l 
 
THE CENTURY SCIENCE SERIES 
 EDITED BY SIR HENRY E. ROSCOE, D.C.L., F.R.S., LL.D. 
 
 JUSTUS VON LIEBIG 
 
The Century Science Series. 
 
 EDITED BY 
 
 SIR HENRY E. ROSCOE, D.C.L., F.R.S., M.P. 
 
 By Ci 
 
 of the 
 
 _ John Dalton and the Rise of Modern Chemistry. 
 
 By Sir HENRY E. ROSCOE, F.R.S. 
 
 Major Eennell, F.R.S., and the Rise of English 
 Geography. 
 
 CLEMENTS R. MARKHAM, C. B., F.R.S., President 
 Royal Geographical Society. 
 
 Justus von Liebig : his Life and Work. 
 
 By W. A. SHENSTONE, Science Master in Clifton College. 
 
 The Herschels and Modern Astronomy. 
 
 By Miss AGNES M. CLERKE, Author of "A Popul ir 
 History of Astronomy during the igth Century," c. 
 In Preparation. 
 
 ... Michael Faraday: his Life and Work. 
 
 By Professor SILVANUS P. THOMPSON, F.R.S. 
 
 Clerk Maxwell and Modern Physics. 
 
 By R. T. GLAZEBROOK, F.R.S., Fellow of Trinity College, 
 Cambridge. 
 
 Charles Lyell : his Life and Work. 
 
 By Rev. Professor T. G. BONNEY, F.R.S. 
 
 - Humphry Davy. 
 
 By T. E. THORPE, F.R.S., Principal Chemist of the 
 Government Laboratories. 
 
 Pasteur : his Life and Work. 
 
 By ARMAND RUFFER, Director of the British institute of 
 Preventive Medicine. 
 
 Charles Darwin and the Origin of Species. 
 
 By EDWARD B. POULTON, M.A., F.R.S., Hope Professor 
 pf Zoology in the University of Oxford. 
 
 Hermann von Helmholtz. 
 
 By A. W. RUCKER, F.R.S., Professor of Physics in the 
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 CASSELL & COMPANY, LIMITED, London; P ar is &> Melbourne. 
 
Photo by Pram ffanfstaengl, Munich. 
 
 JUSTUS VON LIEBIG. 
 
 Born May 12, 1803. 
 
 Died April 18, 1873. 
 
THE CENTURY SCIENCE SERIES 
 
 JUSTUS VON LlEBIG 
 
 HIS LIFE AND WORK 
 
 (1803-1873) 
 
 BY 
 
 W. A. SHEISI STONE, F.I.C. 
 
 Lecturer on Chemistry in Clifton, College 
 
 MACMILLAN & CO. 
 
 1902 
 
PREFACE. 
 
 THE name of Liebig is doubtless familiar to most of 
 us, but I fear that very few have any clear idea what 
 he did, why chemists admire and esteem him, or, 
 indeed, are aware that they do admire and esteem 
 him. As the result of many inquiries, made among 
 cultivated people, I have found the prevailing im- 
 pression concerning Liebig to be that he was a man 
 who gained a large fortune by making "extract of 
 meat." Now and then one meets someone who "seems 
 to have heard " of his name in connection with agri- 
 culture. Scarcely anyone, now, seems to know that 
 he was one of the greatest of that class in whose work 
 Mr. Balfour finds " the causes which, more than any 
 others, conduce to the movements of great civilised 
 societies." I have therefore made it my object, in 
 writing this little book, not so much to dwell upon 
 Liebig's private life as to tell what he was, what 
 he did, and why all chemists and aU those who are 
 versed in the history of science admire and esteem 
 him so greatly. 
 
 Fortunately for my purpose, most of Liebig's work 
 is not only of great general interest, but it lends itself 
 admirably to a non-technical method of treatment. 
 Consequently, I have only found it necessary to 
 employ the language of chemistry in parts of two 
 chapters. As I have been careful to explain technical 
 terms when I have used them, and as I have not very 
 
Univ, Library, UC Santa Cruz 199>0 
 
 vi PREFACE. 
 
 often employed them, I do not think they will be a 
 real source of difficulty or repel anyone. 
 
 If any chemist should read this life of Liobig, he 
 may not improbably feel disposed to complain that 
 it does something less than justice to Liebig's labours 
 in pure chemistry. I admit that this is very true. 
 But it is right that it should be so, for, vast as 
 were Liebig's services to pure chemistry, they lack in 
 some degree the splendour of his contributions to 
 some other departments of equal intrinsic importance 
 and of far wider general interest. 
 
 In concluding these few introductory words, I 
 desire to express my thanks to several very kind 
 helpers : to Liebig's son, Dr. Georg Baron Liebig, who 
 has assisted me most graciously in several ways ; to 
 my friend and colleague, H. Clissold, who has most 
 carefully read the proofs for me; and to my wife, 
 who has very materially lightened my task by helping 
 me to go through the greater part of the numerous 
 bulky volumes which contain Liebig's published 
 correspondence. 
 
 W. A. S. 
 
 Clifton, May, 181)5. 
 
CONTENTS. 
 
 PAGE 
 
 CHAPTER I. INTRODUCTION 
 
 II. LlEBIG AND WOHLER 
 
 III. CHEMICAL DISCOVERIES 
 IV. LIEUIG AND DUMAS 
 
 V. FERMENTATION 
 
 VI. CHEMISTRY OF AGRICULTURE 
 VII. PHYSIOLOGICAL CHEMISTRY . 
 
 VIII. EDUCATIONAL AND OTHER WORK . 173 
 
 IX. CHARACTER AND LATER YEARS . 19" 
 
JUSTUS VON LlEBIG: 
 
 HIS LIFE AND WORK. 
 
 CHAPTER I. 
 
 Introduction Early Life and Tastes His "Wander- Year Appoint- 
 ment at Giessen Method of Organic Analysis Some other 
 Contributions to Chemical Method. 
 
 IT is remarkable that in spite of the epoch-making 
 character of Liebig's contributions to chemistry, to 
 agriculture, to physiology, and to the advancement 
 of education, and in spite of the fact that his name 
 is still a household word over a large part of two 
 continents, no comprehensive or popular account of 
 his life and work has yet been written, though it is 
 more than twenty years since death robbed us of one 
 of the greatest men of this or perhaps of any other 
 century. 
 
 Of Faraday who lived and worked like Liebig, 
 one might almost say with Liebig, when, to men of 
 science, the times were young we have already two 
 lives, those of Dr. Bence Jones and of Dr. Gladstone. 
 Of Pasteur, Liebig's great opponent on the question 
 of the cause of fermentation, whose personality 
 stands out to-day only less distinctly than did that 
 of Liebig fifty years ago, we have a delightful, if 
 rather one-sided, account written by his son-in-law, 
 M. Valery Radot. But of Liebig, perhaps the greatest 
 and the most many-sided of all, we have as yet 
 
10 JUSTUS VON LIEBIG: 
 
 only the memorial addresses of August Vogel on 
 his work in agricultural chemistry, of Emil Erlen- 
 meyer on his contributions to pure chemistry, and 
 of Theodor L. W. von Bischoff on his work and in- 
 fluence on physiology, together with the celebrated 
 Faraday lecture of his brilliant and distinguished 
 pupil A. W. Hofmann, some too brief fragments of 
 autobiography, Moriz Carriere's account of his friend- 
 ship with Platen the poet, and some more fugitive 
 but still interesting contributions, such as Sir Henry 
 Roscoe's obituary notice in Nature of May 8th, 1873. 
 
 But this apparent neglect is only apparent, and 
 is easy to understand. Liebig, owing to his un- 
 rivalled gift of popular exposition, was his own 
 prophet. As he had no need of an interpreter 
 while he lived, so there was no immediate need of 
 a monument after his death. His memorable 
 " Familiar Letters on Chemistry," by means of 
 which he conveyed his teachings to the people, 
 informed those of his own and the succeeding 
 generation what his life-work had been ; and they 
 remained as a sufficient memorial of him for many 
 years after he was gone. 
 
 But with the progress of science the time has 
 now come, as it was sure to come, when the majority 
 of readers can no longer safely betake themselves to 
 Liebig's own writings, in order to learn the part he 
 played in the development of human knowledge. And 
 it is a question whether the immense value of his 
 services is not already more than half forgotten owing 
 to the absence of any suitable review of his Avork. 
 This is especially true of his educational work. How 
 many educated men of to-day nay, how many of the 
 younger chemists are aware that it may fairly be 
 
HIS LIFE AND WORK. 11 
 
 said that it was he who paved the way for the edu- 
 cational revolution which will be for long associated 
 with the second half of the nineteenth century by 
 establishing in 1825 at Giessen his famous labora- 
 tory for giving instruction to all comers in practical 
 chemistry ? 
 
 Justus von Liebig was born on May 12th, 1803, at 
 Darmstadt, where his father dealt in colours, which 
 he also frequently manufactured according to the pro- 
 cesses then prescribed in works on chemistry, some- 
 times with the aid of his small son. As a schoolboy, 
 Liebig was not a success from the pedagogic point 
 of view ; his bent of mind was so distinctly that of 
 the experimenter that, as he tells us, his position at 
 school was very deplorable. Like many other lads of 
 this type, he had no ear memory, and could retain 
 little or nothing of what he learned through the sense 
 of hearing, with the result that he found himself in 
 as uncomfortable a position as a boy could possibly 
 occupy. Not only were all the acquirements that led 
 to praise and honour in the school utterly out of his 
 reach, but once the good Rector of the gymnasium, on 
 the occasion of examining Liebig's class, made a most 
 cutting and public remonstrance with him, reproached 
 him for want of diligence, told him he was the 
 plague of his teachers and the sorrow of his parents, 
 and ended by asking him what did he think was to 
 become of him. Liebig, who, though so ignorant in 
 the linguistic studies of the place, was already pretty 
 widely read in science and versed in the operations of 
 chemistry, replied, amid the uncontrollable laughter 
 of the good rector and of the whole school, " That he 
 would be a chemist." No one at that time had any 
 
12 JUSTUS VON LIEBIG : 
 
 idea that chemistry was a subject that could be 
 studied for itself. To most it was a mere accessory 
 subject, at best a handmaid to medicine and phar- 
 macy ; the idea of the study of chemistry being- 
 adopted as a career seemed preposterous.* 
 
 It was plain, however, that none of the ordinary 
 careers open to a gymnasium student were possible 
 for Liebig, and owing, doubtless, to his inclination for 
 chemistry, he was taken by his father to an apothecary 
 at Heppenheim. Here he soon became acquainted 
 with the various applications of the multifarious con- 
 tents of a druggist's shop, but pill-making did not 
 please him ; he wished to be a chemist, not a druggist. 
 Soon, therefore, he began to make experiments of a 
 non-pharmaceutical character privately in his attic ; 
 before long, accidents occurred, and at last one day 
 the attic window-sash was blown out. At the end of 
 ten months, the alarmed apothecary of Heppenheim 
 was so sorry with his bargain that he sent the lad 
 home again to his father. 
 
 Liebig was now about sixteen, at which age in 
 those days most lads were beginning to take life 
 seriously, and it was plainly difficult to know what to 
 do with him, or to foresee what he would do for him- 
 self. His time had not, however, really been wasted. 
 The interest he had taken in his father's work had led 
 him long before he left school to read with passionate 
 interest the books used for guidance in the manu- 
 
 * The above and many other personal details concerning Liebig 1 s 
 early life are taken, frequently in his own words, from an auto- 
 biographical fragment, which was discovered by his son, Dr. Georg 
 Baron von Liebig, a few years ago, and published in the Deutsche 
 Rundschau for January, 1891. A translation of this sketch, by Prof. 
 J. Campbell Brown, was read at Liverpool on March 19th in the same 
 year, and published in the Chemical News on June 5th and 12th. 
 
HIS LIFE AND WORK. 13 
 
 facture of colours. In fetching these books from the 
 Court Library he had become acquainted with the 
 Librarian, Hess, through whose kindness he soon 
 had the run of the library, where he read in anyhow 
 fashion such works as Macquer on Chemistry ; Basil 
 Valentine's Triumphal Car of Antimony ; Stahl's 
 Phlogistic Theory ; together with numerous essays 
 and treatises, including the writings of Kir wan and 
 Cavendish. course, as he tells us, he did not in 
 this way gain much exact knowledge from his reading, 
 but it led him to attempt to carry out the experiments 
 which he read about as far as his means would allow, 
 and, these being very limited, to make countless 
 repetitions of such of them as he was able to perform, 
 with the result that, boy as he was, he had already 
 acquired, in a considerable degree, that power of 
 perceiving the resemblances and differences between 
 things and between phenomena which he has called 
 sight- or eye-memory. 
 
 Every one of the numerous white precipitates 
 known to the chemist has some quality or qualities 
 peculiar to itself which should be recognised by those 
 who have once fully studied it. Liebig's early habit 
 of experimenting helped to develop in him the eye- 
 memory which makes this possible. This power was 
 afterwards possessed by him to such an extent that in 
 later years he was sometimes able to recognise, by their 
 appearances alone, eleven rare chemicals many years 
 after having once worked with them, with such cer- 
 tainty that he was not even misled by the results 
 of the analysis of impure specimens. 
 
 Though Liebig had not mastered the lessons of the 
 gymnasium, he had gained a knowledge of most of the 
 processes carried on in his neighbourhood. He had 
 
14 JUSTUS VON LIEBIG: 
 
 watched the soap-boiler, and had made soap on his own 
 account. " In the workshop of the tanner and dyer, 
 the smith and brass founder, he was at home and 
 ready to do any hand's turn." Nay, as we shall see, 
 he had even drawn inspiration from a peripatetic 
 cheapjack who visited Darmstadt, and plied his trade 
 in the market place, as he watched him prepare ful- 
 minating silver for his pea- crackers, and clean coat 
 collars for the country folk. Left to himself, he had 
 gained extensive stores of information, and a deep- 
 seated desire for more, and at the age of sixteen, by 
 persistent importunity, he induced his father to per- 
 mit him to go to the University of Bonn, whence he 
 afterwards followed his professor, Kastner, to Erlangen 
 on the removal of the latter to that university. 
 
 There had arisen, Liebig tells us, about that time 
 at the then newly-established University of Bonn, an 
 extraordinary quickening of scientific life, which, how- 
 ever, was unfortunately most perniciously affected by 
 the philosophical methods of investigation as they had 
 been embodied by Oken and by Wilbrand, which had 
 led alike in lecture and in study to a want of appre- 
 ciation of experiment and of an unprejudiced obser- 
 vation of nature that was ruinous to many talented 
 young men " From the professorial chair the pupils 
 received an abundance of ingenious contemplations ; 
 but, bodiless as they were, nothing could be made 
 of them." 
 
 At this time Sir Humphry Davy and others in 
 England ; Berzelius, the great Swede ; and in France 
 a whole galaxy of brilliant experimenters, includ- 
 ing Gay-Lussac, Dulong, Arago, and Chevreul, were 
 rapidly opening out new spheres of investigation of 
 almost boundless importance, but their inestimable 
 
HIS LIFE AND WORK. 15 
 
 acquisitions found no soil, and could bear no fruit 
 in Germany, where, as Liebig says, " It was then a 
 wretched time for chemistry." 
 
 At Bonn and Erlangeftftherefore, Liebig soon learnt 
 that he was not in the way to become a chemist. 
 But here he also discovered, from his intercourse with 
 other students, his own ignorance of many subjects 
 with which they had gained an acquaintance at school. 
 This was something, and since he could learn no 
 chemistry, he exerted all his energies to make up 
 for his previously neglected school studies ; whilst by 
 organising and working with a small band of students, 
 who formed themselves into a cheinico-physical union, 
 he gained some practice in composition and in the 
 art of speaking. But this was all, and before long he 
 returned to Darmstadt, persuaded that he could not 
 become a chemist in Germany. 
 
 Up to this time Liebig's career had certainly been 
 calculated to give a good deal of anxiety to his rela- 
 tions, especially as latterly he had come into conflict 
 with the authorities, and was even at one time under 
 arrest for supposed political offences, though he was 
 not, as we are told by Platen, conscious of any real 
 fault of his own. But there is no doubt that even at 
 this early stage he showed, to those who were capable 
 of judging him, an extraordinary degree of promise. 
 He had not only attracted the interest of Hess, but 
 he was the favourite pupil of Kastner the chemist, 
 whilst the poet Platen may almost be said to 
 have fallen in love with him at sight. The latter 
 wrote of him in his diary on March 13th, 1822 : 
 " The day before yesterday I made an interesting 
 acquaintance. This is a young chemist from Darm- 
 stadt, who is named Justus Liebig, the same student 
 
 l*^\&u^^ 
 
16 JUSTUS VON LIEBIG: 
 
 whom I met some time since at Kastner's. Billow had 
 already described him to me as Kastner's favourite, as he 
 has, particularly in chemistry, very sound knowledge." 
 Before long Platen and Liebig met again, walked 
 in the country, and afterwards adjourned to Liebig's 
 dwelling-place. Platen afterwards declared his new 
 friend to be clear, definite, and solid in everything, 
 and, above all, on the side of the affections, open and 
 confiding. "Never before," said he, "have I been treated 
 with such affection upon so brief an acquaintance." 
 Owing to various accidents, the meetings of these two 
 were very few, but Platen has left us a delightful 
 description of Liebig's personality at this period. 
 " Liebig," he said, writing after a walk with him, " was 
 never more beautiful. Of slender form, a friendly 
 earnestness in his regular features, great brown eyes 
 with dark shady eyebrows, which attracted one in- 
 stantly Oh that I might, after so many 
 
 deceptions, find happiness and peace in this friendship, 
 which seems to open up new future possibilities ! " 
 
 In 1822, at the age of nineteen, Liebig took the 
 degree of Doctor of Philosophy at Erlangen, and at 
 about the same time he published the result of his 
 first attempt at an investigation in a paper on the 
 composition of fulminating mercury, which was 
 remarkable for the clearness and precision of its 
 language. This paper and some analyses of certain 
 colouring matters which he had also already performed 
 fortunately attracted the attention of people who pos- 
 sessed influence with Louis the First, the then reign- 
 ing Grand Duke of Hesse-Darmstadt. Before long 
 Liebig's chance came. He had the good fortune to be 
 provided by Duke Louis with the necessary means for 
 prosecuting his studies abroad. 
 
