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Liebig's Chemical Letters



My dear Sir,

    Having attempted in my last letter to explainto you the simple and admirable office subserved by the oxygen of the atmospherein its combination with carbon in the animal body, I will now proceed to presentyou with some remarks upon those materials which sustain its mechanisms in motion,and keep up their various functions, - namely, the Aliments.

    If the increase in mass in an animal body, thedevelopment and reproduction of its organs depend upon the blood, then those substancesonly which are capable of being converted into blood can be properly regarded asnourishment. In order then to ascertain what parts of our food are nutritious, wemust compare the composition of the blood with the composition of the various articlestaken as food.

    Two substances require especial considerationas the chief ingredients of the blood; one of these separates immediately from theblood when it is withdrawn from the circulation.

    It is well known that in this case blood coagulates,and separates into a yellowish liquid, the serum of the blood, and a gelatinous mass,which adheres to a rod or stick in soft, elastic fibres, when coagulating blood isbriskly stirred. This is the fibrine of the blood, which is identical in all itsproperties with muscular fibre, when the latter is purified from all foreign matters.

    The second principal ingredient of the bloodis contained in the serum, and gives to this liquid all the properties of the whiteof eggs, with which it is indeed identical. When heated, it coagulates into a whiteelastic mass, and the coagulating substance is called albumen.

    Fibrine and albumen, the chief ingredients ofblood, contain, in all, seven chemical elements, among which nitrogen, phosphorus,and sulphur are found. They contain also the earth of bones. The serum retains insolution sea salt and other salts of potash and soda, in which the acids are carbonic,phosphoric, and sulphuric acids. The globules of the blood contain fibrine and albumen,along with a red colouring matter, in which iron is a constant element. Besides these,the blood contains certain fatty bodies in small quantity, which differ from ordinaryfats in several of their properties.

    Chemical analysis has led to the remarkableresult, that fibrine and albumen contain the same organic elements united in thesame proportion, - i.e., that they are isomeric, their chemical composition - theproportion of their ultimate elements - being identical. But the difference of theirexternal properties shows that the particles of which they are composed are arrangedin a different order. (See Letter V).

    This conclusion has lately been beautifullyconfirmed by a distinguished physiologist (Dénis), who has succeeded in convertingfibrine into albumen, that is, in giving it the solubility, and coagulability byheat, which characterise the white of egg.

    Fibrine and albumen, besides having the samecomposition, agree also in this, that both dissolve in concentrated muriatic acid,yielding a solution of an intense purple colour. This solution, whether made withfibrine or albumen, has the very same re-actions with all substances yet tried.

    Both albumen and fibrine, in the process ofnutrition, are capable of being converted into muscular fibre, and muscular fibreis capable of being reconverted into blood. These facts have long been establishedby physiologists, and chemistry has merely proved that these metamorphoses can beaccomplished under the influence of a certain force, without the aid of a third substance,or of its elements, and without the addition of any foreign element, or the separationof any element previously present in these substances.

    If we now compare the composition of all organisedparts with that of fibrine and albumen, the following relations present themselves:-

    All parts of the animal body which have a decidedshape, which form parts of organs, contain nitrogen. No part of an organ which possessesmotion and life is destitute of nitrogen; all of them contain likewise carbon andthe elements of water; the latter, however, in no case in the proportion to formwater.

    The chief ingredients of the blood contain nearly17 per cent. of nitrogen, and from numerous analyses it appears that no part of anorgan contains less than 17 per cent. of nitrogen.

    The most convincing experiments and observationshave proved that the animal body is absolutely incapable of producing an elementarybody, such as carbon or nitrogen, out of substances which do not contain it; andit obviously follows, that all kinds of food fit for the production either of blood,or of cellular tissue, membranes, skin, hair, muscular fibre, &c., must containa certain amount of nitrogen, because that element is essential to the compositionof the above-named organs; because the organs cannot create it from the other elementspresented to them; and, finally, because no nitrogen is absorbed from the atmospherein the vital process.

