MICHEL E U G ~ N ECHEVREUL (1786-1889) PIERRE LEMAY Fontenay Sous Bois (Seine), France RALPH E. OESPER University of Cincinnati, Cincinnati, Ohio
ONAUGUST31, 1786, in Angers (2 Rue des Deur bered the pleasant days he spent there in his youth. Haies) a son was born to Etiennette Magdeleine and He witnessed there a bloody battle, la Roche des Murs, Michel Chevreul, master in surgery and doctor of medi- between the Vendeans and the Republicans. Howcine. The next day the child was baptized Michel ever, his parents did not yield to the unnatural psychoEugkne. The ceremony took place a t the church of sis that had taken possessiou of so many of their fellow St. Pierre; the officiant was Huchelon Desroches, cur6 townsmen. At the close of the siege of Angers in Deof St. Julien, assisted by Robin, cur6 of St. Pierre, and cember 1793, the self-styled "representatives of the Lemay, vicar of St. Pierre. Those who signed the regis- people" conceived the horrible scheme of exposing on ter were the father; the great uncle, Gilles Chevreul, the ramparts the heads of the Vendeans killed before master in surgery, who acted as godfather; the god- the city. The surgeons were ordered to perform the mother, Etiennette Delmont Deslisle, wife of Claude grisly decapitations, but at the risk of their lives, Bachelier, master of surgery, who, as maternal grand- Michel Chevreul and his colleagues neglected (?) to father, also graced the occasion with his presence. carry out this order. Thus, the young Chevreul found in his home brave and humane spirits. The life there EARLY LIFE AND EDUCATION was healthy and free of the current passions. This The boy came of long-lived stock on both sides; his atmosphere shielded him during the impressionable father died at 91 and his mother at 93. From them he years from the dangerous examples that were so prevainherited the robust health which enabled him to reach lent, and kept him, to a great extent, from develthe age of almost 103. He thus added a rather rare oping a nervous excitability that might easily have example of longevity to his accomplishments as a scien- been the sequela of the events to which he was exposed. tist. Ostwald once remarked that Chevreul carried on His mother, for whom he always retained a deep his researches at a pace which indicated that he had a affection, was his first teacher. She did not spoil him, prescience that he would live to be very old. and he often recalled how she firmly insisted when he The home environment was eminently favorable to balked at eating fish or drinking undiluted milk or the development of his intellectual faculties. His wine, for which he had a life-long aversion. father, correspondent of the Academy of Medicine, When he grew old enough to go to school, all the high was not only an able practitioner and a distinguished schools and colleges had been disbanded and there reteacher, he also had a facile pen. He was one of the mained only private teachers. He was sent to two of first in France actively to campaign for the reconcili- these in turn: Jean Perrier, brutal and severe; Maation of medicine and surgery, sister professions that thurin Papin, kinder and more understanding. In 1795, for centuries had been jealously inimical to each other. the Convention decreed that 6coles centrales should be Using himself as example, he maintained that a sur- opened in the capital cities of the various departments geon could be the equal of a physician only if he was and Chevreul was enrolled in the one a t Angers in 1799. trained in the same disciplines and held the same de- The program of studies included literature, science, and gree. technical subjects. Mineralogy was taught by the The moral surroundings mere just as excellent. The pharmacist and surgeon Renou, who had studied a t early boyhood of Chevreul fell within a period of frighe Gottingen. Heron, exoratorian, and formerly proful events: first the Revolution and then the Reign of fessor a t the college a t Anjou, was the instructor in Terror. ' I n 1794, a guillotine was set up permanently chemistry and physics. , Chevreul's scholastic abilities in the Place de Ralliement, only a fev steps from his quickly became evident; he received first prizes in home. He might easily have seen there the horrible Latin, Greek, physics, chemistry, and mineralogy. crimes committed by irresponsible men who sent to Two of his schoolmates, like himself, were destined to death children, young girls, women, old men, the inno- have distinguished careers: Bochard, who became a cent as well as the guilty, chosen a t random or often to member of the Academy of Medicine, and the sculptor, satisfy artisan or urivate hatreds. The bov heard his David d'Ane~ri: ~-~~ elders discussing the mass massacre of more" than 3000 prisoners of war, the murder of civil prisoners, the CHOICE OF CAREER burning of titles of nobility, of university diplomas, of The young man reached the stage where he had to cultural objects, etc. His parents had a summer home choose a career. Despite the family tradition, he was a t Murs, a neighboring village, and Chevreul remem- not attracted toward surgery or medicine. Rather, at ~