HIS LIFE AND WORK. 17 
 
 But whither should he go ? 
 
 In those early years of the nineteenth century the 
 younger men among the German chemists had already 
 many of them repaired to Stockholm in search of 
 inspiration and instruction in the modest laboratory 
 of Berzelius. Mitscherlich, the discoverer of iso- 
 morphism; H. Rose, the analyst; and, later on, Wohler, 
 whose production of urea from materials of purely in- 
 organic origin afterwards revolutionised the views of 
 chemists on organic chemistry, and finally established 
 the idea of isomerism in the science ah* visited 
 Sweden to become the pupils and friends of the great 
 Northern chemist, and it is probable that Liebig, to 
 whom the writings of Berzelius had already been " as 
 springs in the desert," would have followed their 
 footsteps had not Paris offered him opportunities of 
 wider study that were an irresistible attraction. 
 Therefore, to Paris he went, to sit at the feet of the 
 great masters who adorned the French capital at that 
 time. There he found the opportunities he desired. 
 The lectures consisted of a wisely-arranged succession 
 of experiments, whose connection was completed by 
 oral explanations. " The experiments," says Liebig, in 
 his autobiographic fragment, " were a real delight to 
 me, for they spoke to me in a language I understood, 
 and they united with the lecture in giving a definite 
 connection to the mass of shapeless facts which lay 
 mixed up in my head without order and without 
 arrangement," whilst the lectures, as a whole, made a 
 most marked impression on his mind by their intrinsic 
 truth, by the absence of pretence. 
 
 At the time of Liebig's arrival in France that is 
 to say, in 1822 there did not exist in all Paris nor in 
 all the world one such public laboratory for workers 
 
18 JUSTUS VON LIEBIG: 
 
 in chemistry or physics as may now be found in 
 every provincial town of the first class. Though the 
 lectures were so excellent, public places of instruction 
 in analysis and experimenting generally were still as 
 completely non-existent in France as in Germany, 
 and admittance into a chemical laboratory was then 
 a difficult thing indeed for a stranger to attain. By 
 the kind assistance of Thenard, however, Liebig was 
 permitted to continue his researches on explosives in 
 the private laboratory of Gaultier de Claubry, and he 
 soon published another paper on this subject. But he 
 was not even then able to feel complete confidence 
 in his results, and he was meditating yet further ex- 
 periments, when by a happy chance, in the summer 
 of 1823, he made the acquaintance of Humboldt 
 the traveller. From that day Liebig found all doors 
 and all laboratories open to him as by magic, and he 
 was soon at work in the laboratory of Gay-Lussac, 
 revising his analyses of the fulminates, with the result 
 that early in the next year he brought them to a 
 successful conclusion, and was rewarded by the dis- 
 covery of isomerism and by the honour of a waltz 
 round the laboratory with his distinguished teacher, 
 who was in the habit of relieving his feelings, as 
 he explained to his young friend, when discoveries 
 were made, by such-like terpsichorean exercises. 
 
 The account of Liebig's meeting with Humboldt, 
 and of his introduction to Gay-Lussac, was told by 
 him years afterwards to Mr. E. K. Muspratt in the 
 Munich Laboratory, and is so interesting that it 
 deserves to be repeated. 
 
 One day in the summer of 1823 he gave 
 an account of his earlier analyses of fulminating 
 silver before the Academy of Sciences. Having 
 
HIS LIFE AND WORK. 19 
 
 finished his paper, as he was packing up his pre- 
 parations a gentleman caine up to him and ques- 
 tioned him as to his studies and future plans, and, 
 after an exacting examination, ended by asking him 
 to dinner on the following Sunday. Liebig accepted 
 the invitation, but, through nervousness and con- 
 fusion, forgot to ask the name and address of his 
 interviewer. Sunday came, and poor Liebig was in 
 despair at not being able to keep his engagement. 
 
 The next day a friend came to him and said, 
 " What on earth did you mean by not coming to dine 
 with von Humboldt yesterday, who had invited Gay- 
 Lussac and other chemists to meet you ? " "I was 
 thunderstruck," said Liebig. " I rushed off as fast 
 as I could run to von Humboldt's lodgings, and made 
 the best excuses I could." The great traveller, satis- 
 fied with the explanation, told him it was unfortu- 
 nate, as he had several members of the Academy at 
 his house to meet him, but thought he could make 
 it all right if he would come to dinner next Sunday. 
 He went, and then made the acquaintance of Gay- 
 Lussac, who was so struck with the genius and 
 enthusiasm of the youth that he took him into his 
 private laboratory, and continued in conjunction with 
 him the investigation of the fulminating compounds. 
 
 Nor did Humboldt's assistance stop here, for it was 
 on his recommendation that Liebig was afterwards 
 appointed Professor in the little University of Giessen, 
 where he established a school, whose achievements 
 in pure and applied chemistry must have far tran- 
 scended the most sanguine of his youthful aspirations. 
 From the most modest beginning and the scantiest 
 means came results which fill one of the most 
 splendid pages in the history of chemistry. It was 
 
20 JUSTUS VON LIEBIG : 
 
 in Gay-Lussac's laboratory that Liebig, conscious 
 of what he owed to the guidance and friendship 
 of his master, and conscious too of his own growing 
 insight and power, conceived the idea of founding 
 in Germany a school where he should be to his 
 younger fellow workers that which Gay-Lussac had 
 been to himself. A glorious dream gloriously ful- 
 filled, as we shall presently see. 
 
 Liebig was appointed Extraordinary Professor of 
 Chemistry at Giessen in 1824, and Ordinary Pro- 
 fessor two years later. He was called to Munich in 
 1852, and died there on April 18th, 1873. 
 
 Liebig was essentially a pioneer in science. In 
 the course of his life he took the lead in no less 
 than four great departures. The first was in organic 
 chemistry, the second and third in the applications 
 of chemistry to agriculture and to physiology, the 
 fourth, as will presently appear, was the outcome of 
 his labours as a teacher. His work, like that of other 
 pioneers, was, of course, not always correct in all 
 points of detail. But it had all the greater merits 
 of good pioneering work in a most marked degree. 
 It almost always pointed the right way, and its re- 
 markable influence in determining the direction of 
 subsequent research has been singularly permanent. 
 
 But a pioneer in science, like a traveller, must not 
 set out till he is fully equipped. It was useless for 
 Liebig or anyone else to attempt to explore the depths 
 of organic chemistry before a method for the analysis 
 of organic substances was at his command. This 
 Liebig knew full well, and accordingly we find that 
 his leisure during his first years at Giessen was 
 devoted to contriving such a method. 
 
 When the chemist discovers a new substance, 
 
HIS LIFE AND WORK. 21 
 
 whatever its nature may be, whether it is composed 
 of the so-called mineral elements, such as the metals, 
 sulphur, phosphorus, chlorine, etc., or whether it 
 belongs to the so-called organic group of compounds 
 in which the presence of carbon, oxygen, hydrogen, 
 nitrogen, are especially characteristic, it is necessary 
 in the first place to learn its ultimate composition 
 that is to say, to find out by analysis what elements 
 it contains, and in what exact proportions they are 
 present. 
 
 In organic analysis the chemist has usually only 
 to deal with some eight or ten elements, and often 
 with only three or four viz. with compounds of 
 carbon, with hydrogen, oxygen, or nitrogen; hence 
 the operations of ultimate organic analysis are 
 characterised by their comparative simplicity ; but 
 success in this branch of work has only been attained 
 as the result of many failures, owing to the great 
 initial difficulties whiph for half a century prevented 
 chemists from making much progress. 
 
 Lavoisier was one of the first to attempt to analyse 
 organic compounds. He knew that carbon, when 
 burnt, yields the well-known heavy, suffocating car- 
 bonic acid gas, and he had established, with some 
 approach to precision, the relative proportions in 
 which carbon and oxygen enter into its composition. 
 
 Similarly from the work of Cavendish, and from 
 his own experiments, Lavoisier knew approximately the 
 relative proportions in which hydrogen and oxygen 
 unite to form water. Armed with these three facts, 
 he endeavoured to ascertain the proportions in which 
 carbon, hydrogen, and oxygen are present in alcohol 
 and oil, by burning weighed quantities of them in small 
 lamps placed under receivers, standing over mercury, to 
 
22 JUSTUS VON LIEBIG: 
 
 which such additional measured volumes of oxygen as 
 might be necessary could be added during the process. 
 At the end of the process he measured the carbonic 
 acid gas formed, and the volume of the air which 
 remained. From the data so obtained he endeavoured 
 to calculate the composition of the substance burnt in 
 his lamp ; but, unfortunately, neither his knowledge of 
 the composition of the compounds produced, nor of 
 the relative densities of the gases concerned, was 
 sufficiently accurate for his purpose, and hence the 
 results of his experiments had no permanent value. 
 Later he made other experiments, in which he collected 
 and weighed the products of burning oil, but these 
 were cut short by his pseudo-judicial murder in 1794. 
 It was not until Liebig attempted to solve the 
 problem that success was really attained ; and Liebig 
 only achieved complete success after devoting many 
 years to thinking and experimenting. 
 
 The method of organic analysis given to chemistry 
 by Liebig, like that of Lavoisier, consists in completely 
 burning, or oxidising, the substance which is to be 
 analysed, and then collecting and measuring the car- 
 bonic acid gas and water formed. This process is at once 
 admirable in its conception and in its execution. It 
 is a model of simplicity and accuracy. It remains 
 in use, almost unchanged, to this day ; it has served 
 for the analysis of countless numbers of organic sub- 
 stances, and has thus formed the very foundation on 
 which the whole vast structure of modern organic 
 chemistry is based. It may fairly be said to be one of 
 Liebig's greatest gifts to science. The details of this 
 beautiful process cannot be here described, but may 
 be found in every treatise on organic chemistry. In 
 contriving it, Liebig selected with unerring judgment 
 
HIS LIFE AND WORK. 23 
 
 all that was best in the ideas of his predecessors ; 
 added his famous absorption apparatus, "Liebig's 
 potash bulbs;" and embodied the whole in a form 
 which remains nearly intact after half a century. 
 
 Modifications of Liebig's process that were suitable 
 for bodies containing other elements, such as nitrogen, 
 chlorine, and sulphur were soon introduced, some of 
 them by Liebig or his pupils, and thus the fundamental 
 difficulty which had retarded the progress of organic 
 chemistry to an almost inconceivable extent for 
 nearly fifty years was overcome, and Liebig and his 
 pupils were able to carry out the numerous investiga- 
 tions which have given the little Hessian University a 
 world- wide reputation. 
 
 By the analysis of a compound, however, the 
 chemist only learns the relative proportions in which 
 its constituents have combined to form it. Analysis 
 alone tells him nothing about the weight of its 
 molecule (page 32) ; and until he knows this, as well 
 as the percentage composition of a substance, he can 
 neither tell in what numbers the atoms of its 
 component elements occur in its molecules, nor 
 proceed to the important work of investigating its 
 constitution that is to say, the relations of these 
 various atoms to one another. 
 
 The methods of weighing molecules are, some of 
 them physical, some of them chemical Liebig made 
 important contributions to the latter class, by teaching 
 us two elegant processes, suitable for the group of 
 compounds known as the organic bases. The first 
 is no longer much in use, but its successor, which 
 consists in the analysis of the compounds they form 
 with chloride of platinum, is still frequently used, and 
 is highly valued for its accuracy. 
 
24 JUSTUS VON LIEBIG : 
 
 In concluding this brief sketch of some of Liebig's 
 chief contributions to chemical method, all of which 
 were characterised by simplicity, elegance, and ac- 
 curacy, one cannot refrain from alluding first to the 
 process of gas analysis, in which he employed the 
 long-known power of alkaline solutions of pyrogallic 
 acid to absorb oxygen, for the purpose of measuring 
 the volume of that gas in air and other gaseous mixtures 
 containing it, and also took advantage of the necessary 
 use of potash in this process to combine the measure- 
 ment of oxygen with that of carbonic acid gas. And, 
 secondly, to the " Liebig's Condenser." This, however, 
 will be already familiar to nearly every one who has 
 visited a chemical laboratory. No single instrument 
 has done better service to experimental chemistry 
 than this. It is, as we all know, in daily and hourly 
 use in every laboratory. It has become almost as 
 essential to the work of every student of chemistry as 
 the test-tube. 
 
HIS LIFE AND WORK. 25 
 
 CHAPTER II. 
 
 LIEBIG AND WOHLER. 
 
 Liebig and Wohler Wdhler's Early Life His Visit to Berzelius 
 The Composition of Fulminic Acid and Cyanic Acid Isomerism 
 Liebig and Wohler meet They Propose to Work Together 
 Researches on Oil of Bitter Almonds The Benzoyl Theory 
 Study of Uric Acid and its Derivatives The Characters of Liebig 
 and Wohler Contrasted Their Mutual Esteem. 
 
 IT would be impossible to tell the story of Liebig and 
 his work without soon referring to his joint labours 
 and life-long intimacy with Friedrich Wohler. Their 
 joint work and their friendship will be remembered, 
 and will link the names of Liebig and Wohler through 
 all time in the mind of every student of chemistry, 
 equally for the purity and warmth which charac- 
 terised their historic alliance, and for the epoch- 
 making results which were the outcome of their 
 mutual endeavours. 
 
 Friedrich Wohler was born in the neighbourhood 
 of Frankfort on the 31st of July, 1800. Like Liebig, 
 he showed at an early age a passion for experiment- 
 ing, which is said to have been the cause of frequent 
 neglect of his studies at the gymnasium. His scientific 
 tastes were fostered and directed by Dr. Buch, a 
 physician who sympathised with his inclinations, and 
 who had himself devoted time to the study of 
 chemistry and physics. His father, who was a citizen 
 of some position in Frankfort, on the other hand, 
 encouraged in him a taste for drawing, for the litera- 
 ture of his country, and, above all, indoctrinated him 
 with a love of outdoor life and exercise, which was 
 
26 JUSTUS VON LIEBIG: 
 
 probably in a great degree the cause of the almost 
 constant good health* that he enjoyed throughout a 
 long and active life. 
 
 After obtaining his degree in 1823, a year later 
 than Liebig, Wohler determined, on the advice of 
 Gmelin, to place himself under the direction of 
 Berzelius, at Stockholm, who was then at the summit 
 of his fame alike on account of his achievements in 
 analysis and for his contributions to chemical theory ; 
 and Berzelius having been applied to, and having 
 consented to his desire, Wohler was soon on his way 
 to the famous laboratory in Sweden. The following 
 account of his reception, and of the laboratory and 
 life at Stockholm, will be interesting to everyone who 
 has visited a laboratory of to-day : " With a beating 
 heart," he says, "I stood before Berzelius's door and rang 
 the bell. It was opened by a vigorous and portly man. 
 This was Berzelius himself. As he led me into his 
 laboratory I was as in a dream, doubting if I could 
 really be in the classical place which was the object 
 of my aspirations. ... I was then the only one 
 in the laboratory. . . . The laboratory consisted 
 of two ordinary rooms, furnished in the simplest 
 possible way ; there were no furnaces or draught 
 places, neither gas nor water supply. In one of the 
 rooms were two common deal tables ; at one of these 
 Berzelius worked, the other was intended for me. 
 On the walls were a few cupboards for reagents ; in 
 the middle was a mercury trough, whilst the glass 
 blower's lamp stood on the hearth. In addition was 
 a sink with an earthenware cistern and tap standing 
 over a wooden tub, where the despotic Anna, the cook, 
 had daily to clean the apparatus. In the other room 
 were the balances, and some cupboards containing 
 
HIS LIFE AND WORK. 27 
 
 instruments ; close by was a small workshop fitted 
 with a lathe." 
 
 " In the adjacent kitchen, in which Anna prepared 
 the meals, Avas a small and seldom used furnace and a 
 never cool sand-bath." Anna appears to have picked 
 up the elements of nomenclature, for Wohler tells 
 us that he was not a little surprised on one occasion 
 to hear Berzelius chide her for saying that some 
 apparatus she was cleaning smelt strongly of oxy- 
 muriatic acid, saying, " Hearest thou, Anna, thou 
 must no longer speak of oxymuriatic acid ; thou must 
 call it chlorine ; that is better." 
 
 After a year at Stockholm, Wohler's visit to 
 Berzelius was brought to a conclusion by a short 
 period of travel in Southern Sweden and Norway, 
 which was partly devoted to collecting geological 
 specimens and partly to sport. In the course of his 
 travels he met Davy, who, like Wohler, had a marked 
 taste for active outdoor exercises. 
 
 This tour completed, Wohler returned to Germany, 
 having contracted a friendship with Berzelius which 
 continued without interruption throughout the life of 
 the latter, in spite of the fact that Wohler's subsequent 
 work with Liebig brought him into occasional conflict 
 with the rather tightly held convictions of Berzelius 
 on various points of chemical theory. 
 