    The substance of the brain and nerves containsa large quantity of albumen, and, in addition to this, two peculiar fatty acids,distinguished from other fats by containing phosphorus (phosphoric acid?). One ofthese contains nitrogen (Frémy).

    Finally, water and common fat are those ingredientsof the body which are destitute of nitrogen. Both are amorphous or unorganised, andonly so far take part in the vital process as that their presence is required forthe due performance of the vital functions. The inorganic constituents of the bodyare, iron, lime, magnesia, common salt, and the alkalies.

    The nutritive process is seen in its simplestform in carnivorous animals. This class of animals lives on the blood and flesh ofthe graminivora; but this blood and flesh are, in all their properties, identicalwith their own. Neither chemical nor physiological differences can be discovered.

    The nutriment of carnivorous animals is derivedoriginally from blood; in their stomach it becomes dissolved, and capable of reachingall other parts of the body; in its passage it is again converted into blood, andfrom this blood are reproduced all those parts of their organisation which have undergonechange or metamorphosis.

    With the exception of hoofs, hair, feathers,and the earth of bones, every part of the food of carnivorous animals is capableof assimilation.

    In a chemical sense, therefore, it may be saidthat a carnivorous animal, in supporting the vital process, consumes itself. Thatwhich serves for its nutrition is identical with those parts of its organisationwhich are to be renewed.

    The process of nutrition in graminivorous animalsappears at first sight altogether different. Their digestive organs are less simple,and their food consists of vegetables, the great mass of which contains but littlenitrogen.

    From what substances, it may be asked, is theblood formed, by means of which of their organs are developed? This question maybe answered with certainty.

    Chemical researches have shown, that all suchparts of vegetables as can afford nutriment to animals contain certain constituentswhich are rich in nitrogen; and the most ordinary experience proves that animalsrequire for their support and nutrition less of these parts of plants in proportionas they abound in the nitrogenised constituents. Animals cannot be fed on mattersdestitute of these nitrogenised constituents.

    These important products of vegetation are especiallyabundant in the seeds of the different kinds of grain, and of peas, beans, and lentils;in the roots and the juices of what are commonly called vegetables. They exist, however,in all plants, without exception, and in every part of plants in larger or smallerquantity.

    These nitrogenised forms of nutriment in thevegetable kingdom may be reduced to three substances, which are easily distinguishedby their external characters. Two of them are soluble in water, the third is insoluble.

    When the newly-expressed juices of vegetablesare allowed to stand, a separation takes place in a few minutes. A gelatinous precipitate,commonly of a green tinge, is deposited, and this, when acted on by liquids whichremove the colouring matter, leaves a grayish white substance, well known to druggistsas the deposite from vegetable juices. This is one of the nitrogenised compoundswhich serves for the nutrition of animals, and has been named vegetable fibrine.The juice of grapes is especially rich in this constituent, but it is most abundantin the seeds of wheat, and of the cerealia generally. It may be obtained from wheatflour by a mechanical operation, and in a state of tolerable purity; it is then calledgluten, but the glutinous property belongs, not to vegetable fibrine, but to a foreignsubstance, present in small quantity, which is not found in the other cerealia.

    The method by which it is obtained sufficientlyproves that it is insoluble in water; although we cannot doubt that it was originallydissolved in the vegetable juice, from which it afterwards separated, exactly asfibrine does from blood.

    The second nitrogenised compound remains dissolvedin the juice after the separation of the fibrine. It does not separate from the juiceat the ordinary temperature, but is instantly coagulated when the liquid containingit is heated to the boiling point.

    When the clarified juice of nutritious vegetables,such as cauliflower, asparagus, mangelwurzel, or turnips, is made to boil, a coagulumis formed, which it is absolutely impossible to distinguish from the substance whichseparates as a coagulum, when the serum of blood, or the white of an egg, dilutedwith water, are heated to the boiling point. This is vegetable albumen. It is foundin the greatest abundance in certain seeds, in nuts, almonds, and others, in whichthe starch of the gramineae is replaced by oil.