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an early age chemistry appealed strongly to him. This choice probably was a reflection of Heron's exceptional teaching talents. However, Chevreul had exhausted the possibilities a t Angers, and he dreamed of going to Paris where Fourcroy and his associate Vauquelin were lecturing with eclat. The latter, to quote Dumas, "was a chemist through and through every day of his life and throughout each day"; he was a prominent member of the Institute and held teaching posts a t the Museum of Natural History (Jardin des Plantes), a t the Ecole des Mines, and at the Ecole Polytechnique. I n 1801 he succeeded Darcet a t the College de France and in 1803 he was appointed Director of the newly erected Ecole de Pharmacie. In addition, he managed the chemical works of Fourcroy, Deserres and Vauquelin at 23 Rue du Vieux Colombier. It was to him that Chevreul applied in 1797, armed with a recommendation from Proust, who had been born a t Angers in 1754 and hence knew the bri!liance and promise of his young townsman. Vauquelin took the boy into his laboratory, where he worked side by side with another student, Thenard. Chevreul quickly showed that he, too, was worthy of every attention, and before long he was on an intimate footing with Vauquelin. Soon he spent most of his evenings a t Vauquelin's apartment at the Museum. He was allowed to collaborate in the master's researches and so had a small part in the discovery of chromium in 1798. He was scarcely twenty years old when Vauquelin put him in charge of the laboratory. Their mutual esteem was deeply rooted and Chevreul never failed to acknowledge the great debt he owed to Vauquelin. He especially appreciated the latter's scientific genius and his ability to work both intensely and accurately. This was in contrast to his opinion of Fourcroy, of whom he remarked to Armand Gautier "You may have absolute confidence in Vauquelin, but be suspicious of Fourcroy." Chevrcul's first teaching appointment was a t the Lycee Charlemagne. In 1810 he was made assistant naturalist at the Museum; in 1821 he became examiner in chemistry a t the Ecole Polytechnique. When in 1824 he was made director of dyeing a t the national tapestry works of Gobelins he was only 38. ELECTION TO THE ACADEMY OF SCIENCES
Figure
la.
Obverse of Medal
cow, Philadelphia, and London. He received many decorations, including the Grand Cross of the Legion of Honor. Despite these distinctions, he remained an unassuming scientist, striving only to merit the title he loved best: "Dean of Students." Though he never took part in p-olitics, he was an ardent patriot. I n 1814, Czar Alexander offered him the headship of the Polytechnic a t St. Petersburg, but Chevreul refused the post. During the Franco-Prussian war of 187C-71, he calmly continued his researches during the bombardment which devastated the galleries and greenhouses near his laboratory. He entered a strong protest to the German ruler against such needless destruction in the minutes of the Academy (January 9, 1871) and this gained.wide publicity. He even enlisted in the National Guard, and despite his 8.5 years insisted on taking his turn as guard on the fortifications. The celebration of his centenary in 1886 was a great national event. A medal, by Roty, mas presented to the celebrant and on the back'was the dedication: "La jeunesse franpaise au Doyen des 6tudiants." A volume containing a complete bibliography of Chevreul's publications was issued a t this occasion. In his Introduction, C. Brogniart declared, "This volume mky be regarded as the history of modern chemistry." Besides a complete account of the elaborate ceremonies, the volume also contained reproductions of the congratulatory addresses sent by various learned societies from all parts of the world. Among these was one from the American Association for the Advancement of Science, which made Chevreul an honorary member.'
In 1818, Chevreul was a candidate for election to the Academie des Sciences, but graciously withdrew to insure that Proust would be elected. At the latter's death in 1826, Chevreul was named to fill the vacancy. He was President of the Academy in 1839 and again in 1867. Finally in 1830 he was elected successor to Vauquelin as Administrative Professor of the Museum of Natural History. He was reelected six successive times, the last term beginning in 1861. He then became honorary director. He spent almost 70 years in the service of this world famous scientific in- PRIVATE LIFE stitution. He also was a member of the Royal Society Chevreul's private life was pleasant and exemplars. of Agriculture and served several terms as its president. He wss a rneml~rri ~ fi ~ ~ n n w (f~~ur it . i wimtifi(' p r ~ r p ~ u i - ' MALLOIZEL, G., "Oeu~resscientifiques de M. E. Chevreul, z:itions, w c h US Copenhagen, S;;ruckhulrn,Hrrlin, llus- 1806-1886," Paris, 1886.