 After his return to Germany, Wohler accepted a 
 post at the then recently founded trade school in 
 Berlin, and thus obtained possession of a laboratory 
 of his own ; he stayed in Berlin about six years, and 
 during this period, besides less important work, he 
 succeeded for the first time in isolating aluminium 
 by a process which has only lately been superseded ; 
 and accomplished the transformation of ammonium 
 
28 JUSTUS VON LIEBIG: 
 
 cyanate, an inorganic substance, into urea, one of the 
 most characteristic of animal products, thereby break- 
 ing down the imaginary barrier between the products 
 of the chemical and the so-called vital forces, and 
 opening out a new field of investigation which is still 
 inexhausted, and seems inexhaustible. 
 
 Above all, during these years he found Liebig. 
 The mode in which they were brought together is 
 one of the romances of science. Whilst still a boy, 
 Liebig's attention had been drawn to the fulminates, 
 a class of bodies which owe their name to their 
 violently explosive character, when watching a peri- 
 patetic dealer in odds-and-ends make fulminating 
 silver for fire-crackers in the market-place at Darm- 
 stadt, by dissolving silver in nitric acid, and then 
 adding a liquid which smelt of brandy with which 
 he also cleaned dirty coat collars for the rustics 
 to the product. Liebig's attention was so strongly 
 drawn to this body, that he afterwards, as has already 
 been mentioned, made repeated examinations and 
 analyses of it, which were only brought to a success- 
 ful issue by the work done in Gay-Lussac's laboratory 
 in 1823-24. 
 
 At almost the same time Wohler was occupied 
 with his investigations of another and very dissimilar 
 substance, cyanic acid, in the laboratory of Berzelius. 
 When Liebig had finally satisfied himself as to the 
 composition of fulminic acid, he was surprised to 
 find that his results coincided with the analyses of 
 cyanic acid made and published by Wohler somewhat 
 earlier. 
 
 Cyanic acid and fulminic acid were so obviously 
 different that no one could confuse them. Such 
 a result, therefore, seemed almost absurdly impossible. 
 
HIS LIFE AND WORK. 29 
 
 How could the same elements combine in the same 
 proportions to form dissimilar compounds ? It was 
 contrary to the fundamental principles of chemistry ; 
 there must be a mistake somewhere. 
 
 Confident that his own results were correct, Liebig 
 repeated the investigation of Wohler. But this was 
 found to be correct too. These two compounds, so 
 different from each other, were indeed identical in 
 their composition. 
 
 Berzelius and some other chemists did not at 
 first accept these concluions, in spite of the fact 
 that some compounds were already known among 
 inorganic bodies, which were opposed to the axiom 
 that substances having the same qualitative and 
 quantitative composition must exhibit the same 
 properties. But Gay-Lussac not only accepted the 
 results, but pointed out that the new facts might be 
 accounted for by assuming a difference in the manner 
 in which the constituent elements are combined in the 
 two substances. And before very long it became 
 impossible for any one to doubt the existence of the 
 phenomenon of isomerism, as it was named by 
 Berzelius, which was thus first recognised through 
 the work of these two Liebig and Wohler. On the 
 very threshold of their careers, these two men had 
 made a discovery of the first order. It w^as inevitable 
 that they should become rivals or friends. It was 
 characteristic of them that they became friends. Not 
 indeed at once, but as soon as an opportunity offered 
 itself. They met, some time after, in the house of a 
 common friend at Frankfort, and the acquaintance 
 then formed soon ripened into friendship, and resulted 
 in a frequent and free intercourse, never afterwards 
 interrupted. Their friendship was soon cemented by 
 
30 JUSTUS VON LIEBIG: 
 
 their undertaking the joint labours which have in- 
 separably united their names in the annals of science. 
 
 The proposal that they should thus join hands 
 came first from Wohler, in the following characteristic 
 letter : 
 
 " SACROW, near POTSDAM, 8th June, 1829. 
 
 "DEAR PROFESSOR, The contents of your last letter to 
 Poggendorff have been communicated to me by him, and I am 
 glad that they afford me an opportunity of resuming the 
 correspondence which we began last winter. It must surely be 
 some wicked demon that again and again imperceptibly brings 
 us into collision by means of our work, and tries to make the 
 chemical public believe that we purposely seek these apples of 
 discord as opponents. But I think he is not going to succeed. 
 If you are so minded, we might, for the humour of it, undertake 
 some chemical work together, in order that the result might be 
 made known under our joint names. Of course, you would 
 work in Giessen and I in Berlin, when we are agreed upon the 
 plan, and we could communicate with each other from time to 
 time as to its progress. I leave the choice of subject entirely 
 to you. 
 
 " I am very glad that you have also determined the identity 
 of pyro-uric and cyanic acids. Grnelin would say : ' God be 
 thanked, there is one acid the loss.' . . . Yours, 
 
 " WOHLER." 
 
 Liebig expressed his joyful assent to this proposition 
 at once, and a research upon mellitic acid the acid 
 of honeystone was selected, and carried to a success- 
 ful issue. Soon after publishing their results they 
 again joined forces, at Wohler's suggestion, to make 
 an investigation of an acid, cyanuric acid, obtained 
 by him from urea, in the course of which Wohler 
 observed the remarkable examples of molecular 
 re-arrangement, which occur in the spontaneous 
 transformation of the liquid cyanic acid into its iso- 
 meride, the solid cyanuric acid, and the re-conversion 
 of the latter, on distillation, into cyanic acid once 
 
HIS LIFE AND WORK. 31 
 
 more. It was next proposed that fulminic acid 
 should form the subject of a new and joint attack ; 
 indeed, Liebig evidently at one time commenced 
 operations. These, however, were soon abandoned, 
 for he wrote on November 18th, 1830 : " The fulminic 
 acid we will allow to remain undisturbed. Like you, 
 I have vowed to have nothing more to do with this 
 stuff. Some time back I wanted, in connection with 
 our work, to decompose some fulminating silver by 
 means of ammonium sulphide; at the moment the 
 first drop fell into the dish, the mass exploded under 
 my nose. I was thrown backwards, and was deaf for 
 a fortnight, and became almost blind." 
 
 It would be impossible and unfitting in this book 
 to enter at any length into the details of the fifteen 
 memoirs published jointly by Wohler and Liebig, but 
 the results of several of their researches are of such 
 remarkable importance, and have played so great a 
 part in the growth of chemistry, that it is impossible 
 not to refer to some of them. 
 
 First in importance stands their research on oil 
 of bitter almonds, concerning which Berzelius wrote 
 to them : " The facts put forward by you give rise 
 to such considerations that they may well be deemed 
 the beginning of a new day in vegetal chemistry." 
 
 Well might Berzelius commend this splendid piece 
 of work. It was one of the most prolific of their joint 
 investigations. It not only gave several new and im- 
 portant compounds to chemistry, but it also exercised 
 a most important influence on the growth of chemical 
 theory, by establishing in organic chemistry the con- 
 ception of what are called compound radicles. 
 
 When these pioneers in the new organic chemistry 
 took the field, in 1832, oil of bitter almonds was 
 
32 JUSTUS VON LIEBIG: 
 
 already known as a volatile liquid rendered familiar by 
 its characteristic smell, and remarkable for its power 
 when exposed to the air of absorbing oxygen and 
 changing into a beautifully crystalline substance, 
 then as now known as benzole acid. 
 
 Armed with Liebig's method of organic analysis, 
 the two chemists soon ascertained the true com- 
 position of these substances. They arrived at the 
 formulae by which, in effect, we still represent them, 
 and made the exact relation of the one to the other 
 easy to understand. 
 
 For the sake of those of my readers who are not 
 acquainted with chemistry, I must here explain that, 
 according to the conceptions of chemists, the larger 
 masses of matter with which we are familiar may 
 be supposed to be built up of exceedingly small 
 and indestructible particles, called atoms. These 
 atoms are so exceedingly small as to be quite beyond 
 our powers of seeing, even when we are assisted 
 by the most powerful microscopes. The atoms of 
 a given chemical element are supposed to be in 
 every way identical in their properties, including 
 their weight. Those of different elements, however, 
 are, on the other hand, unlike in their properties. 
 Atoms are supposed to be endowed with the power 
 of attracting one another in varying degrees, to 
 form the molecules of compounds or of elements. 
 
 Chemists are in the habit of representing these 
 atoms by means of conventional symbols. Thus the 
 letter H represents one atom of hydrogen ; one 
 atom of oxygen ; C one atom o'f carbon ; N one atom 
 of nitrogen ; Cl one atom of chlorine ; and so on. 
 
 Compounds are represented by placing the sym- 
 bols of the combined elements close to each other. 
 
HIS LIFE AND WORK. 33 
 
 Thus, there being good reason to conclude that each 
 molecule of water contains two atoms of hydrogen 
 united with one atom of oxygen, we give the formula 
 HoO to this substance. 
 
 The theory thus briefly sketched is known as 
 the " Atomic Theory." We owe it to an Englishman 
 John Dalton, of Manchester.* 
 
 When Liebig and Wohler had worked out the 
 formulae of oil of bitter almonds and of benzoic acid, 
 they found that the former might be looked upon as 
 a compound of an atom of hydrogen with a group of 
 atoms, or radicle, C-ELO, which we call benzoyl i.e. 
 as C-ELO, H ; and that the latter might be considered 
 to contain the same radicle united with another 
 group or radicle, hydroxyl. This view is represented 
 by the following formulae, in which benzoyl. C 7 H 5 O, 
 is common to both substances : 
 
 OH of bitter almonds ... ... C 7 H 5 O, H 
 
 Benzoic acid ... ... ... C 7 H 5 O, OH 
 
 By other experiments they discovered several more 
 new substances which might also be regarded as com- 
 pounds of benzoyl. These are given in the following 
 table ; they show well what a clearness of ideas was 
 gained by conceiving the presence in all these com- 
 pounds of a common group of atoms, which could 
 be transferred from one compound to another, as it 
 were, like a single atom : C Rr 
 
 Oil of bitter almonds ... ... C 7 H 5 0, H 
 
 Benzoic acid ... ... ... C 7 H 3 O.(OH) 
 
 chloride C 7 H 5 0', Cl 
 
 bromide ... ' ... ... C 7 H 5 O, Br 
 
 iodide ... ... ... C 7 H 5 O, I 
 
 cyanide ... ... ... C 7 H 5 0, CN 
 
 * Sec " John Dalton and the Rise of Modern Chemistry " in this 
 series. 
 
 C 
 
34 JUSTUS VON LIEBIG: 
 
 This idea of compound radicles was not, it is true, 
 entirely new when Liebig and Wohler applied it in 
 the above case, for Berzelius had, as early as in 1820, 
 attempted to compare the constitutions of organic 
 substances with those of inorganic compounds by the 
 use of such a conception, whilst it was known from 
 the researches of Gay-Lussac that cyanogen, a com- 
 pound of nitrogen and carbon, acts very much like 
 an element. 
 
 But a real appreciation of the existence of a con- 
 nection between the properties of substances and the 
 radicles they contain was mainly brought about, in 
 the first instance, by this memorable research " upon 
 the radicle of benzoic acid." 
 
 Well might these investigators modestly congratu- 
 late themselves on this achievement. They had 
 discovered a true path into the almost unknown 
 regions of organic chemistry by their " Benzoyl 
 Theory." 
 
 But Liebig and Wohler did not stay their hands at 
 this point. It was known that oil of bitter almonds 
 does not exist ready formed in the almonds them- 
 selves, and that the almonds contain a beautifully- 
 crystalline substance amygdalin. 
 
 In 1836 Wohler was selected to succeed Stromeyer 
 as Professor of Chemistry at Gottingen. The choice lay 
 between Liebig and Wohler on this occasion. As soon as 
 he was ready for fresh work, he wrote to Liebig : " I 
 am like a hen which has laid an egg and straightway 
 sets up a great cackling. I have this morning found 
 how bitter oil of almonds containing prussic acid may 
 be obtained from amygdalin, and would propose that 
 we jointly undertake the further investigation of the 
 matter, as it is intimately related to the benzoyl 
 
HIS LIFE AND WORK. 35 
 
 research." A few days later lie wrote that he had made 
 a remarkable discovery in relation to ainygdalin. 
 Bitter almond oil can be obtained from ainygdalin 
 and also from almonds, and it occurred to him that 
 the oil might be produced from the ainygdalin by 
 an action similar to that of a ferment. Experiments 
 showed that this hypothesis was well borne out by the 
 facts, for he found that an emulsion of sweet almonds 
 which contain no arnygdalin, causes the formation oi 
 the oil and of prussic acid, when it acts upon 
 amygdalin. From this they inferred the presence 
 both in bitter and sweet almonds of a "kind of soluble 
 ferment," to which they gave the name emulsin. Nor 
 was this all. From their analyses of amygdalin, and 
 of oil of bitter almonds and prussic acid, they satisfied 
 themselves that in the transformation of the former 
 something besides the oil and prussic acid must be 
 produced; something had been missed. They soon 
 discovered this complementary product. It was sugar. 
 And thus they made known to chemistry, for the first 
 time, a member of another new and most important 
 and interesting group of substances viz. the glucos- 
 ides, and at the same time made an important contri- 
 bution to our knowledge of ferments. 
 
 The joint work of Liebig and Wohler was continued 
 till 1838, when their grand investigation of uric acid 
 was published. Their experiments soon showed that 
 the interest of uric acid to the chemist is hardly 
 interior to that which this substance excites in the 
 physiologist. The readiness with which it takes part 
 hi chemical change is such that, in a single research, 
 they added no less than sixteen new and most remark- 
 able substances to the list of organic compounds, 
 concerning which it is notable that in the course of 
 
36 JUSTUS VON LIEBIG: 
 
 nearly half a century only one of them disappeared 
 from the science. On this occasion, as on that of 
 their work on oil of bitter almonds, their memoir 
 was not only remarkable for the number of the 
 new substances which it introduced into chemistry, 
 it was again distinguished both by the masterly 
 interpretation of their results, in which they again 
 made use of their conception of organic radicles, and 
 by the prescience, which enabled them to foresee the 
 direction in which organic chemistry was about to 
 advance. From these researches, they said: "The 
 philosophy of chemistry must draw the conclusion 
 that the synthesis of all organic compounds which 
 are not organised must be looked upon not merely as 
 probable, but as certain of ultimate achievement. 
 Sugar, salicin, morphine will be artificially prepared. 
 As yet, we are ignorant of the road by which this end 
 will be reached, since the proximate constituents 
 required for building up these substances are not 
 yet known to us; but these the progress of science 
 cannot fail to reveal." This was the last great re- 
 search undertaken by these two friends. Liebig soon 
 afterwards turned his attention to the problems 
 of agricultural and physiological chemistry, whilst 
 Wohler thereafter devoted himself chiefly to inorganic 
 chemistry. 
 
 It is natural, nay inevitable, that reference should 
 be made to the human side of the friendship between 
 these two men, whose names are so entwined with 
 one another and with the history of chemistry. 
 Hofmann, the pupil of Liebig and the editor of their 
 correspondence, has left us a picture of the men in 
 which each figure stands clearly before us " Liebig, 
 fiery and rash, seizing a new idea with enthusiasm, 
 
HIS LIFE AND WORK. 37 
 
 readily giving free rein to his imagination, tenacious 
 of his opinions, yet open to the recognition of error, 
 sincerely grateful, indeed, to those who made him 
 conscious of it. Wohler, calm and deliberate, ap- 
 proaching a new problem with temperate considera- 
 tion securely guarded against over-hasty conclusions ; 
 but both equally inspired by the same invariable 
 love of truth. Liebig, irritable, easily offended, hot- 
 tempered, hardly master of his emotions, which often 
 found vent in bitter words that involved him in 
 long and painful quarrels. Wohler, unimpassioned, 
 even under the most malignant provocation, disarm- 
 ing the bitterest opponent by the sobriety of his 
 speech, the sworn foe of quarrels and dissension, yet 
 both animated by the same unerring sense of right." 
 Can we wonder that between two such natures, so 
 different and yet so complementary, there should 
 ripen a friendship that they might count among the 
 best harvests of their lives. The following letter 
 from Wohler, on the occasion of one of Liebig's fits of 
 annoyance, indicates the manner in which Wohler's 
 influence was exerted on Liebig : 
 
 " GOTTINGEN March 9th, 1843. 
 
 " To make war against Marchand, or, indeed, against, anybody 
 else, brings no contentment with it and is of little use to science. 
 . . . Imagine that it is the year 1900, when we are both 
 dissolved into carbonic acid, water, and ammonia, and onr ashes, 
 it may be, are part of the bones of some dog that has despoiled 
 onr graves. Who cares then whether we have lived in peace or 
 anger ; who thinks then of thy polemics, of the sacrifice of thy 
 health and peace of mind for science ? Nobody. But thy good 
 ideas, the new facts which thou hast discovered these, sifted 
 from all that is immaterial, will l)e known and remembered to 
 all time. But how comes it that I should advise the lion to 
 eat sugar ? " 
 
38 JUSTUS VON LIEBIG: 
 
 Nor was this the only occasion on which Wohler 
 sought to moderate the occasional in temperance, under 
 provocation, of his friend. Thus in a letter of 
 March 3rd, 1834, on the occasion of another dispute, 
 he warns his friend that he may be right and may 
 be doing a service to knowledge, but that he em- 
 bitters his life and ruins his health for nothing. 
 
 On the other hand, it was to Liebig and Giessen 
 that Wohler turned when, in 1832, he lost his young 
 wife, and it was by working in Liebig's company that 
 he sought for consolation and forgetfulness after 
 his loss. On which occasion he wrote after his 
 return to Cassel " I am here back again in my 
 darkened solitude . . . How happy was I that 
 we could work together face to face." 
 
 And again on another occasion he wrote : " The 
 days which I spend with Liebig slip by like hours, 
 and I count them among my happiest." 
 
 As it has sometimes been suggested that Wohler 
 received something less than his fair share of credit 
 for the work done with Liebig, the following extract 
 from Liebig's autobiographic sketch may fitly close 
 this brief account of their labours and friendship. 
 