    The third nitrogenised constituent of the vegetablefood of animals is vegetable caseine. It is chiefly found in the seeds of peas, beans,lentils, and similar leguminous seeds. Like vegetable albumen, it is soluble in water,but differs from it in this, that its solution is not coagulated by heat. When thesolution is heated or evaporated, a skin forms on its surface, and the addition ofan acid causes a coagulum, just as in animal milk.

    These three nitrogenised compounds, vegetablefibrine, albumen, and caseine, are the true nitrogenised constituents of the foodof graminivorous animals; all other nitrogenised compounds occurring in plants, areeither rejected by animals, as in the case of the characteristic principles of poisonousand medicinal plants, or else they occur in the food in such very small proportion,that they cannot possibly contribute to the increase of mass in the animal body.

    The chemical analysis of these three substanceshas led to the very interesting result that they contain the same organic elements,united in the same proportion by weight; and, what is still more remarkable, thatthey are identical in composition with the chief constituents of blood, animal fibrine,and albumen. They all three dissolve in concentrated muriatic acid with the samedeep purple colour, and even in their physical characters, animal fibrine and albumenare in no respect different from vegetable fibrine and albumen. It is especiallyto be noticed, that by the phrase, identity of composition, we do not here intendmere similarity, but that even in regard to the presence and relative amount of sulphur,phosphorus, and phosphate of lime, no difference can be observed.

    How beautifully and admirably simple, with theaid of these discoveries, appears the process of nutrition in animals, the formationof their organs, in which vitality chiefly resides! Those vegetable principles, whichin animals are used to form blood, contain the chief constituents of blood, fibrineand albumen, ready formed, as far as regards their composition. All plants, besides,contain a certain quantity of iron, which reappears in the colouring matter of theblood. Vegetable fibrine and animal fibrine, vegetable albumen and animal albumen,hardly differ, even in form; if these principles be wanting in the food, the nutritionof the animal is arrested; and when they are present, the graminivorous animal obtainsin its food the very same principles on the presence of which the nutrition of thecarnivora entirely depends.

    Vegetables produce in their organism the bloodof all animals, for the carnivora, in consuming the blood and flesh of the graminivora,consume, strictly speaking, only the vegetable principles which have served for thenutrition of the latter. Vegetable fibrine and albumen take the form in the stomachof the graminivorous animal as animal fibrine and albumen do in that of the carnivorousanimal.

    From what has been said, it follows that thedevelopment of the animal organism and its growth are dependent on the receptionof certain principles identical with the chief constituents of blood.

    In this sense we may say that the animal organismgives to the blood only its form; that it is incapable of creating blood out of othersubstances which do not already contain the chief constituents of that fluid. Wecannot, indeed, maintain that the animal organism has no power to form other compounds,for we know that it is capable of producing an extensive series of compounds, differingin composition from the chief constituents of blood; but these last, which form thestarting-point of the series, it cannot produce.

    The animal organism is a higher kind of vegetable,the development of which begins with those substances with the production of whichthe life of an ordinary vegetable ends. As soon as the latter has borne seed, itdies, or a period of its life comes to a termination.

    In that endless series of compounds, which beginswith carbonic acid, ammonia, and water, the sources of the nutrition of vegetables,and includes the most complex constituents of the animal brain, there is no blank,no interruption. The first substance capable of affording nutriment to animals isthe last product of the creative energy of vegetables.

    The substance of cellular tissue and of membranes,of the brain and nerves, these the vegetable cannot produce.

    The seemingly miraculous in the productive agencyof vegetables disappears in a great degree, when we reflect that the production ofthe constituents of blood cannot appear more surprising than the occurrence of thefat of beef and mutton in cocoa beans, of human fat in olive-oil, of the principalingredient of butter in palm-oil, and of horse fat and train-oil in certain oilyseeds.