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On July 22, 1811, he married Sophie Davallet, daughter of a retired tax official. The bridegroom (Figure 2) was , handsome, tall, robust, full of energy, with rather sparkling, mischievous eyes, and unruly hair. His bride was nice looking, intelligent, and, in addition, she had the attraction of being quite well off. She quickly realized that the wife of a scientist must lead a slightly self-effacing life that would not interfere with the work of her husband. During the winter she lived in Paris: the rest of the year she resided on her place at L'Hay, a suburb, about 8 kilometers out of the city. Each Saturday evening, Chevreul walked out to rejoin her, and he returned to the city each Monday morning in the same manner. They were a t home to their friends on Sunday. Their visitors included scientists: Fr6deric Cuvier (brother of Georges), AmpBre, and temporary visitors to France such as Berzelius (who was his neighbor in 1819), Liebig, Oersted; the artists: Horace Vernet, David d'Angers, Delaroche; writers such as Vilemain, Champollion, etc. Chevreul was mayor of L'Hay for thirteen years. He was generous and public spirited and gave the village a town hall, a school, a fountain. and urovided the funds for the restoration of the church. -
the Institute, the Museum, and the Opera, with the President of the Republic in attendance, did not greatly interrupt the even course of Chevreul's life. The next day he quietly resumed his accustomed routine. His last publication, which dealt with the role of atmospheric nitrogen in the vegetable economy, was presented before the Academy on May 22, 1888. He was then almost 102. His first paper, a chemical examination of fossil bones, had appeared in 1806, i. e., more than 80 years earlier. The following letter2 addressed to a foreign scientist sums up exceptionally well Chevreul's remarkable career. The letter, dated Paris, September 1, 1884, was written when Chevreul wyas 98, the mriting is only slightly shaky. "Dear Sir: Permit me this expression, although I have never spoken to you, and above all allow me to add Figvrs lb. Reverse of Medel that this is by no means simply a letter, addressed to a stranger, but a token of true gratitude for the very nice Mme. Chevreul died in 1862. He then gave up practhings you have said about me personally, rather than tically all social life. The estate at L'Hay was turned for the complimentsaddressed to the scientist. If, with over to his son, and Chevreul never made his home there again. He resided a t the Museum, surrounded by eld- respect to the latter, the press has treated me as I would erly servants. His life was almost that of a recluse. never have anticipated, be assured that if I have been He practically gave up newspapers and read only scien- made happy by these compliments, it is because I have tific journals and books. He went out only to scien- the convictions of moral honesty, and it is because of tific meetings. He was perfectly satisfied to spend all my conscience that I am grateful for the kindness of the his time in his laboratory and library. The latter was press. I have never desired either riches, or elevation quite large, and he added to it constantly, buying to a highsocial position, or to be given authority over not only recent but also rare books. E~~~ others. maxim in my scientific career has always the great festivities in connection with his centenary a t 8 Private eoIIeetion of PIERRE LEMAY.
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Two statues of Chevreul can still be seen. The first, (Figure 3) showing him standing, was erected in honor of his centenary in the Jardin des Plantes, Paris, near the Museum where he spent so many happy and fruitful years. The other (Figure 4), a seated figure, is in the Jardin des Plantes at Angers, his birthplace. WORKS
The researches of Chevreul are outstanding because of their number, their importance, and their diversity. His books, papers, and notes number more than one thousand. They are grouped under 540 heads in the bibliography issued a t the centenary celebration. Obviously, not more than a small fraction of this mass of published material can be considered here; only some of the more important and most characteristic of his uchi(wmtmti h a w bwn srlrrtrd for cliscuiiion. .\ dprailed bioeruohv and anulvsis of his writinm wo111dhe >I worth-while project and would constitute a really fitting memorial to this great man. Untilthe 19th century chemistry dealt almost excluU
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"
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Figure 3. The orisina1 is in the Jardin des Plantes, i n Paris, and e Porcelain Reprodurtion in the cons..v.t.iro des Art e* Metie..