 Speaking of his work at Giessen, he says, " I had 
 the great good fortune from the commencement of my 
 work at Giessen to gain a friend of similar tastes and 
 similar aims, with whom, after so many years, I am 
 still knit in the bonds of warmest affection. 
 
 "While in me the predominating inclination was 
 to seek out the points of resemblance in the behaviour 
 of bodies or their compounds, he possessed an unparal- 
 leled faculty of perceiving their differences. Acuteness 
 of observation was combined in him with an artistic 
 dexterity, and an ingeniousness in discovering new 
 
HIS LIFE AND WORK. 39 
 
 means and methods of research or analysis, such as 
 few men possess. 
 
 "The achievement of our joint work upon uric acid 
 and oil of bitter almonds has frequently been praised ; 
 it was his work. I cannot sufficiently highly estimate 
 the advantage which the association with Wohler 
 brought to me in the attainment of my own as well 
 as of our mutual aims, for by that association were 
 united the peculiarities of two schools the good that 
 was in each became effective by co-operation. Without 
 envy and without jealousy, hand-in-hand, we plodded 
 our way ; when the one needed help, the other was 
 ready. Some idea of this relationship will be obtained 
 if I mention that many of our smaller pieces of work 
 which bear our joint names were done by one alone ; 
 they were charming little gifts which one presented 
 to the other." 
 
 Wohler, on the other hand, wrote as follows : " We 
 two, Liebig and I, have dissimilar kinds of talent ; 
 each, when in concert, strengthens the other. No one 
 recognises this more fully than Liebig himself, and no 
 one does me greater justice for my share of our 
 common work than he." 
 
 Assuredly neither of these two undervalued the 
 services rendered to him by the other. Their friend- 
 ship was as nobly unselfish as it was useful. 
 
40 JUSTUS VON LIEBIG : 
 
 CHAPTER III. 
 
 CHEMICAL DISCOVERIES. 
 
 Practical Importance of some of his Discoveries Method of Making 
 Cyanide of Potassium Chloroform and Chloral Experiments 
 on Ammonium Thiocyanate Conflict with Gerhardt How 
 Liebig Missed Discovering Bromine Nature of Acids Theory 
 of the Polybasic Acids Hydrogen Theory of Acids Distinction 
 of Equivalent from Molecular Weights Progress of Radicle 
 Theory Ethyl Theory Compound Radicles. 
 
 IN chemistry, as in the arts and manufactures, there 
 are certain substances which form, as it were, the 
 raw material from which others are fabricated. Thus 
 salt is the raw material for makers of soda and soap. 
 Sometimes the starting-point, so to speak, of a group 
 of manufactures is not a natural product like salt, but 
 one that must itself be prepared on the large scale 
 from other substances. This is the case with yellow 
 prussiate of potash. 
 
 The manufacture of yellow prussiate of potash on 
 the large scale had long been practised, but the nature 
 of the changes by which it is formed was first made 
 clear by the experiments of Liebig, with the result that 
 his discovery led to improvements in the process, 
 which cheapened it and greatly extended its uses. 
 One of its most important applications is in the 
 making of potassium cyanide. Liebig devised an 
 easy, safe, and inexpensive process for preparing the 
 latter salt by melting a mixture of the yellow prussiate 
 of potash with carbonate of potash. The salt was thus 
 
HIS LIFE AND WORK. 41 
 
 made available for many new purposes. The folio whig 
 are some striking examples : 
 
 Potassium cyanide dissolves several silver salts 
 which are insoluble in water alone. This property 
 was very useful in the early days of photography ; 
 photographers made use of it for removing the 
 unaltered silver compounds from their negatives. 
 Cheap C} r anide of potassium thus helped on the de- 
 velopment of this useful and interesting art. 
 
 Again, it is found by electro-platers that silver 
 can best be deposited from a solution of silver cyanide 
 in potassium cyanide. Hence a cheap method of 
 making the cyanide went far to render electroplating, 
 practically speaking, possible ; and finally the low 
 price at which it can now be produced enables miners 
 to use a solution of it for extracting finely divided 
 gold from the rocks, even when the gold occurs, as it 
 often does, only to the extent of less than one ounce 
 to the ton. Thus, without counting the importance 
 of this substance in pure chemistry, the progress of 
 three highly important branches of industry have been 
 immensely promoted by this one simple discovery. 
 
 Xor does this example of the practical value of 
 investigations which at first sight may seem to be 
 of purely chemical interest by any means stand 
 alone. One more instance of even greater import- 
 ance must be mentioned. 
 
 In 1832 Liebig's experiments on the action of 
 chlorine with alcohol resulted in the discovery of a 
 substance of absolutely immeasurable value to man 
 viz. of chloroform, the anaesthetic and of a second 
 substance, chloral, also of great, though not of such 
 supreme, importance as the former. Liebig de- 
 scribed how to prepare these compounds, and gave 
 
42 JUSTUS VON LIEBIG: 
 
 their properties,* and he observed the remarkable 
 fact that chloral when it acts with potash yields 
 chloroform. Fifteen years afterwards, as we all know, 
 chloroform was first employed as an ansesthetic by 
 Simpson, of Edinburgh. But it was not till twenty 
 years later that Oscar Liebreich, inspired by Liebig's 
 observation that chloral yields chloroform under the 
 influence of alkalies, formed the happy idea of study- 
 ing its physiological action, with the hope that 
 the small amount of alkali in the blood would be 
 sufficient to effect the transformation of chloral into 
 chloroform (and formic acid), with the result that 
 he discovered the interesting and unexpected physio- 
 logical qualities of that substance. 
 
 It would be impossible within the space which 
 this part of Liebig's work can claim in this book to 
 give even a superficial account of the numerous 
 substances discovered by him and described in the 
 three hundred and eighteen papers that bear his 
 name ; but those who have even an elementary know- 
 ledge of chemistry will recognise their importance when, 
 to mention only a few, I say that they include such 
 bodies as ferrocyanic acid, aldehyde, meta-aldehyde, 
 thialdine, carbothialdine, and creatinine and sarcosine, 
 the decomposition products of creatine from flesh. 
 
 Neither is it possible to do justice to the almost 
 endless variety of his miscellaneous observations, to 
 the long list of organic substances the composition 
 of which either he or his pupils determined, to the 
 numerous plant-ash analyses that were made in his 
 laboratory, to the processes with which he endowed 
 physiology, to his analyses of German mineral waters, 
 
 * Dumas first correctly determined the composition of these 
 substances. 
 
HIS LIFE AND WORK. 43 
 
 or to his contributions to technology, such as his 
 processes for silvering mirrors, and for making unfer- 
 mented bread. These by themselves might have 
 made a reputation for a not undistinguished man of 
 science. 
 
 There are, however, other contributions to chemistry 
 by Liebig of equal rank with those which have been 
 already discussed. For example, his investigation 
 of the compounds derived from ammonium sulpho- 
 cyanate, and the conflicts with Gerhard t and Laurent 
 to which it gave rise. And, again, the part he played 
 in the discussions of the great theoretical questions 
 which followed the establishment of the Atomic Theory 
 and agitated chemists during the period of his greatest 
 activity as a student of pure chemistry. 
 
 The first of these is all the more interesting because 
 it affords at once an excellent illustration of Liebig's 
 method of work, and also an example of the advan- 
 tages that often flow from those conflicts between 
 experimentally determined facts on the one hand, and 
 theoretical interpretations on the other ; or between 
 old established views and new conceptions, which so 
 often agitate the followers of an active branch of experi- 
 mental science, and whose significance is usually so 
 little understood, or rather so completely misunder- 
 stood, by the world at large, and by the cynics in 
 particular. 
 
 It is true enough that such struggles are some- 
 times conducted in too vehement a manner, but this 
 is not always nor, indeed, very often the case. 
 Besides, is there not, after all, a great element 
 of nobility in every one of these struggles, in 
 which both sides aim equally at truth, and both are 
 equally free, in spite of occasional excess of zeal, from 
 
44 JUSTUS VON LIEBIG: 
 
 all petty and sordid desires for personal advantage ? 
 The struggle in these contests is not to decide who 
 shall gain most advantage from the result, but who 
 shall do most in the service of humanity. 
 
 When examining the effect of heat and certain 
 reagents on ammonium thiocyanate, Liebig found 
 that, instead of breaking up into simpler substances, 
 as he expected, it gave rise to a series of new 
 products of ever- increasing complexity. His experi- 
 ments led him to give the following formuke to the 
 new products : 
 
 1. Melamine . . . C :5 N, (NH ) 3 
 
 2. Ammelinc . . . C 8 N 3 (Nil,)., (OH) 
 
 3. Ammelide . . . (Co, N 3 ), (NH~ 9 ) 3 (OH) 3 
 
 4. Cyanuric Acid . . C 3 N 3 (OH) 3 
 
 Gerhard t, looking at the formulae given in the 
 above table from a purely theoretical point of view, 
 quickly perceived a certain want of symmetry in the 
 relations of the compounds represented, and pointed 
 out that the substance which might have been ex- 
 pected to occupy the third place in the table was 
 a substance with the formula (C 3 N 3 ) (NH.,) (OH) a , 
 melanurenic acid, in which case the series would be 
 written as follows : 
 
 Melamine . . . . C 3 N 3 (NH 2 ) 3 
 
 Ammoline . . . , QJ N 3 (NIL,)'., (OH) 
 
 Melanurenic Acid . . . C. } N 3 (NH.,) (OH)., 
 
 Cyanuric Acid . . . C 3 N 3 (OH) 3 
 
 And he did not hesitate, supported only by theory, 
 to declare that Liebig's analyses must be wrong. 
 Liebig at once entered a solemn protest against this 
 use of theory unsupported by experiment, declared 
 it to be in opposition to all sound principles of 
 scientific inquiry, and smashed the critical part of 
 
HIS LIFE AND WORK. 45 
 
 the case of his antagonist by producing from urea, 
 jointly with Wohler, the compound which Gerhard t 
 had only imagined to exist, and showing that its 
 properties were different from those of ammelide. 
 
 But whilst Liebig's reproaches to Gerhardt were 
 doubtless justified, as regards his method of criticism, 
 the importance of attacking such subjects from the 
 theoretical side is well shown by the subsequent 
 production of Gerhardt's hypothetical melanurenic 
 acid. Gerhardt's fault lay, not in his theorising, but 
 in not subjecting his hypothesis to the test of rigorous 
 experiment before attempting to discredit the experi- 
 mental results obtained by another. 
 
 On the occasion of another discussion Liebig again 
 drew attention to the importance of never trusting an 
 untested hypothesis, by telling a story against himself. 
 Early in his career, speculating without experimenting 
 cost him and Germany the discovery of bromine. 
 
 " No greater misfortune," he said, " can befall a 
 chemist than being unable to disengage himself from 
 preconceived ideas, and yielding to the bias of his 
 mind to account for all phenomena not agreeing with 
 his conceptions by explanations not founded on 
 experiment. . . I know a chemist who, while 
 at Kreuznach many years ago, undertook an investi- 
 gation of the mother-liquor from the salt works. He 
 found iodine in it ; he observed, moreover, that the 
 iodide of starch turned of a fiery yellow by standing 
 overnight. The phenomenon struck him ; he procured 
 a large quantity of the mother-liquor, saturated it 
 with chlorine, and obtained by distillation a consider- 
 able amount of a liquid colouring starch yellow, and 
 possessing the external properties of chloride of iodine, 
 but differing in many of its reactions from the latter 
 
46 JUSTUS VON LIEBIG: 
 
 compound. He explained, however, every discrepancy 
 most satisfactorily to himself ; he contrived for himself 
 a theory on it. 
 
 "Several months later he received the splendid 
 paper of M. Balard, and, on the very same day, he 
 was in a condition to publish a series of experiments 
 on the behaviour of bromine with iron, platinum and 
 carbon ; for Balard's bromine stood in his laboratory, 
 labelled liquid chloride of iodine." 
 
 One of Liebig's most important contributions to 
 chemical theory has already been brought forward in 
 connection with his joint research with Wohler upon 
 oil of bitter almonds. This has justly been termed 
 one of the pillars of the theory of compound radicles. 
 It was largely owing to Liebig's influence, also, that the 
 new ideas regarding the nature of acids first brought 
 forward by our countryman, Sir Humphry Davy, in 
 1809, were, after a period of neglect, once more pro- 
 minently brought under the notice of chemists. 
 
 According to the ideas of the earlier chemists, all 
 acids and all salts must contain oxygen. 
 
 The first blow at this conception concerning acids 
 and salts was struck when Davy, after his famous 
 investigation of hydrochloric acid and chlorine, re- 
 nounced the hypothetical " murium" whose oxide 
 was supposed to exist in hydrochloric acid, and 
 boldly represented this acid as a combination of the 
 element chlorine with hydrogen. 
 
 Having observed that oxide of iodine only becomes 
 an acid after it is dissolved in water, Davy and Dulong 
 subsequently went further, and concluded that hy- 
 drogen, and not oxygen, is the true acidifying 
 element; that hydrogen, in fact, is the essential 
 constituent of acids. 
 
HIS LIFE AND WORK. 47 
 
 The opinions of Davy and Dulong on this subject 
 were opposed by Berzelius, as they appeared not to be 
 reconcilable with the electro-chemical theory in which 
 he had combined and developed the dualistic hypo- 
 thesis of Lavoisier and the electro-chemical conception 
 of our great countryman Davy, and, consequently, 
 the views of Davy and Dulong lost ground, until 
 Daniell's studies in electrolysis led to new ideas of the 
 electro-chemical constitution of acids and salts. 
 
 The final return to the views of Davy and Dulong 
 was greatly helped on by the papers in which Liebig 
 brought forward his theory of the poly basic acids. 
 
 In the earlier years of the nineteenth century 
 most chemists held views on the relations of acids 
 and alkalies which practically involved the assumption 
 that the molecules of all acids are of equal value in 
 their power of neutralising alkalies, until Graham, in 
 1833, published his investigations of the phosphoric 
 acids, and showed that when phosphoric oxide dis- 
 solves in water it can generate three distinct acids, 
 with very different powers of neutralising alkalies. 
 
 Liebig, in 1837, paid a visit to England, when he 
 formed a high opinion of Graham, of whom he says, 
 " Graham . . . modest and without pretence, 
 makes wonderful discoveries." About the same time 
 he visited Paris and Dumas, and from a letter written 
 to Wohler, after his return, it would appear that he 
 then began to form new views on the constitution of 
 the acids. In the same year he published, jointly 
 with Dumas, a paper in which they proposed that 
 the accepted formula for citric acid should be trebled, 
 thus making this a tribasic acid.* Liebig afterwards 
 
 * Note. The neutralising power of a molecule of a dibasic acid 
 is twice as great as that of a molecule of a monobasic acid, and so on. 
 
48 JUSTUS VON LIEBIG: 
 
 returned to the subject, and described experiments 
 with a whole host of acids and their salts, in which 
 the existence of monobasic, dibasic, and tribasic 
 organic acids was clearly indicated, and in the 
 course of his work he soon saw that the hydrogen 
 theory of acids was both probable and convenient. 
 Acids he denned as particular compounds of 
 hydrogen, in which the latter can be replaced by 
 metals. 
 
 It was at one time objected that Davy's theory 
 involved the necessity of admitting the existence 
 of a host of radicles which had not been, and in 
 most cases still have not been, isolated ; but this, 
 as Liebig pointed out, was equally true of the 
 earlier view. Very few of the acid anhydrides Aviiich 
 were supposed to enter into the formation of salts 
 had at that time been discovered, whilst in addition 
 the experimental evidence which could then be 
 brought forward was all against the existence of such 
 hypothetical elements as the " murium," which was 
 supposed to be a constituent of the hypothetical acid 
 anhydride of the chlorides. To support the earlier, 
 and at that time more orthodox, view it was necessary 
 not only to admit the existence of non-isolated 
 radicles, but also to invent non-existent elements 
 from which to construct them. Hypothesis had to 
 be supported by hypothesis. It was natural, there- 
 fore, that Liebig, who was ever forward in denouncing 
 such a use of theory in science, should be amongst 
 the foremost of those who supported the new con- 
 ceptions which were more securely based on the 
 proved truths of chemical science. 
 
 The chemists of to-day nay, even comparatively 
 young students of the subject are accustomed to 
 
HIS LIFE AND WORK. 49 
 
 distinguish with precision between the chemical 
 conceptions of the molecule, the atom, and the 
 equivalent. When Graham and Liebig worked on 
 phosphoric and the organic acids, far less distinct 
 notions prevailed on the subject, and especially on 
 the proper employment of the terms "equivalent" 
 and "atom." 
 
 The term atom was, as has previously been ex- 
 plained, introduced into chemistry by John Dalton 
 to designate certain very small indivisible particles 
 of which matter is supposed to be composed. Dalton 
 considered that chemical compounds were formed by 
 the uniting or approximating of atoms of different 
 elements, and that the atoms of each element were 
 exactly alike in all their properties, including their 
 weight. He published certain tables in which he 
 professed to give the relative weights of the atoms. 
 These numbers were calculated from the relative pro- 
 portions of the elements found in their compounds. 
 Wollaston, in 1808, and afterwards Davy and Gay- 
 Lussac, denied that Dalton's atomic weights were 
 really the relative weights of the atoms, and Wollaston 
 proposed for them the name chemical equivalents. 
 The use of this term was not, however, always con- 
 fined to its original purpose ; it came to be extended 
 to compound substances as well as to the elements. 
 Thus it happened that when the word was applied to 
 elements, it was apt to be used very much in the 
 early sense of the term atomic weight; whilst as 
 applied to compounds, it often signified more nearly 
 what we now define as the molecular weight. The 
 clearing - up of the confusion thus created was 
 initiated by Liebig, to whom we owe the first 
 precise expressions of the distinction between the 
 
50 JUSTUS VON LIEBIG: 
 
 equivalent weights and the molecular weights* of 
 substances. 
 
