other causes; and while water is essential to the process of digestion, it is also rapidly absorbed by the blood-vessels of the alimentary canal, and conveyed through the portal system, holding many substances in solution, respecting which our information is at present very defective.

2. The mucilaginous principle. Numerous vegetable substances employed as food contain gum, in proportions varying from 1, as in rice, to 19 37 per cent., as in the kidney-bean, or even more; its constituents are carbon and the elements of water, and it is regarded by Liebig as an element of respiration. Experience shows that in its isolated state this substance is by no means easily digested in the human stomach.

3. The saccharine principle. This is also composed of carbon and of the elements of water in somewhat variable proportions. Respecting the use of this principle in the economy, the greatest difference of opinion prevails. Dr. Prout regards it as a nutritious substance, and the type of one class of his alimentary principles, and indeed of vegetable aliments generally; holding, that during digestion and secondary assimilation it is convertible into the oleaginous and albuminous principles, and into blood. Liebig admits that it may be formed into fat, but denies that it is ever converted into azotized compounds, or employed in the nutrition of the azotized tissues; he classes it with gum as an element of respiration, its carbon being employed as fuel for the production of animal heat.