been: "Strive diligently to attain perfection without laying claim to it." My rule of conduct has been: "To accept the obligations of the position in which I find myself." Thanks to these inborn propensities, I am happy in the kindness which some scientists foreign to France have shown me, and among the French my teacher Vauquelin, ArnpBre, Frederic Cuvier, Gay-Lussac, and Thenard. Dear Sir, I trust that your kindness will not find this letter too long since it has been dictated by a cratitude which all who know me will find entirely natural." However, even the best and the longest careers must come to a close. Chevreul's vitality declined, he spoke only with difficulty. He received practically no one, and went out only when his son took him for a daily drive. The son died on March 27, 1889, and though this news was carefully concealed from the father, he seemed to be drawn to the tomb. He had contracted a bad cold, while viewing the building of the EiffelTower, and collapsed suddenly. After lingering several days he died on April 9, 1889, aged 102 years, 7 months, 9 days. He was given a State funeral withmilitary honors. At his request, there was no elegy at the tomb. He rests in th6 cemetery a t L'Hay.
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Figuro 4.
Statve of Cheveul in t h e Jardin des Plantee, Anwe..
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sively with inorganic materials. Lavoisier, near the close of the 18th century, had made it an exact science based on qualitative and gravimetric data, but the domain of organic chemistry, the chemistry of living things and their products, still remained almost untouched. The work in this field was mostly limited to producing certain salts and oils by distillation of organic materials, a method extensively employed by Rouelle, Macquer, and especially by pharmacists, who thus prepared alleged panaceas for all sorts of ailments. In 1774, Lavoisier remarked in his laboratory journal, "What is meant by 'oil'? It appears that i t can be converted into a gas by combustion, but we know nothing beyond this." The organic materials found in nature consist usually of an aggregate of diverse substances and hence ultimate analysis can give no real information about such systems. The treatment is too drastic and far-reaching; it is necessary first of all to resolve the native materials and to separate the proximate principles. "This initial step," said Berthelot, ' will be the starting point of all later studies and will constitute the foundation of organic chemistry." It was here that Chevreul entered the picture. Although Vauqueli's researches had been mainly inorganic he, together with Fourcroy, had studied some organic materials, especially those related to medicine. Hence it was not strange that Chewed's first studies, beginning in 1807, dealt with such substances as logwood, cork, bones cartilage, indigo, wood, the liquids contained in the invertebral cavities, litmus, etc. These, along with a number of minor studies of inorganic topics, occupied him until 1813, a notable date since it marks the start of his systematic investigation of fatty materials. BehaI states that Vauquelin set Chevreul on this path by bringing to him for examination a sample of spoiled fat, while Graebe declares that, in 1811, Chevreul became interested in the crystals that separated, after long standing, from a water solution of a potassium soap that he was analyzing. In any event, the fruits were abundant; a new field of organic chemistry was inaugurated. This work was the real beginning of Chevreul's brilliant career.