 One of the most interesting discussions in which 
 Liebig assisted related to the constitution and relations 
 of alcohol and ether. 
 
 L According to Liebig, the relation of these organic 
 compounds to the inorganic substances is very simple 
 and intelligible. They contain a compound radicle, 
 ethyl (CoIL^ alcohol being its hydroxide and ether 
 xide. L.thyl may be compared to the element 
 potassium, and its compounds to those of potassium. 
 Thus 
 
 Ether (C 2 H 5 ) 2 corresponds to the oxide K 2 O. 
 Alcohol (C 2 H 5 ) OH ,, hydroxide KHO. 
 
 Ethyl chloride (C 2 H 5 )C1 chloride KC1. 
 
 He did not, it is true, at once arrive at the 
 "ethyl theory" exactly as stated above. Still, the 
 modern view is in all its essentials founded solely 
 on the views of Liebig, according to which organic 
 chemistry was defined as the "chemistry of organic 
 radicles," whilst these radicles were compared 
 with the elements, and their combinations with 
 the corresponding inorganic substances. " Organic 
 chemistry," said Liebig and Dumas, " possesses its own 
 elements, which sometimes play the part of chlorine 
 or oxygen, sometimes that of a metal. Cyanogen, 
 amidogen, benzoyl, and the radicles of ammonium com- 
 pounds, of fats, and of alcohol and its derivatives, con- 
 stitute the true elements of organic nature " 
 
 * Liebig used the word atom in his writings where we use mole- 
 cule, for the exact distinction between atoms and molecules was 
 accomplished later by his successors. 
 
HIS LIFE AND WORK. 51 
 
 CHAPTER IV. 
 
 L1EBIG AND DUMAS. 
 
 Dumas's Early Life Dumas at Geneva His Meeting with Hum- 
 boldt Paris Substitution Conflict with Dualism Li ebig 
 accepts Substitution Theory Chemistry of Vinegar-making 
 Aldehyde. 
 
 THE two chemists who were chiefly associated with 
 Liebig in directing the course of organic chemistry 
 during the third and fourth decades of the nineteenth 
 century were Wohler, in Germany, and Dumas, in 
 France. Wohler and Liebig almost from the time 
 of their first meeting were, as we have seen, closely 
 knit in friendship, and for many years were intimately 
 associated in the prosecution of common studies. 
 
 The relations of Liebig and Dumas were not 
 always equally harmonious ; these two sometimes 
 found themselves in opposite camps. Both of them 
 were men who could take as well as give hard 
 knocks, however, and hence their frequent scientific 
 encounters never prevented either of them from appre- 
 ciating the high qualities of the other, and on several 
 occasions they worked in unison for a common 
 object. When Liebig dedicated a German edition of 
 his " Familiar Letters on Chemistry " to Dumas, in 
 1851, Avith characteristic open-heartedness he ad- 
 dressed the following note to his old opponent : 
 
 " MY DEAR DUMAS, It was by a strange coincidence that 
 for more than a quarter of a century our labours in the cause 
 
52 JUSTUS VON LIEBIG: 
 
 of the science to which our lives have been devoted were 
 prosecuted in the same direction. 
 
 " If the roads by which we endeavoured to attain the goal 
 were often different, in the proximity of that goal we always met 
 in order to shake hands with each other. 
 
 " Not only your country, but the whole scientific world, 
 acknowledges the range, the depth, and the importance of your 
 researches and discoveries, but no one knows better than myself 
 the obstacles which your genius had to surmount in order to 
 achieve those inestimable conquests which, in a measure, con- 
 stitute the foundation of modern science. Though contending 
 with difficulties of every kind, )ou never descended into the 
 arena without leaving it as conqueror. 
 
 " Permit me, in recognition of the services which you have 
 rendered to science and to mankind at large, to dedicate to you 
 this little work, in which I have ventured to sketch for an 
 enlarged circle of readers the onward movement of scientific 
 and applied chemistry, to which you have so much contributed. 
 Your approbation would be the highest reward I could possibly 
 hope for. " LIEBIG. 
 
 " Giessen, 1851." 
 
 Whilst Durnas. not less generous than his old 
 friend and opponent, when referring to their labours 
 in organic chemistry in his commemorative speech on 
 Pelouze, said : 
 
 " Into this as yet uncultivated domain we had 
 plunged, Liebig and I, with most living ardour. The 
 number of organic substances, nowadays immense, was 
 even then very considerable. Their study, however, 
 if we except the group of bodies selected by Chevreul 
 for his researches, had not as yet elicited results of any 
 great importance. The nature of most compounds 
 was unknown ; their differences, their analogies, 
 their connections had still to be unveiled." 
 
 " To find our way through these unexplored terri- 
 tories, we had neither compass nor guides, neither 
 method nor laws. Each of us had been led to form 
 
HIS LIFE AND WORK. 53 
 
 ideas and to elaborate views peculiar to himself, which 
 he defended with warmth and even with passion but 
 without any feeling of envy or jealousy. The dis- 
 coveries to be made appeared to us without limit, 
 and each was satisfied with his harvest. What we 
 both had at heart was to stake the ground and open 
 roads, nor have I any doubt that in reading my 
 papers Liebig felt the same pleasure which the 
 perusal of his afforded me. If a new step had been 
 taken, it was of little moment whether it had been 
 made by the one or by the other, since it served us 
 both on the road to truth." 
 
 Jean Baptiste Andre Dumas was born at Alais, in 
 the department of the Gard, July 14th, 1800, and, like 
 Liebig. became apprentice to an apothecary ; but not 
 finding much opportunity for scientific progress in his 
 position he soon migrated to Geneva, travelling there 
 on foot. Here he found both employment and means 
 of education, and very quickly developed in a surpris- 
 ing degree his talent for experimental investigation. 
 
 The turning-point in the career of Liebig was, as 
 we have seen, probably his meeting with Alexander 
 von Hurnboldt and his introduction by the latter to 
 Gay-Lussac in 1823. Singularly enough, it was a day 
 spent with the great traveller which induced Dumas 
 to turn his face to Paris at a moment when there was 
 much to induce him to settle in Geneva. 
 
 The story of their meeting was told by Dumas 
 himself to Hofmann, and it illustrates so well the 
 fascination that Humboldt could exercise on a youth 
 of genius that, though it is rather foreign to the 
 purpose of this book, it must not be omitted : 
 
 " One day," said Dumas, " when I was in my study 
 completing some drawings at the microscope, and, it 
 
54 JUSTUS VON LIEBIG : 
 
 must be added, rather negligently attired to enable 
 me to move more freely, some one mounted the 
 stairs, stopped on my landing, and .gently knocked at 
 the door. ' Come in/ said I, without looking up from 
 my work. On turning round I was surprised to find 
 myself face to face with a gentleman in a bright 
 blue coat with metal buttons, a white waistcoat, 
 nankeen breeches, and top boots. This costume, 
 which might have been the fashion under the Direc- 
 tory, was then quite out of date. The wearer of it, his 
 head somewhat bent, his eyes deep set but keen, 
 advanced with a pleasant smile, saying, ' Monsieur 
 Dumas ? ' ' The same, sir ; but excuse me/ ' Don't 
 disturb yourself. I am M. de Humboldt, and did not 
 wish to pass through Geneva without having had the 
 pleasure of seeing you.' Throwing on my coat, I 
 hastily reiterated my apologies. I had only one chair ; 
 my visitor was pleased to accept it, whilst I resumed 
 my elevated perch on the drawing stool. Baron 
 Humboldt had read the paper published by M. Prevost 
 and myself, on blood, and was anxious to see the 
 preparations I had by me. His wish was soon grati- 
 fied. ' I ara going to the Congress at Verona/ said he, 
 1 and I intend to spend some days at Geneva, to see 
 old friends and make new ones, and more especially 
 to become acquainted with young people who are 
 beginning their career. Will you act as my cicerone ? 
 I warn you, however, that my rambles begin early 
 and end late. Now, could you be at my disposal, 
 say, from six in the morning till midnight ? ' This 
 proposal, which was, of course, accepted with alacrity, 
 proved to me a source of unexpected pleasure. Baron 
 Humboldt was fond of talking; he passed from one 
 subject to another without stopping. He obviously 
 
HIS LIFE AND WORK. 55 
 
 liked being listened to, and there was no fear of his 
 being interrupted by a young man who, for the first 
 time, heard Laplace, Berthollet, Gay-Lussac, Arago, 
 Thenard, Cuvier, and many others of the Parisian 
 celebrities, spoken of with familiarity. I listened with 
 a strange delight ; new horizons began to dawn upon 
 me. . ." 
 
 " At the end of a few days Baron Humboldt left 
 Geneva. After his departure the town seemed empty 
 to me. I felt as if spellbound. . . I had been more 
 especially impressed with what he told me of Parisian 
 life, of the happy collaboration of men of science, and 
 of the unlimited facilities which the French capital 
 offered to young men wishing to devote themselves to 
 scientific pursuits. I began to think that Paris was 
 the only place where, under the auspices of the leaders 
 of physical and chemical science, with whom, I had no 
 doubt, I should soon become acquainted, I might 
 hope to find the advice and assistance which would 
 enable me to carry out the labours over which I had 
 been pondering for some time. My mind was made 
 up : I must go to Paris." 
 
 In the year 1823, therefore, Dumas went to Paris. 
 Before long he obtained the appointment of Repetiteur 
 de Chimie to Thenard's course of Lectures in the ficole 
 Polytechnique. From this time his attention was 
 directed to the study of chemical phenomena. 
 
 Such were the first steps of Liebig's great colleague 
 and rival, Dumas. 
 
 If Liebig had so frequently the happiness of wit- 
 nessing the triumph of his ideas, and had so often the 
 gratification of observing the development of organic 
 chemistry proceed along the lines which he himself 
 had laid down, on the other hand, he occasionally 
 
56 JUSTUS VON LIEBIG: 
 
 found it necessary at the end of a discussion to 
 accept the conclusions of others or to remain opposed 
 to the truth. At such times he never hesitated; 
 once let him be convinced of the justness of the views 
 of an opponent, and he was among the very foremost 
 to welcome an advance, or the discovery of a new 
 road, into the unknown. This quality of his mind 
 is well illustrated by his attitude to the theory of 
 substitution, which in the hands of Dumas and his 
 successors effected the final overthrow of the dualistic 
 view of chemical phenomena, and greatly modified 
 the early form of the theory of compound radicles 
 itself. 
 
 The dualistic idea was introduced into chemistry 
 by Lavoisier ; it reached its highest development in 
 the hands of Wohler's master, Berzelius. According 
 to Lavoisier, however great may be the complexity of 
 a compound we may always detect in it evidence of 
 two constituent parts. These may be either simple, 
 as in the oxides e.g. oxide of calcium, or quicklime, 
 which contains a non-metal, oxygen, united with a 
 metal, calcium or compound, as in the salts, which 
 he regarded as produced by the union of an oxide of 
 a metal, on the one hand, with an acid (usually the 
 oxide of a non-inetal) on the other. Early in the 
 century Berzelius, by his electro-chemical theory, 
 had offered an explanation of dualistic combina- 
 tion which was consistent with the knowledge of 
 electrolysis then possessed by chemists, and thus re- 
 established the position of dualism in the science at 
 a moment when it seemed to be seriously threatened 
 by the new facts which had lately been brought to 
 light by the investigations of Davy. 
 
 Berzelius started with the assumption that the 
 
HIS LIFE AND WORK. 57 
 
 atoms are themselves electric, and possess at least 
 two poles whose quantities of electricity are in most 
 cases unequal. Thus the elements could be classed as 
 positive and negative according to which electricity 
 prevailed. Chemical combination, according to this 
 theory, consists in the attraction of the dissimilar 
 poles of the atoms, and consequently in the neutral- 
 ising of the two electricities. As, however, these 
 were not always equal in amount, the compound pro- 
 duced was itself frequently electric, and therefore 
 capable of entering into further combinations. Thus, 
 according to Berzelius, each compound consists of two 
 different parts, as suggested by Lavoisier, which attract 
 each other in consequence of their different states of 
 electrification. 
 
 In accordance with these ideas Berzelius repre- 
 sented the composition of salts by such formulae as 
 the following, in which the electro-chemical form of 
 the dualistic view of chemical combination will at 
 
 once be recognised : 
 
 i 
 
 Barium sulphate Ba O S0 3 
 
 + - 
 Calcium sulphate Ca O SO 3 
 
 + 
 Copper sulphate Cu O SO 3 
 
 When organic chemistry began to develop in the 
 hands of Liebig, Wohler, and the French chemists, 
 Berzelius directed his attention especially to the 
 task of bringing the radicle theory of organic 
 chemistry into agreement with the fundamental 
 ideas of his electro-chenrical form of dualism. 
 According to him compound radicles were unalter- 
 able groups, and he thought that organic chemistry, 
 like inorganic chemistry, must accept oxygen as the 
 
58 JUSTUS VON LIEB1G: 
 
 supreme ruler among the elements, and give it the 
 place which it had held in the mineral world since 
 the time of Lavoisier. 
 
 But, meanwhile, new departures were imminent 
 which were fated to work great changes in the radicle 
 theory almost before it had come to maturity. 
 
 One evening, at a soiree at the Tuileries, during the 
 reign of Charles X., the pleasure of the entertain- 
 ment was seriously marred by the fact that the wax- 
 candles emitted very unpleasant, irritating fumes, and 
 were remarkable for the smokiness of their flames. 
 The investigation of the cause of their peculiar be- 
 haviour was entrusted to Dumas. The irritating 
 fumes were found to be hydrochloric acid, and Dumas 
 had no difficulty in discovering that they were 
 due to the candles having been made from 
 wax which had been bleached by chlorine. This 
 circumstance led him to investigate the action of 
 chlorine upon organic bodies. He soon found that, 
 when chlorine acts on compounds containing hydro- 
 gen, the hydrogen may be removed and replaced by 
 an equivalent quantity of chlorine. This observation 
 was not, it is true, a new one ; Gay-Lussac, Faraday, 
 and Liebig and Wohler had all observed that hydro- 
 chloric acid is emitted, and chlorine fixed by organic 
 bodies. But Dumas first systematically examined 
 this kind of action, laid down the rules which it 
 follows, and, jointly with Laurent, who first perceived 
 their significance, developed their consequences. How 
 was it possible, they asked, to continue to accept 
 the electro-chemical hypothesis, with its rigidly ap- 
 pointed and opposite functions for electro-positive and 
 electro-negative elements, when it was thus shown 
 to be possible for an electro-negative atom, like that 
 
HIS LIFE AND WORK. 59 
 
 of chlorine, to replace an electro-positive atom of 
 hydrogen ? 
 
 Dumas' earliest results were soon splendidly con- 
 firmed hy the discovery of trichloracetic acid. By 
 suitably treating acetic acid with chlorine Dumas 
 produced from it another acid, which differed from 
 the first by containing three atoms of chlorine in 
 place of three atoms of hydrogen in every molecule. 
 " It is chlorinated vinegar," said Dumas ; " but 
 what is very remarkable, at least for those who 
 refuse to find in chlorine a body capable of replacing 
 hydrogen in the precise and complete sense of the 
 word, this chlorinated vinegar is jxist as much an 
 acid as common vinegar itself. Its acid power is not 
 changed. It saturates the same quantity of alkali as 
 before, and saturates it equally well, and the salts to 
 which it gives rise exhibit, when compared with 
 acetates, resemblances full of interest and gener- 
 ality." 
 
 " Here, then, is a new organic acid, containing a 
 very considerable quantity of chlorine, and exhibiting 
 none of the reactions of chlorine ; its hydrogen has 
 disappeared, and has been replaced by chlorine, and 
 yet this remarkable substitution has produced only 
 a slight change in its properties, all its essential 
 characters remaining unaltered." 
 
 " If its internal properties are modified, this modi- 
 fication becomes apparent only when, through the 
 intervention of a new force, the molecule itself is 
 destroyed and transformed into neAv products . . . 
 It is evident that, in confining myself to this system 
 of ideas dictated by facts, I have not in any way 
 taken into consideration the electro-chemical theories 
 on which Berzelius has generally based the idea 
 
60 JUSTUS VON L1EBIG: 
 
 predominating in the opinions which this illustrious 
 chemist has endeavoured to enforce." 
 
 " But do these electro-chemical ideas, this special 
 polarity attributed to the molecules of elementary 
 bodies, rest upon facts so evident that it is necessary 
 to erect them into articles of faith ? Or, if they must 
 be regarded as hypotheses, have they the power of 
 lending themselves to facts, of explaining and fore- 
 seeing them with so complete a certainty as to have 
 afforded important assistance in chemical researches ? 
 It must plainly be allowed that this is not the case." 
 
 At first Berzelius received these new ideas with 
 something like disdain. Such assertions, put forward 
 as they were at first by Laurent, a beginner, were still 
 without authority ; they appeared to him unworthy 
 of serious refutation. But when Dumas came into 
 the field, Berzelius energetically defended his opinions. 
 In this he was at first vigorously supported by Liebig, 
 who admitted, indeed, the fact of substitution, but 
 protested against the wide conclusions Dumas drew 
 from his results, and met him with an ironical re- 
 joinder in the form of a letter,* purporting to come 
 from S. C. H. Windier, which ran much as follows : 
 
 " The last great discovery from Paris shows that it 
 has been found possible to replace in acetate of man- 
 ganese first the atoms of hydrogen by chlorine, then 
 the oxygen, then the manganese, and at last even the 
 carbon, so that a body was formed which contained 
 only chlorine, but retained still the properties of the 
 original substance." He continued, after alluding to 
 the method of bleaching cotton goods by chlorine : " I 
 understand that there are already in the London shops 
 stuffs made of chlorine thread much approved in 
 
 * Tliis letter was written by Wohler, and published by Liebig. W 
 
HIS LIFE AND WORK. 61 
 
 the hospitals and preferred to all others for night- 
 caps, under-garments, etc." 
 
 However, further facts, such as the re-converting 
 of chloracetic acid into acetic acid by the action of 
 nascent hydrogen, and especially the production of the 
 chlorine and bromine derivatives of aniline in his 
 own laboratory by his pupil, Hofmann, before long 
 convinced Liebig that the character of a chemical 
 substance does not depend so much as he had supposed 
 on the nature of its constituent atoms, but very largely 
 also on the manner in which these atoms are arranged, 
 and he declared that the interpretations proposed by 
 Dumas, of the facts relating to substitution, appeared 
 to him to afford the explanation of a great number of 
 phenomena in organic chemistry. 
 
 Some years afterwards, at a dinner given by the 
 French chemists to chemical visitors to the Exhibition 
 of 1867, Liebig made his defeat on this occasion the 
 source of a happy retort to Dumas, who had asked 
 him why of late years he had devoted himself exclu- 
 sively to agricultural chemistry. " I have withdrawn 
 from organic chemistry," said Liebig, "for with the 
 theory of substitution as a foundation, the edifice of 
 chemical science may be built up by workmen : masters 
 are no longer needed" 
 
 Of course, this must be taken in the spirit of 
 the after-dinner speech ; but the reply shows the 
 completeness with which Liebig extended his ad- 
 miration to a great achievement, even when it had 
 not been reached without some little opposition from 
 himself and some damage to his own ideas. 
 
 To conclude this long list of some of Liebig's chief 
 contributions to pure chemistry, which, as will pre- 
 sently be seen, only represents a part of the field 
 
62 JUSTUS VON LIEBIG: 
 
 covered by his work, we must now glance at 
 his inquiry into the chemistry of the change of 
 alcohol into vinegar. This subject closes most 
 suitably an account of Liebig's labours in chemistry, 
 because it leads us by a natural transition to a 
 chemico-biological inquiry in which he took a promi- 
 nent part viz. to the question of the nature of 
 fermentation. 
 
 It has long been a familiar fact that moderately 
 dilute solutions of alcohol, such as wine or beer, when 
 exposed to the air, become, under certain conditions, 
 converted into vinegar, but the exact nature of the 
 chemical changes involved for long remained unex- 
 plained. 
 
 Liebig soon dispelled the obscurity in which this 
 subject had so long remained, by showing that the 
 change from alcohol to vinegar (acetic acid) takes 
 place in two stages, viz. that when alcohol is 
 submitted to the action of an oxidising agent to 
 the action of a substance which readily parts with 
 oxygen, that is to say it first loses hydrogen, which 
 is removed in the form of water, by which change a 
 substance called aldehyde is produced ; and, secondly, 
 that the aldehyde takes up oxygen to form acetic 
 acid. Frequently, of course, these two changes pro- 
 ceed simultaneously, so that they become indistin- 
 guishable, but by suitable methods Liebig secured 
 the intermediate product, and by doing so presented 
 to us a new substance, which has given its name to a 
 class the aldehydes and still remains in the minds 
 of chemists as the aldehyde par excellence. One 
 useful quality of aldehyde, which Liebig was the 
 first to observe and employ, must be mentioned. If 
 one adds a few drops of aldehyde to a flask or 
 
HIS LIFE AND WORK. 63 
 
 test-tube containing a solution of lunar caustic 
 nitrate of silver rendered slightly ammoniacal by 
 an addition of ammonia, and then gently warms the 
 mixture, at once the glass becomes coated with a film 
 of silver, reflecting more perfectly than an ordinary 
 mercury mirror. This reaction affords both a useful 
 test for aldehyde . and a ready and simple method of 
 preparing mirrors which is often useful, and makes it 
 possible to avoid the danger to health and life attached 
 to the older process in which mercury is used. 
 
64 JUSTUS VON LIEBIO: 
 
 CHAPTER V. 
 
 FERMENTATION. 
 
 Liebig's Theory of Fermentation Supposed Influence of Oxygen 
 Difference between Fermentation and Decay Fermentation 
 of Alcohol " Quick Vinegar Process " The Vitalistic Theory 
 revived by Pasteur Discussion between Liebig and Pasteur 
 Lactic Ferment Vinegar Plant Enzymes. 
 
 FERMENTATION. The word fermentation is derived 
 from fervere, " to boil," and may be supposed to owe its 
 origin to the effervescence which occurs when saccharine 
 liquids are left to themselves in contact with air, or 
 placed in contact with a ferment such as yeast. 
 Naturally, such a wonder-working process as fermen- 
 tation has always attracted the interest of the observant, 
 and numerous indeed have been the conjectures 
 hazarded in the various attempts which have been 
 made to fathom its mystery. It was impossible, how- 
 ever, as will soon be seen, that much real progress 
 should be made by the earlier thinkers on the subject, 
 for, only when the microscope had been brought to a 
 state of considerable efficiency, and when, at least, a 
 good start had been made in organic chemistry, was 
 it possible to get any light on this absorbing yet 
 bewildering subject. 
 
 To many the word fermentation still implies very 
 little ; to most it probably merely connotes the useful 
 process by which the sugar of malt or of grapes is 
 converted into beer or wine, or by which flour and 
 water are made to yield bread ; or again, the pernicious 
 
HIS LIFE AND' WORK. 65 
 
 change by which badly made preserves are apt to lose 
 their attractiveness of flavour and become both dis- 
 tasteful and unwholesome. It is, therefore, necessary 
 to explain that these are only a few instances from a 
 very large class of diverse changes, all of them brought 
 about by substances or organisms known to chemists 
 and biologists as the "ferments." 
 
 According to Liebig, fermentation is to be con- 
 sidered an essentially chemical phenomenon ; accord- 
 ing to his opponents, fermentive changes depend, in 
 many cases at least, on the life-processes of certain 
 minute organisms. To the latter, therefore, ferment- 
 ation, in these cases at any rate, is not a chemical, but 
 rather a biological, phenomenon. 
 
 It is one of the distinguishing features of a " fer- 
 ment " that a very little of it goes a long way. A 
 minute fragment of rennet is sufficient to cause the 
 curdling of a relatively large quantity of milk. A 
 single yeast cell may bring about the fermentation of 
 the largest vessel of grape juice, and presently the 
 grape juice will be converted into wine. If but the 
 point of a needle touch a liquid in which the ferment 
 of anthrax has been cultivated, a prick from that 
 needle will be sufficient to communicate anthrax to 
 any animal susceptible to the disease. 
 
 These, and others like them, are the phenomena 
 which must be explained by a theory of fermen- 
 tation. 
 
 An early attempt to explain these fermentive 
 changes chemically was that of Berzelius. Lavoisier 
 having shown that sugar is split up by fermentation 
 into alcohol and carbonic acid gas, Berzelius sug- 
 gested that the action of the yeast is "catalytic" 
 that is, that the ferment brings about the de- 
 
b6 JUSTUS VON LIEBIG: 
 
 composition of the sugar, by mere contact, much as 
 platinum black causes hydrogen peroxide to decom- 
 pose into water and oxygen, or as manganese dioxide 
 causes chlorate of potassium to give up its oxygen 
 at a lower temperature than is required for the decom- 
 position of the salt by heat alone. For some time this 
 so-called explanation of the phenomenon seems to 
 have been accepted as satisfactory, but inasmuch as 
 the nature of a catalytic action was not itself under- 
 stood, it did not in reality throw much, if any, new 
 light on the subject. 
 
 Liebig pointed out that universal experience teaches 
 that all organised bodies after death suffer a change, in 
 consequence of which their remains gradually vanish. 
 From the smallest twig to the largest tree all vege- 
 tables disappear after a few years, whilst animal 
 matters once deprived of life and if exposed to the 
 air are dissipated in a much shorter time, leaving 
 only their mineral matter behind them. This great 
 process, which requires for its progress air and 
 moisture, results finally in the converting of their 
 carbon into carbonic acid gas, of their hydrogen into 
 water, of their nitrogen into ammonia, and of their 
 sulphur into sulphuric acid. They are then in the 
 forms in which they can serve as food for new 
 generations of plants and animals. Those parts 
 which were derived from the air are returned to 
 the air, and the mineral parts which were taken 
 from the earth are returned once more to the soil. 
 The death followed by the dissolution of one genera- 
 tion is the source of life for a succeeding generation. 
 The atoms of carbon and hydrogen, which yesterday 
 formed part of the brain or muscle of an Englishman; 
 may to-morrow contribute to the material parts of a 
 
HIS LIFE AND WORK. 67 
 
 Persian or a native of Japan. The processes which 
 bring about these resolutions of organic matter into 
 the very same simple bodies from which they them- 
 selves were formerly produced, belong to the class 
 which we are considering to the fermentations. 
 They require, in order that they may occur, moisture, 
 and at the earlier stages the presence of air ; after- 
 wards, in many cases, fermentations can proceed even 
 if air be excluded. 
 
 These are the facts on which Liebig's theory of 
 the ferments was founded. Reasoning upon them, 
 he said : " It is obvious that by the contact of these 
 organic compounds with the oxygen of the air, a 
 process begins, in the course of which their constituents 
 suffer a total change in their properties. This change 
 is the result of a change in their composition. Before 
 contact with oxygen, their constituents are arranged 
 together without action on each other. By the 
 oxygen the state of rest or equilibrium of the attrac- 
 tions which keep the elements together has been 
 disturbecyin a particle of the substance, and, as a 
 consequence of this disturbance, a separation or new 
 arrangement of the elements has been brought 
 about. 
 
 " The continuance of these processes, even when 
 the oxygen, the original exciting cause of them, no 
 longer acts, shows most clearly that the state of de- 
 composition which has been produced among the 
 elements of a particle of the mass exerted an influence 
 on the other particles which have not been in contact 
 with the oxygen of the air ; for not only the first 
 particles, but, by degrees, all the rest undergo the same 
 change. 
 
 " All those processes of decomposition," he con- 
 
68 JUSTUS voN LIEBIG: 
 
 tinues, " which begin in a part of an organic substance 
 from the application of an external cause, and which 
 spread through the whole mass, with or without the 
 co-operation of that cause, have been called processes 
 of putrefaction. A putrescible substance, therefore, 
 is distinguished from one not putrescible, because the 
 former, without other conditions than a certain tem- 
 perature, and the presence of water (after exposure, 
 although transient, to the atmosphere), are resolved 
 into a series of new products, while the latter, if un- 
 mixed, do not, under the same circumstances, undergo 
 any change." 
 
 The number of substances, however, which are thus 
 putrescible is few, though they are widely diffused. 
 They are all of them highly complex, containing 
 nitrogen and sulphur such things as albumin, fibrin, 
 and gelatin, for example. 
 
 On the other hand, a great number of substances, 
 such as sugar, starch, and the organic acids which are 
 found in the juices of plants, are not putrescible if 
 pure; if exposed to air and moisture, they do not 
 undergo any perceptible change ; a solution of sugar, 
 for example, when exposed, dries up and deposits 
 crystals which retain their original properties. 
 
 If, however, some sugar, or sugar of milk, &c., be 
 dissolved in water, and if the solution be added to a 
 portion of a putrescible substance already in a putrid 
 condition, these substances will then be fermented, 
 that is to say, will undergo a change. 
 
 Substances like sugar and starch were termed by 
 Liebig the fermentescible substances. The process 
 of their decomposition under the influence of the 
 putrescible substances, according to him, is what we 
 call fermentation. And it will be perceived by this 
 
HIS LIFE AND WORK. 69 
 
 time that the putrescent matters which can thus 
 induce the decomposition of the " fermentescible " 
 substances are the ferments. 
 
 If one examines the juices of vegetables, or fluids 
 of animal origin, one finds present in them always, 
 in greater or less quantity, the instable compounds of 
 the first class (Liebig's ferments) , as well as substances 
 of the second class. This fact explains, according to 
 Liebig's hypothesis^ why all such juices undergo fer- 
 mentation after contact with air, in the course of which 
 they are ultimately reduced to substances of simpler 
 composition than before. 
 
 The products of a fermentation are, as we know, 
 commonly of more simple composition than the 
 substances from which they are derived. Thus, 
 grape sugar, which has the formula C 6 H 1: :>0 6 , when 
 fermented yields chiefly alcohol, C 2 H 6 0, and carbonic 
 acid gas. Sugar of milk may similarly be made to 
 yield a simpler compound lactic acid. In order 
 that such changes as these may occur, it seemed 
 obvious that the atoms of the substance fermented 
 must be set in motion, since, in order to rearrange 
 themselves, it is plain that they must move. From 
 this Liebig concluded that the power of the ferments 
 over fermentable substances depends on a certain state 
 or condition of their atoms, and, further, that this 
 state must be one in which the complex molecules 
 of the ferments undergo resolution into simpler 
 molecules. Thus, the action of a ferment on a 
 fermentable compound, seemed to Liebig not unlike 
 that of heat on an organic compound. In virtue of 
 the motions of the atoms of its own molecules, it 
 disturbs the equilibrium previously existing among 
 the atoms of the molecules of the fermented matter, 
 
70 JUSTUS VON LIEBIG: 
 
 and new and simpler combinations result. This 
 analogy he found to be confirmed by the influence 
 of temperature on fermentation. If an organic 
 substance be submitted to a given temperature 
 certain definite products are obtained, which may, 
 however, be altered if a higher temperature be 
 employed. Similarly, the products of fermentations 
 are markedly influenced by the temperatures at 
 which the fermentations occur. 
 
 But, it will be asked, what is it that starts these 
 internal movements, which changes a merely 
 putrescible substance into an actually putrescent 
 body or ferment ? 
 
 Liebig was of opinion that the oxygen of the air 
 was the first cause of the breaking-up of the nitro- 
 genous substances.* The immediate and most 
 energetic cause of all the alterations and transfor- 
 mations which organic molecules undergo is, he 
 says, " the chemical action of oxygen." That is why 
 exposure to the air is a necessary preliminary 
 condition for the commencement of a fermentive 
 change. 
 
 Fermentation, according to Liebig, is only a 
 consequence of the commencement of a process of 
 decay, in which oxygen plays a part, and it continues 
 till the fermenting substance has resolved itself into 
 a series of new products which undergo no further 
 change, unless as the result of further causes ot 
 alteration. 
 
 But although a state of rest may thus be reached 
 in regard to the attractions among the atoms of the 
 newly-formed substances, yet this condition of 
 equilibrium would not exist with regard to their 
 
 * Familiar letters. 
 
HIS LIFE AND WORK. 71 
 
 attraction for oxygen. The chemical action of oxygen 
 would only cease when the capacity of the elements 
 to combine with oxygen was exhausted. Fermenta- 
 tion, therefore, represents only the first stage of the 
 resolution of complex molecules into simpler ones. 
 The process is completed by decay, which he defined 
 as " a process of combustion taking place at 
 common temperatures, in which the products of the 
 fermentation and putrefaction of plants and of 
 animal bodies combine gradually with the oxygen of 
 the atmosphere," producing carbonic acid, water, and 
 ammonia. 
 
 There are many compounds which, by themselves, 
 are deficient in the property of absorbing oxygen. 
 Alcohol, for example, only combines with oxygen at 
 a comparatively high temperature. For such things 
 contact with a substance itself undergoing change 
 was held by Liebig to be the chief condition of decay. 
 They then behave, he said, as if they were a part of 
 the decaying material, and their oxidation is effected. 
 He offered as an instance the German " quick vinegar 
 process," in which alcohol, in the form of wine or 
 diluted brandy, is allowed to flow slowly over shavings 
 of wood, packed in casks through which a slight 
 current of air also circulates. The alcohol, in spite 
 of its want of attraction for oxygen is, under these 
 circumstances, quickly converted into vinegar. This 
 would not, however, occur if pure spirit were passed 
 over a non-putrescible material, and at the com- 
 mencement of the process it is usual to add to the 
 spirit a small quantity of one of those substances 
 which are capable, unassisted, of undergoing decay 
 such as beer- wort, honey, vinegar then, after the lapse 
 of a short time, the surface of the shavings passes, 
 
72 JUSTUS VON LIEBIG: 
 
 according to this view, into a state of oxidation, and 
 from that time there is no further need of the 
 co-operation of added decaying matter. 
 
 Such was Liebig's explanation of the phenomena 
 of fermentation and decay. 
 
 But not even the authority of a Liebig will for 
 long protect a wide-reaching hypothesis, such as this, 
 from criticism, and soon its great rival, the vitalistic 
 theory of fermentation, was revived by Louis Pasteur, 
 who brought to its support observations and new facts 
 of a most startling character. 
 
 The starting-point of the vitalistic theory of fer- 
 mentation is to be found in the observation made in 
 1680, by Leuwenhoeck, when examining beer yeast 
 with the microscope, that this substance consists of 
 small globules, which are spherical or globular in 
 form, but whose nature he was not able to deter- 
 mine. A century and a half later the observations of 
 Cagniard de Latour, and soon afterwards those of 
 Schwann, at Jena, and Klitzing, at Berlin, showed that 
 yeast consists of a mass of organic globules capable 
 of reproducing themselves by buds ; these globules 
 seemed to them to belong to the vegetable kingdom, 
 and not to be simply organic chemical compounds, as 
 had till then been supposed to be the case. They 
 concluded that probably the converting of sugar into 
 alchohol and carbon dioxode was an effect of their 
 vegetative processes. This view of the action of yeast 
 cells was not, however, widely accepted till it had the 
 support of Pasteur's experiments, and not universally, 
 even then, until after a controversy of the keenest 
 kind. 
 