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acid. Scheele, in 1783 isolated from oils a "sweet principle (6lsuss)," to which Chevreul later gave the modern name, glycerin. However, Scheele's discovery had no effect on the chemistry of the saponification process, which was considered to be merely a direct union of fat (thought to be an acid) and alkali. In passing, it should be noted that Scheele deserves great credit for leading the way of escape from pure empiricism in the preparation of particular types of compounds. His procedure for the preparation of acids is the earliest model of the proximate analysis of organic materials. In this sense, he was the precursor of Chevreul, who became the real organizer and codifier ef this method of attack that has since been so widely applied. Was it mere chance, or was it rather a genuine flair for research that led Chevreul to choose the most fecund path when he opened his study of fats by investigating soaps? His now famous series of seven papers were issued from 1813 to 1818. The results, augmented and systematized, were collected in his "Recherches sur les corps gras d'origine animale" (Figure 5), a book of 500 pages, published in 1823 a t Paris. (It was republished in 1889.) By his masterly experimental studies, he elucidated the process of the formation of soap and showed that fats are combinations of acids with glycerin. These studies yielded a number of other important results. Among these are the discovery and isolation of the fatty acids: oleic, palmitic, stearic, valerianic, caproic, and capric. From biliary calculi and bile, he isolated cholesterine (discovered in 1775 by Conradi) and by saponifying spermaceti he obtained cetyl alcohol. He showed, in contradiction to Fourcroy, that saponification can proceed in a vacuum and hence in the absence of oxygen. He followed quantitatively the saponification of fats by different bases and and found that the weight of the products, i. e., the fatty acids and the glycerin, is 5 or 6per cent greater than that of the fat. These data were quite significant in the development of his theoretical conclusions. In separating the acids derived from the fats, he introduced into chemistry a procedure that has proved to be tremendously important and useful in establishing the identity, purity, or constancy of composition of organic compounds. "The stearates and margarates were considFATS AND OILS ered to be pure, when after being dissolved five sucLittle was then known about the greasy materials cessive times in boiling alcohol, the precipitate, formed variously designated as oils, butters, greases, fats, during the cooling, contained an acid that had the same etc. Lavoisier, in 1794, found that "the 'fixed oils' fusibility (melting point) as that of the acid which recontained 21 parts of hydrogen and 79 parts of carbon." mained dissolved in the alcohol." Up to then melting "Perhaps," he wrote in 1789, "the solid oily materials points (and likewise boiling points) had been very much may contain also a little oxygen, to which they owe neglected as identifying characteristics of materials. their solid state. ' By 1805 Fourcroy had not gotten Chevreul studied fats from all available sources, inmuch further. Various ultimate analyses had contrib- cluding man and many wild and domesticated animals. uted little if anything, though proximate analyses pro- Such origins are revealed in the names he coined for the duced somewhat more information. As early as the new acids: butyric (butter); capric, caproic (goat); 17th century Tacken, (Tachenius) a German chemist delphinic, phocenic (dolphin, porpoise), etc. Margaric Living in Venice, made the important observation that acid (from margarita= a pearl), which he originally "in saponification it is an acid which combines with the called margarine, was given this name because it was alkali, because the oil or fat contains a hidden acid." obtained by adding hydrochloric acid to the mother-ofIn 1745, Macquer pointed out that rancid oils contain pearl like crystals (matiere nacrde) that he, as already
FEBRUARY; 1948
stated, had observed to deposit from his original soap A good many of his terms have become a permanent part of both the chemical and the everyday vocabulary. He showed that all natural fats are mixtures of a comparatively few chemical individuals. Each of these species in itself conformsto Proust's law of definite composition, but by being present in different proportions confer on the numerous and varied fats, oils, butters, greases, etc., their differencesin fusibility, odor, taste, etc. Sapo~fication,to him, was simply a decomposition of a fat in which the inorganic bases took the place of the anhydrous glycerin, which by taking up water became the glycerin that could be recovered from the sweet water. "This," declared Colson, "was a discovery of fundamental importance, a veritable scientificimpulsion, whose effects are still evident in organic chemistry." To Chevreul it was then evident that the soaps are true salts, comparable to inorganic salts, he concluded "from this point of view, the fats, in many respects, are quite analogous to the estek, which are considered to be compounds of acids and alcohol." Berzelius wrote: "It may be said that this constitutes the best and most complete chemical study that chemistry can exhibit, and which leaves scarcely a question to be answered. It is a model for younger chemists who propose to investigate any of the less known parts of chemistry." These masterly researches by Chevreul led subsequently to the famous studies, in 1853 and 1854, by Berthelot, who synthesized the glycerides from their fission products, to the study (1828) by Dumas and Boullay of esters, and to the discovery (1858) by Wurtz of glycol. From the technological standpoint Chevreul's findings resulted in: rational improvementsin the manufacture of soap; the large-scale production of glycerin; the manufacture of stearin (i. e. stearic acid) candles. Tallow candles were in common use a t this time. They were soft, needed constant attention, and their smoky flames g$ve off unpleasant odors. In 1818 Braconnota tried in association with Simonin, a pharmacist a t Nancy, to improve them, by removing the more fluid portions of the tallow. Gay-Lussac, who was always looking for profitable applications of science, suggested to Chevreul that it might be possible to commercialize candles made of stearic acid. They took out a joint French patent in 1825, and a little later had a similar patent taken out in England by the industrialist Moses Poole. However, they never got beyond the laboratory stage because the process was too complicated and the wicks did not work properly. One month after the granting of their patent, an engineer, Jules Cambaceres, also took out a patent and started a factory. His product was of poor quality and did not sell well. Two young physicians, Motard and de Milly (Demilly) bought Cambaceres' patent in 1829, im-
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. solution.