 Liebig did not deny the organised nature of yeast, 
 nor its power of multiplication by budding, but he 
 
HIS LIFE AND WORK. 73 
 
 was of opinion that the living cells are always accom- 
 panied, also, by dead cells, and that it was the mole- 
 cular motions of the decaying matter of these dead 
 cells which was communicated to the sugar and 
 brought about its decomposition by fermentation. 
 
 To put this idea to the test of experiment, Pasteur, 
 who was not one whit behind Liebig himself in his 
 conviction that experiment is the final court of appeal 
 on all scientific questions, sowed almost imponderable 
 portions of fresh yeast cells in solutions of pure sugar, 
 to which he had added small quantities of such 
 mineral salts as are necessary for their growth, with the 
 result that the cells thus sown multiplied, and the sugar 
 fermented as before. From this result he concluded 
 that the process mainly took place between the sugar 
 and a ferment germ, which owed its life and develop- 
 ment to the nutritive matter he supplied to it. The 
 most important of these nutritive substances was the 
 sugar. That fermentation, in short, is simply a pheno- 
 menon of nutrition, in which the organism assimilates 
 one part of the fermentable matter, using it for its 
 growth and for the production of new individuals, and 
 converts the rest into the well-known products of the 
 change. This attempt at a crucial experiment by 
 means of the fermentation of pure sugar, in associa- 
 tion with mineral salts, does not, by itself, as a close 
 examination shows, really finally overthrow Liebig's 
 hypothesis, for it is manifestly reasonable to sup- 
 pose, first, that even the small amount of ferment 
 taken, however carefully purified, must have been 
 accompanied by a certain amount of what Liebig 
 called putrescible material ; and, secondly, that 
 the growth and life of the yeast would be soon 
 accompanied by the death of some part of it. This 
 
74 JUSTUS VON LIEBIG: 
 
 would supply the putrescible matter even if it were 
 absent in the first instance. 
 
 It is scarcely surprising, therefore, that Liebig was 
 not for some time convinced of the dependence 
 of fermentation on the existence of these organisms 
 by the facts brought forward by his opponent ; and he 
 made merry over these minute beings which feed on 
 sugar, and secrete alcohol, whose appearance simul- 
 taneously with the fermenting of the sugar, had been 
 held to support the idea that fermentation is a result 
 of their vital processes. To him the presence of 
 animalculso in putrefying matter appeared to be of the 
 nature of an accident, their number, when large, 
 being the result of the fact that these organisms find 
 in such matters the most favourable conditions for 
 their nutrition and development, their presence being 
 often very beneficial, since it resulted in a more rapid 
 oxidation of the material concerned. Fortunately for 
 science, Pasteur did not confine himself, as his pre- 
 decessors had done, to the investigation of a single 
 case of fermentation. He carried his researches 
 into other fields, and thereby enriched science with 
 many new facts, some of which are of incalculable 
 importance, in consequence of the light they have 
 thrown on the nature of several of the most virulent 
 diseases by which animals and men are afflicted, 
 and because they stimulated other observers to make 
 equally productive exertions in many new directions. 
 
 When Liebig's earliest opinions were formed on 
 the subject of fermentation, only the fermentation of 
 sugar by yeast had received any considerable atten- 
 tion on its biological side, but subsequently Pasteur 
 extended the range of his investigations, and was able 
 to connect the existence of other organisms with 
 
HIS LIFE AND WORK. 75 
 
 other fermentive changes. Thus he found that the 
 lactic fermentation, in which sugar of milk pro- 
 duces lactic acid, is associated with the growth of 
 a grey substance which may be easily overlooked, 
 partly because the amount produced is usually re- 
 latively small, partly on account of the difficulty of 
 distinguishing it from the other materials present. 
 
 The grey matter in question, when separated from 
 other substances, appears, as seen under the micro- 
 scope, to be formed of small globules or points smaller 
 than those of beer yeast. Pasteur showed a fairly 
 complete analogy between the known facts relating 
 to these two fermentations, and proved further, that 
 whilst in the presence of both ferments both fermenta- 
 tions can proceed under suitable conditions, lactic acid 
 is never produced in normal alcoholic fermentation. 
 
 Vinegar, as has previously been explained (p. 71), 
 is produced by exposing wine or diluted brandy to 
 the air ; oxygen is absorbed by the alcohol present in 
 these liquids, and the alcohol is thereby converted 
 into acetic acid. Pure alcohol will not undergo 
 this oxidation; it is necessary, as will be remem- 
 bered, to add a little beer- wort, meat-juice, or some 
 such putrescible body, in order that "acetic fer- 
 mentation" shall set in. It is necessary, in fact, 
 to have a " ferment," and Liebig, as we know, be- 
 lieved the ferment to be nitrogenous matter in a 
 state of change. Pasteur, on the other hand, attri- 
 buted the acetous fermentation to a plant, which 
 had long been known under the name of "flower 
 of vinegar," a little fungus which floats on the 
 surface of wine during the process which transforms 
 it into vinegar. The yeast plant sometimes forms a 
 hardly visible veil over the surface of the liquid, at 
 
76 JUSTUS VOX LIEBIG: 
 
 others it exists as a wrinkled film, very thin and 
 unctuous to the touch. This little organism, which 
 can only exist if it be plentifully supplied with its 
 proper aliments and at a moderate temperature, 
 possesses, Pasteur contended, the power of condensing 
 considerable quantities of oxygen from the air and of 
 fixing this gas on alcohol. It is one of those so-called 
 spontaneous productions which, like moulds, almost 
 always make their appearance on liquids suitable for 
 their growth; they or their germs appear to exist 
 everywhere around us, so that if one wants them it is 
 generally only necessary to expose a suitable nutrient 
 liquid in this case a mixture of wine and vinegar 
 will do well in a warm place, and they will soon 
 make their appearance. 
 
 Pasteur showed that, owing to the sensitiveness of 
 the vinegar plant, it is destroyed by a temperature of 
 a little above 60 C., and that wine heated to that 
 temperature, even if afterwards exposed to filtered air, 
 refuses to become vinegar. This temperature, he 
 argued, must have left intact the albuminous and 
 nitrogenous substances in the wine, and hence these 
 cannot be regarded as the source of the fermentation ; 
 in short, the ferment in this case must be the living 
 vinegar plant. 
 
 Such experiments as those which have been quoted, 
 and perhaps still more the constant connection which 
 has been shown by Pasteur and others to exist between 
 certain fermentive diseases and definite organisms, 
 have now for some time past satisfied nearly everyone 
 that fermentation, including in that term also putre- 
 faction and decay, is, as Pasteur has so vehemently 
 insisted, in many cases connected with the existence 
 of organisms, and that each kind of fermentation is 
 
HIS LIFE AND WORK. 77 
 
 dependent on the growth and development of a 
 particular organism. To this very considerable 
 extent Pasteur has fixed the vitalistic theory on 
 so secure a base, that even Liebig practically ad- 
 mitted its truth in his later writings. And yet, in 
 spite of the success of the vitalistic theory, it cannot 
 be said that this theory has overthrown its rival 
 Although the connection of many fermentations with 
 the life of definite organisms has been clearly proved, 
 there remain a number of changes, which may also be 
 described as fermentations, the occurrence of which, 
 it seems almost certain, does not depend upon such 
 organisms. And, besides, though it is clear that hi 
 many cases fermentation depends on the presence 
 of micro-organisms, we do not yet know how these 
 organisms act. We do not know whether they live 
 on the fermentable matter and excrete the products 
 of the fermentation, or whether the microbes produce 
 soluble ferments, such as will be presently mentioned, 
 which afterwards bring about the fermenting of the 
 fermentable substances in some manner more or less 
 like that which Liebig suggested. As regards the 
 first alternative, it may be pointed out that the 
 amount of the products of a fermentation is usually 
 very great, in proportion to the mass of the 
 organisms concerned and to the time they are in 
 action. On the other hand, as regards the second 
 alternative, though attempts to isolate soluble 
 ferments from the organisms have not often been 
 successful, yet it has been done. Hence, a priori, 
 there is nothing improbable in the second suggestion, 
 as the past failure to isolate these substances may 
 well have been due to want of knowledge and experi- 
 ence. The difficulty is always to begin. 
 
78 JUSTUS VON LIEBIG: 
 
 This brings us again to the further important fact, 
 that besides the organised ferments whose existence 
 was established by Pasteur and others, there are yet 
 other ferments of quite a different nature ; such are 
 the active agent (emulsin) in the change by which 
 the amygdalin of bitter almonds yields sugar, prussic 
 acid, and oil of bitter almonds (see p. 35), and the 
 ptyalin of saliva, by which starch may be transformed 
 into a sugar. These latter ferments are called enzymes. 
 They are believed to be unorganised chemical sub- 
 stances which result from the activity of living cells. 
 The changes which they bring about are analogous to 
 those induced by the organised ferments themselves. 
 Like the latter, they appear to be independent of 
 any change in the agent which produces them, 
 and in both cases great effects are produced by 
 what seem to us, at first sight, very trivial causes. 
 Whilst Liebig was wrong in denying the existence 
 of any connection between the organisms and the 
 fermentations, it must be admitted, on the other 
 hand, that his opponents have not yet succeeded in 
 explaining their mode of action, so that it is not 
 impossible that some modification of the theory of 
 Liebig may yet be found useful. The rival theories of 
 fermentation which have here been discussed have 
 jointly and severally done a splendid work, by 
 stimulating and guiding workers in this field of 
 science, from which rich crops have already been 
 gathered, and which seems to promise results in the 
 future, such as the earlier workers would scarce have 
 dared to dream of when they began their labours. 
 
 It now seems possible that the difference between 
 an organised and an unorganised ferment may be 
 this that the active agent of the organised ferment 
 
HIS LIFE AND WORK. 79 
 
 is one which only acts within the cells in which 
 it is formed, and which has not yet, with one or 
 two exceptions, been separated from the cells which 
 contain it ; whilst the unorganised ferments are those 
 which can act outside the cells which produce them. 
 
 Whatever may be the final fate of Liebig's theory 
 of the ferments, I fear it must be said, after nearly 
 half a century of interesting, active work, that it has 
 not yet been replaced by a really general and satis- 
 factory hypothesis. 
 
80 JUSTUS VON LIEBIG: 
 
 CHAPTER VI. 
 
 CHEMISTRY OF AGRICULTURE. 
 
 Commemorative Addresses Components of Plants Relations of 
 Plants and Animals Davy's Lectures Boussingault's Labora- 
 tory The Humus Theory Opinions in Germany and England 
 Overthrow of Humus Theory Evidence that Air is Source 
 of Carhon of Vegetables Plants Source of Carbon for Animals 
 The Real Use of Humus Sources of Components of Plants 
 Other than Carbon Origin of Nitrogen of Plants Mineral 
 Theory of Manures Liebig's Experiments in Agriculture 
 Errors Made in Applying the Mineral Manures, and how they 
 were Corrected " Ground Absorption " of Soils Object of 
 Liebig's Practical Work in Husbandry " The Natural Laws 
 of Husbandry " Deterioration of Land in Western Countries ; 
 How to Avoid it Liebig's Influence on Education of Agri- 
 culturists. 
 
 SPEAKING generally, the first twenty years of Liebig's 
 life, after his return from Paris, were devoted to 
 pure chemistry. The next period was distinguished 
 by his remarkable contributions to chemistry as 
 applied to agriculture and physiology. Perhaps some 
 idea of the range and depth of his work over the whole 
 field of pure and applied chemistry may be gained 
 from the fact that, on his death, in 1873, it was felt 
 to be impossible that any one man could sufficiently 
 comprehend all the subjects he had advanced and his 
 share in their advancement, and that, therefore, not 
 one, but three of his colleagues, all men of great emi- 
 nence, were charged with the duty of delivering 
 commemorative addresses, each one of them under- 
 taking that part of the great master's work with which 
 
HIS LIFE AND WORK. 81 
 
 he was himself most familiar from his own life-work. 
 (See p. 9.) 
 
 Liebig's first writings on agricultural chemistry 
 and on fermentation were presented in 1840 * to the 
 Members of the British Association, as part of a 
 Report upon the Present State of Organic Chemistry, 
 in fulfilment of a task which had been imposed upon 
 him by its chemical section at a previous meeting. 
 From the work which he then began, in response 
 to the duty thus laid upon him, he may be said 
 never to have withdrawn his hand. The subject 
 was worthy of the worker. " Perfect agriculture," as 
 he says, in his preface to this book, " is the true 
 foundation of all trade and industry, it is the founda- 
 tion of the riches of states. But a rational system of 
 agriculture cannot be formed without the application 
 of scientific principles, for such a system must be 
 based on an exact acquaintance with the means of 
 nutrition of vegetables, and with the influence of soils 
 and actions of manure upon them. This knowledge 
 we must seek from chemistry, which teaches the 
 mode of investigating the composition, and of studying 
 the character of the different substances from which 
 plants derive their nourishment. . ." 
 
 These were the convictions which impelled Liebig 
 to bring his unique and now highly- trained faculties to 
 bear on the task of creating a science of agriculture, 
 and to enter a field which had lain fallow since the time 
 of Davy, who was the first chemist to occupy himself 
 with the study of the application of chemical principles 
 to the growth of vegetables and to organic processes. 
 
 * Liebig's "Chemistry in its Applications to Agriculture and 
 Physiology." Published in- Brunswick, 1840. English Edition 
 Edited by Dr. Lyon Playfair, London, 1840. 
 F 
 
82 JUSTUS VOX LIEBIG: 
 
 When Liebig began this, his second great work, 
 he was not far from the zenith of his career. 
 No one was better trained than he in the methods 
 of organic analysis, which he had brought to per- 
 fection and applied to their purpose, as Wohler says, 
 with almost pedantic exactness. He was surrounded, 
 too, by a number of pupils eager and well qualified 
 to aid him in his undertaking ; among them at this 
 moment was Lyon Playfair, to whom was entrusted 
 the duty well carried out of editing Liebig's first 
 book, for publication in England. 
 
 Liebig's first book on chemistry, in its applications 
 to agriculture and vegetable physiology, quickly 
 passed through a number of editions. Twenty-two 
 years later, after he had studied in minute detail 
 the various questions involved in this subject, and 
 taken part in numerous discussions with agriculturists 
 and others, at home and abroad, he published an em- 
 bodiment of his researches in " The Natural Laws of 
 Husbandry," a work which has been truly described 
 by Hofmann as the first perfect construction of the 
 philosophy of agriculture Avhich had ever appeared 
 up to that date. This work was originally issued in 
 two parts. The second part was published in an 
 English translation under the above title. 
 
 Liebig spared no pains in order to qualify him- 
 self on the technical side for this new undertaking. 
 Besides studying the practice of husbandry at 
 home, he paid a visit to Great Britain, and made 
 a journey through the agricultural districts of 
 England and Scotland, in order that he might 
 acquaint himself personally with the various 
 practices of different districts in farming, in butter- 
 making in cheese-making, and so on. 
 
HIS LIFE AND WORK. 83 
 
 Side by side with his labours on behalf of 
 agriculture, Liebig undertook another and equally 
 searching investigation into the applications of 
 chemistry to physiology and pathology. It was a 
 happy inspiration which led him to combine 
 in his researches these two lines of work, each so 
 important to the welfare of mankind, each so 
 closely bearing upon the other. The study of agri- 
 culture, consists largely in the study of the science 
 of manuring. Liebig's strict investigation of the 
 processes of nutrition in the animal organism, and of 
 the origin of animal excrements, enabled him in 
 the end to understand, better than those who 
 preceded him, the cause of the beneficial effects of 
 these excrements on the growth of vegetables, and 
 enabled him to trace among the multitudinous 
 phenomena of life processes, a few simple yet funda- 
 mental laws for the guidance of practical farmers. 
 
 Interesting as it would be to follow the details of 
 Liebig's agricultural investigations during the years 
 of his chief activity in this direction, it is impossible 
 to attempt it. Were we to do so, we should see 
 him now at his desk, now working in his laboratory, 
 now guiding the older students in their share of his 
 studies ; at another time we should have to follow 
 him to the factory, where his mineral manures were 
 prepared; then go with him to his experimental 
 plot to watch their effects ; or accompany him to 
 some gathering of agriculturists, scientific or other- 
 wise, to join in discussions, which were often warm 
 and always animated, on the agricultural topic of 
 the moment. In no part of all these varied duties 
 did Liebig fail to take his full share. The work 
 of the porter in the manure factory, that of the 
 
<S4 JUSTUS VON LIEBIG : 
 
 labourer on the farm, and of the student at his analysis 
 was all done under his close guidance, and was as 
 fully the outcome of his inspiration as that part 
 which was done with his own hands and head 
 alone. But, delightful as it would be thus closely 
 to watch the activities of such a man, all that can 
 be even attempted is to try to gain a clear view 
 of the ideas which animated Liebig's work at its 
 inception, of the main features of the labours he 
 undertook, of the difficulties he met and overcame, 
 and, finally, of the position to which agricultural 
 science had been raised when these labours were 
 brought to a conclusion. 
 
 For the benefit of the uninitiated, it must here 
 be pointed out, that there is a beautiful connection 
 between the organic and the inorganic kingdoms of 
 nature. It is inorganic matter mainly which affords 
 food to plants, and they, on the other hand, yield the 
 means of subsistence to animals. 
 