a Heni-i Braconnot (1781-1855) from 1807, director of the Botanical Gardens and professor of natural hihistory a t Nancy was involved in a heated priority debate with Chevreul. For details see R. JAGNAUX, "Histoirc de la. Chirnie,"Vol. 2, Paris, 1891, p. 639.
Figure S
proved the plaited wicks, and substituted lime for potash in the saponification. They took out patents in their own names in 1831, and began to manufacture "Bougies de I'Etoile," so-called because their factory was near the barrier of that name. This term for stearin candles persisted for many years. They dissolved their partnership in 1835; Motard started a stearinerie in Berlin, and de Milly moved his factory to St. Denis. He built a second plant in 1836 at Marseilles. I t as de Milly who made an industrial success of saponification by means of lime, who perfected hot pressing to eliminate the liquid acids, who used wicks impregnated with boric acid (or ammonium borate). He and his son-in-law, Bouis, professor a t the School of Pharmacy, invented the saponification of fats under pressure and a t elevated temperatures in the presence of a small amount of lime as catalyst. The stearin candle inaugurated a new era in lighting. The jury of thehternational Exposition of 1855 recognized the merits of the theory and likewise of its practical application by awarding n grand medal of honor to Chevreul, and amedal of honor to de Milly. The prize of 12,000 francs instituted by
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'he Marquis d'Argenteuil "for the encouragement of national industry" was given to Chevreul in 1852. PROXIMATE ANALYSIS
The great success that issued from his studies of fatty materials strengthened in Chevreul his ideas regarding proximate principles and the application of appropriate means of separating them without altering their natures. The use of neutral or indifferent solvents is a h e example. By such means he brought to a focus one of his capital achievements, namely the creation of and instruction in his methods of proximate organicanalysis. Theseproceduresweresoclear-cutand accurate that they soon entirely supplanted the old, faulty, and confused notions. They provided rules that could be applied to isolated chemical species with the objective of rigorously defining such individuals. Berthelot wrote: "He thereby imposed on chemists and physiologists an inflexiblediscipline, hitherto unheard of in studies of vegetable and animal chemistry." Chevreul's doctrine is developed in his "ConsidBrations gBnBrales sur l'analyse organique et sur ses applications," published a t Paris in 1824. In the introduction to this volume of more than 250 pages, he stresses the importance of his new approach. "Without a determination of the proximate principles that make up plants and animals, organic chemistry cannot he counted among the sciences, since it has rules neither for itself, nor for those who wish to make planned applications of it to the arts or to some branch of knowledge that deals with organized beings. A method that will provide precepts for determining organic species will be truly fundamental." Chevreul appreciated thoroughly that a proximate analysis should precede a useful ultimate analysis, and that in the prevailing state of the science it was impossible to carry out, as a control, syntheses of organic compounds as could be done in the inorganic field. However, the possibility of organic syntheses was foreseen by Chevreul as early as 1824. "I have shown that among the materials which constitute organized beings it is admissibleto distinguish between organic and inorganic compounds, because there exist numerous compounds that are found only in animals and plants, and which cannot be produced by any chemical process now known. However, to regard this distinction as absolute and invariable would be contrary to the spirit of science; this would be to admit the futility of all the attempts that have for their objective to produce compounds identical with or analogous to those that are now considered as peculiar to organized beings." Therefore he heldit incorrect to assume "that life or the so-called vital force can alone produce compounds absolu~elydistmct from inorganic compounds." Before the year ended, Wohler produced oxalic acid from cyanogen, and four years later achieved his classic synthesis of urea. This field of preparative, synthetic organic chemistry was subsequently developed in extensive fashion by Berthelot. In the introduction to this, his second book, Chev-
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reul rigorously imposed on himself the following rules: (a) to define exactly the words employed; (b) to accept as true only the data derived from experiments; (c) to dissect difficulties so as to solve them better; (d) to proceed from the simple to the compound; ( e ) to make enumerations so complete and reviews so general that there would be every assurance that nothing had been omitted; (f) to avoid all useless distinctions and considerations which would tend to slow down the progress of the science, but to establish, on the contrary, all those that might favor such advance by multiplying and revealing the relations in regard to which the products of the organism might be considered. He defined "species" as "a collection of simple materials with identical properties, and particularly in organic chemistry, identical in their composition, and physical, chemical, and organoleptic properties." Heaplied the term "variety" to samples of the same organic species that differ in their secondary crystalline forms or in other less important properties from the compound that is accepted as typical of the species. A I'genus," according to Chevreul, is a collection of organic species which have one or more properties in common, and which are very important or very remarkable. With this as foundation, he proceeded to develop his subject. He showed first of all that proximate analysis is a delicate procedure. For instance, ce~tainmaterials distill without decomposition, whereas others decompose when exposed to the action of heat. Similarly, oxygen behaves differently, according to the temperature, or whether little or much water is present, or foreign materials, such as alkalis The solvents (water, alcohol, ether, acids, bases) may alter or decompose the proximate principles; the same is true of precipitants. It is well to be thoroughly acquainted with these modifying or deleterious effects, so that a reagent may be applied either without introducing an error, or with advantage. Under such enlightened conditions, the changeableness of organic mixtures is no hmger an insurmountable obstacle. In short, Chevreul conferred on proximate organic analysis a great measure of scientific rigor and certainty. The soundness of this methodology was reflected almost a t once, and the founding of the chemistry of alkaloids quickly followed that of the fatty materials. In 1817 Sertiirner described in detail morphine, which he had discovered in 1806. Pelletier and Caventou isolated strychnine in 1818, brucine in 1819, quinine in 1820, etc. Then came the capital discovery of the alcohols, of which only ethanol was really known up to 1835. This idea that proximate principles preexisted in ms, terials had circulated before Chevreul. Nevertheless, to h i belongs the credit for the conception of proximate principles as applied to well-defined materials, that display constant physical and chemical properties, and which cannot be separated without being denatured. Solutions and mixtures had always been confused with real compounds, but before Chevreul, no one had tried to characterize the latter by an exact criterion. Berthelot wrote: "As long as this idea, so simple and yet
W R U A R Y , 1948
so concealed, was not understood and accepted in the science, the progress of organic chemistry remained deprived of all precise basis." Chevreul himself defined chemistry as "the science which reduces materials to species that are characterized by their properties." As early as 1823 he foresaw the existence of isomers. "Substances are known which give on analysis the same elements united in the same proportion, but which are far from having the same properties. If one stops a t the limits of experiment, it is evident that there is no other way of conceiving this case except by referring it to different arrangements of the atoms or particles." Chevreul was an excellent quantitative analyst. He contributed significantly to the development of combustion analysis of organic compounds. For instance, he burned his fatty acids by means of cupric oxide; the water was weighed and the carbon dioxide determined by volume. When corrected by modern atomic weights his data for stearic acid and cholesterin are in quite acceptable agreement with the calculated values. DYEING AND COLORS
In 1824, Chevreul was appointed Director of Dyeing a t the tapestry works at Gobelins and Beauvais, and of the carpet factory of Savonnerie. In these famous establishments, the dyeing was conducted according to unscientific recipes, that were often jealously guarded secrets handed down from father to son. Hence there was need of a competent chemist who could put the art on a rational basis and also improve the fastness and the brilliance of the colors. Chevreul thus found himself obliged to leave a field that had bornemuch fruit, and t o take up a new line of endeavor. He returned to the study of fats only twice throughout the many years that still stretched ahead. At the request of the government (1850-1856) he investigated the drying of oils. He proved that this important characteristic is due not to drying in the usual sense but to the ability of oils, linseed especially, to combine with oxygen. His other late study of fats dealt with lanolin. The new field of work was not entirely novel to him because, as has been mentioned, he had studied indigo and wood at the beginning of his career. He set up a laboratory and got it in running order. His innovations were received with hostility by some of the workers, hut he nevertheless made such rapid progress in dye chemistry that he was able to publish, in 1829 to 1830, a two volume work "Le~onsde chimie appliqu