 The conditions necessary for animal and veget- 
 able nutrition are essentially different. An animal 
 requires for its development, and for the sustenance 
 of its vital functions, a certain class of substances 
 which can be generated only by certain organic 
 beings possessed of life viz. by the plants. 
 Although many animals are entirely carnivorous, 
 yet their nourishment is ultimately derived from 
 plants, for the animals on which the carnivora 
 feed receive their nutriment from vegetable mat- 
 ter. Plants, on the other hand, find new nutritive 
 material only in inorganic substances. Hence, one 
 great end of vegetable life is to generate matter 
 adapted for nourishing the animals out of inorganic 
 substances which are not fitted for that purpose. 
 
HIS LIFE AND WORK. 85 
 
 These are almost the very words with which Liebig 
 opened his book on " Chemistry in its Appli- 
 cation to Vegetable Physiology and Agriculture." 
 He devoted the first part of this book to an 
 examination of the matters which supply nutriment 
 to plants, and of the changes which these undergo 
 in the living organism. 
 
 The second part of the book deals with fermenta- 
 tion, putrefaction, and decay ; but this department of 
 Liebig's work has already come before us. 
 
 All parts of all plants contain carbon and hydrogen ; 
 these are among the constituents of all their organs ; 
 they are absolutely essential to their existence. 
 
 The substances of which the greater part of 
 vegetables are composed also contain oxygen ; the 
 larger part of these substances are compounds in 
 which carbon is present, together with hydrogen and 
 oxygen in the proper proportions for forming water. 
 Sugar, starch, gum, and the materials of which the 
 outer walls of plants are formed are all of this character ; 
 such substances are called by chemists carbohydrates. 
 Other oxidised substances also occur, in which the 
 proportion of oxygen is greater ; many of these are 
 sour bodies, such as the vegetable acids. Wax, resin, 
 and the fixed oils usually contain these three elements, 
 but in their case oxygen is either absent or is present 
 in smaller proportion than that which is found in the 
 carbohydrates. 
 
 Besides this, the juices of plants contain small 
 quantities of mineral matter which may be detected 
 in their ashes, whilst nitrogen is present, especially 
 in the seeds. The proportion of nitrogen is small, but 
 it is rarely altogether absent from any part of a plant ; 
 it is a component of such substances as the gluten of 
 
86 JUSTUS VON LIEBIG: 
 
 wheat, vegetable albumen, and of the alkaloids, such 
 as quinine and strychnine. Finally, plants also con- 
 tain a little sulphur and phosphorus. 
 
 From these facts it follows that the nourishment 
 of a plant must contain carbon, nitrogen, phosphorus, 
 and sulphur in forms suitable for their assimilation. 
 Water and mineral matter must also be supplied to it. 
 
 From what sources are these obtained ? Which of 
 them, if any, come from the air, which from the soil ? 
 Are the sources of supply exhaustible or inexhaustible, 
 practically speaking ? Does nature provide modes of 
 replenishing the supplies of all or any of them ? How 
 far do the lives of animals and the arrangements of 
 mankind supplement or oppose themselves to such 
 replenishment ? If the latter tend to exhaust the 
 supply of any constituent or constituents, how can 
 these arrangements be modified so as to prevent this, 
 or, better, so as to produce enrichment in place of 
 exhaustion ? These are some of the great questions 
 for the agriculturist and the agricultural chemist. 
 
 How can we educate the agriculturist and the 
 man of science, so that they shall face these problems 
 with an open mind, and co-operate in attempting to 
 solve these and similar questions ? This is one of the 
 great problems of technical education ; considering 
 the vital importance of agriculture, it is perhaps the 
 great question of technical education. 
 
 It was said above that Liebig first, after Davy, 
 attempted to construct a theory of agricultural 
 chemistry; but it should be mentioned that very 
 shortly after the commencement of Davy's work, De 
 Saussure published his " Chemical Researches on 
 Vegetation," and that from 1834 Boussingault also 
 worked, with devotion, at agricultural chemistry. 
 
HIS LIFE AND WORK. 87 
 
 The results these two investigators published are 
 frequently referred to by Liebig in the course of his 
 writings, and there is no doubt at all that the data 
 they supplied contributed very largely to form the 
 basis of Liebig's historic generalisations. 
 
 The foundations of the chemistry of agriculture 
 were of necessity laid somewhat late, for, from what 
 has been said, it will be seen that it was impossible to 
 gain any true conceptions on the subject until an 
 accurate knowledge of the composition of air and 
 water had been attained. 
 
 When Liebig commenced the studies which ended 
 in the overthrow of the humus theory, the composi- 
 tion of the air, of water, and of carbonic acid gas were 
 known. The importance of minerals to plants was 
 admitted, and the advantage of adding them, in 
 the form of marl to manure had been suggested, 
 more than a century earlier, but their mode of 
 action was still under discussion. Thaer, the agri- 
 culturist, regarded thein as non-essential, or at best 
 useful as a kind of stimulant, whilst De Saussure and 
 Davy were opposed to this view. Priestley had ob- 
 served that plants possess the faculty of purifying air 
 that had been vitiated by the breathing of animals or 
 by combustion, and it had been discovered that the 
 bubbles of gas which growing plants emit when 
 plunged under water consist chiefly of oxygen. 
 
 Ingenhousz had shown that these phenomena 
 demand the presence of sunlight, and Sennebier that 
 the oxygen emitted by plants finds its source in the 
 carbon dioxide absorbed from the air. De Saussure, 
 who worked at the end of the last century, had, by 
 experiments that were approximately quantitative, 
 shown that in sunlight plants increase the quantity of 
 
88 JUSTUS VON LIEBIG: 
 
 carbon, hydrogen, and oxygen in their tissues, and 
 that this is done at the expense of carbon dioxide and 
 water ; by an important experiment he had also proved 
 that the increase of carbon and of the components of 
 water correspond pretty closely to what we know to 
 be the proportions of them in the carbohydrates. 
 (See p. 85.) He had also recognised the usefulness of 
 minerals, that they must come from the soil, and had 
 called attention to the probability that the origin of 
 the mineral constituents of the animals was to be 
 found in these incombustible substances extracted by 
 plants from the earth. In regard to the already much 
 debated point of the origin of the nitrogen of plants, 
 De Saussure was of opinion that nitrogen was rather 
 given to the air than taken from it, and that the 
 sources of nitrogen were probably the nitrogenous 
 substances in the soil and the ammonia of the air, 
 which is largely brought down to the earth by rain. 
 On the whole, though he does not seem to have 
 distinctly disowned the humus theory (see below), he 
 regarded the air as the main source of the carbon, 
 hydrogen, and oxygen of plants, and the earth as 
 valuable in affording supplies of mineral matter and 
 nitrogenous substances. 
 
 The humus theory. Owing to the fact that 
 virgin soils are often particularly well suited for the 
 cultivation of plants, and that these virgin soils have 
 been found to be very rich in vegetable mould, or 
 humus, it had come to be supposed, prior to the 
 date of Liebig's earliest writings on agricultural 
 chemistry, by many chemists and agriculturists, that 
 this vegetable mould was the source of the fertility 
 of these soils. By an extension of this idea many 
 vegetable physiologists ascribed the fertility of all 
 
HIS LIFE AND WORK. 89 
 
 soils to its presence, and even regarded it as the 
 chief nutriment of the plants ; it was supposed by 
 them that the humus was extracted from the soil 
 by the roots of the growing plants. 
 
 In Germany this view Avas very widely accepted. 
 Even De Saussure is said by Ernst von Meyer, in his 
 history of chemistry, not to have kept himself quite 
 clear of this error, though he realised much better 
 than many others that the air, more than the earth, was 
 the source of the carbon and hydrogen of plants. In 
 England, as Dr. Playfair pointed out long ago, this 
 idea of the function of humus was by no means so 
 unreservedly accepted when Liebig wrote; but one 
 rises from a careful examination of what Davy, 
 Daubeny, Thomson, and Brande wrote during the early 
 part of the century, very far from convinced that 
 these authorities unreservedly accepted the alternative 
 hypothesis suggested by Priestley's observations on 
 the purifying of the air by plants. In fact, even in 
 England, a clear perception of the source of the carbon 
 of plants was still to seek. Thus Brande, writing as 
 late as 1836, still doubted whether plants could 
 efficiently remove the carbonic acid gas from the air, 
 and considered that the carbon of manure, reduced by 
 putrefaction to a soluble state, was an important source 
 of supply. 
 
 Liebig, in dealing with this subject, went straight 
 to the root of the matter. Humus is of very vari- 
 able composition, and is only soluble in water when 
 freshly precipitated ; it becomes quite insoluble after 
 drying, and after it has been exposed to a freezing 
 temperature. Both the heat of summer and the cold 
 of winter, therefore, render this substance insoluble. 
 This is confirmed by the fact that water does not 
 
90 JUSTUS VON LIEBIG: 
 
 dissolve anything more than a mere trace of organic 
 matter from good garden soil, and that the decayed 
 wood of various trees, which largely consists of humus- 
 like matter, is also practically insoluble. 
 
 But, in order that humus shall be absorbed by 
 plants, it must first become capable of entering into 
 solution. It follows, that humus, as it occurs in the 
 earth, cannot serve to nourish plants. 
 
 This was not denied by physiologists. And in 
 order to overcome the difficulty, they assumed that 
 the lime and the alkalis found in the ashes of vege- 
 tables render the humus soluble, and so fit it for 
 being assimilated by the roots of plants. 
 
 It is true that alkalis and alkaline earths (e.g. lime) 
 do exist in different soils in sufficient quantities to 
 form compounds with humus, and that humus is 
 rendered soluble in water by their presence. One 
 kilogramme of lime, for example, will combine with 
 10*9 kilos, of humic acid. 
 
 To test the validity of the hypothesis based upon 
 these facts, Liebig made various calculations, the 
 results of which clearly showed that the amount of 
 humus which could possibly be obtained by the plants 
 on a given area is far too small to account for the 
 vegetables actually produced, even after making all 
 sorts of impossible assumptions in favour of the theory 
 
 He supported the facts brought to light by his 
 calculations by others. 
 
 Thus, he showed that the proportion of carbon 
 produced on one acre of cultivated land from such 
 various crops as fir-wood, pine-wood, beech-wood, 
 beet-root, rye and hay, is remarkably constant under 
 dissimilar conditions of cultivation. 
 
 Also, that carbon may be removed to some extent 
 
HIS LIFE AND WORK. 91 
 
 from a forest or meadow in the form of wood or 
 hay, and that, in spite of this, the soil will become 
 richer, not poorer, in carbon. And finally, he clinched 
 the argument by pointing out that since it is uni- 
 versally admitted that humus is only produced by 
 the decay of plants, no primitive humus can have 
 existed for the first plants, for plants must have pre- 
 ceded the humus. 
 
 These were the facts and arguments by which, 
 once and for all, Liebig rendered the humus theory 
 utterly untenable by any reasonable human being. 
 
 Anarchy, however, is no more acceptable in 
 science than in society, and Liebig was the last man 
 to be satisfied with the mere overthrow of an 
 erroneous theory. He was bound by his nature to 
 ask himself the question Whence, then, did the first 
 vegetables obtain their carbon ? 
 
 Having thus demolished the humus theory, Liebig 
 proceeded to investigate its rival. Ever since 
 Priestley's observations, in 1771, on the influence of 
 plants on the air, vegetable physiologists had, more 
 especially in England, shown an inclination to 
 suppose that part at least of the carbon of plants 
 is derived from the carbonic acid gas of the air, 
 and as early as 1807 Thomson had observed that if 
 plants are deprived of this gas they droop their 
 leaves, which then refuse to fulfil their functions. 
 De Saussure and others had abundantly confirmed the 
 English observer's conclusions, and it was beginning 
 to be generally admitted in this country that prob- 
 ably a large quantity of carbon is obtained by plants 
 from this source, and that oxygen is simultaneously 
 supplied to the air. But further progress was retarded, 
 partly by the apparent existence of an alternative 
 
92 JUSTUS VON LIEBIG: 
 
 source of supply in the humus of the soil, and partly 
 by the discovery by Ingenhousz that, although in 
 sunlight plants absorb carbonic acid gas and give 
 out oxygen, in the dark, on the contrary, they rather 
 injure the air than improve it, as they then absorb 
 oxygen, and in some cases emit carbonic acid gas. 
 The oxygen given out in the light was, moreover, 
 even said to correspond to that absorbed in the dark. 
 Naturally, under these circumstances it seemed 
 doubtful whether, on the whole, very much carbon 
 was really derived from the air by plants, and the 
 uncertainty on this point helped to maintain the 
 position of the humus theory until, as we have 
 seen, Liebig demolished it in 1840. But one of the 
 most remarkable facts about the composition of the 
 air is this, that in spite of the vast consumption of 
 oxygen by animals, by combustion, and in decay, 
 the proportion of oxygen in pure air is, at all times 
 and in all climates, practically uniform. The analysis 
 of air always shows that one hundred volumes of air 
 contain twenty-one volumes of oxygen. And yet, vast 
 as is the store of oxygen in the air, so great is the con- 
 sumption thereof that the supply cannot be considered 
 to be inexhaustible. Liebig, indeed, made a calculation 
 Avhich showed that at the rate at which we now use it 
 the air would become quite unfit for us in less than 
 300,000 years, owing to loss of oxygen alone, if the 
 supply were not renewed. How is it possible, he 
 asked, that with so vast a consumption of oxygen the 
 supply should remain so nearly constant; that the 
 analysis of air taken from jars buried in Pompeii 
 nearly 2,000 years ago should, under these circum- 
 stances, present so nearly the same composition as a 
 sample taken from the air it may be yesterday. 
 
HIS LIFE AND WORK. 93 
 
 Again, it had been found in Liebig's day, from the 
 analyses of De Saussure, that the proportion of car- 
 bonic acid gas in the air was actually not above one 
 part in a thousand.* What then has become of the 
 vast quantities of carbonic acid gas that have been 
 poured into the air during countless .ages by the 
 breathing of animals, by the process of combustion, 
 and in the decay of organic substances quantities so 
 great that, according to the above-mentioned calcula- 
 tion, in a period of 100,000 years they must have 
 caused an accumulation of this gas amounting to 
 several parts by volume in every one hundred volumes 
 of air ? 
 
 We have even reason to believe that the propor- 
 tion of carbonic acid may once have been greater than 
 now, and yet, in spite of the enormous volumes of 
 this gas which are daily poured into the air, there is 
 to-day only a trace of it present. It is evident, said 
 Liebig, that the invariable proportions of carbonic 
 acid gas and oxygen in the atmosphere and the mere 
 trace of the former that is found there, in spite of the 
 enormous and never-ending supplies, must be the 
 result of some cause which prevents the increase of 
 carbonic acid gas by removing that which is con- 
 stantly forming ; and that there must be some means 
 of replacing the oxygen removed from the air by the 
 processes of combustion and putrefaction, as well as by 
 the respiration of animals. 
 
 He found the answer to these questions in the 
 data supplied by his predecessors, Priestley, Sennebier, 
 and De Saussure. Both these causes are united in the 
 processes of vegetable life. 
 
 * The more refined methods of modern analysis place the propor- 
 tion still lower. 
 
94 JUSTUS VON LIEBIG: 
 
 The carbon of plants cannot come from the earth ; 
 it must be derived from the atmosphere. The only 
 form of carbon which exists in the atmosphere is 
 carbonic acid gas, or oxide of carbon ; it follows that 
 this must be the source of the carbon of vegetables. 
 
 But, further, the analysis of plants shows that the 
 greater part of their constituents are composed of 
 carbon, together with hydrogen and oxygen, in the 
 proportions in which they form water ; from this it 
 follows that the oxygen required for the nutrition of 
 a plant can be mainly supplied by the water which it 
 assimilates, since hydrogen enters into the food of 
 plants almost entirely in this form, and if this be so 
 the oxygen of the carbonic acid gas is quite unneces- 
 sary for plants, and must escape into the air in the 
 gaseous form. As carbonic acid gas contains its own 
 volume of oxygen, the atmosphere must thus receive 
 for every volume of carbonic acid gas decomposed by 
 the plants an equal volume of oxygen. 
 
 This view was not only in accordance with the 
 facts already mentioned, but De Saussure had shown 
 that the gain in weight of a growing plant is greater 
 than can be accounted for by the carbon assimilated, 
 which agrees with the supposition that the elements of 
 water are assimilated at the same time as the carbon. 
 
 " The life of plants," said Liebig, " is closely con- 
 nected with that of animals, in a most simple manner, 
 and for a wise and sublime purpose. 
 
 " The presence of a rich and luxuriant vegetation 
 may be conceived without the concurrence of animal 
 life, but the existence of animals is undoubtedly de- 
 pendent upon the life and development of plants. 
 
 " Plants not only afford the means of nutrition for 
 the growth and continuance of animal organisation, 
 
HIS LIFE AND WORK. 95 
 
 but they likewise furnish that which is essential for 
 the support of the important vital process of respira- 
 tion ; for, besides separating all noxious matters from 
 the atmosphere, they afford an inexhaustible supply 
 of pure oxygen, and they thus make up to the air the 
 loss constantly sustained by it. Animals, on the 
 other hand, expire carbonic acid gas, whilst plants 
 inspire it, and thus the composition of the atmosphere, 
 the medium in which both live, is maintained con- 
 stantly unchanged." 
 
 It was only necessary to prove that the amount of 
 carbonic acid gas in the air, at most only one- tenth 
 per cent., is sufficient to supply the wants of the 
 whole vegetation on the surface of the earth, in order 
 to finally establish the doctrine so clearly enunciated 
 in the above paragraphs. This was not difficult, for 
 the total mass of carbon in the air is immense, 
 although its proportion is small relatively to that 
 of the other constituents. But is it possible that it 
 can be absorbed sufficiently rapidly from so dilute a 
 mixture ? 
 
 This seemed to Liebig quite likely to be the case 
 when he considered the wide area which the leaves 
 offer to the gas, and the rapidity with which a lime- 
 wash, spread on a wall, will absorb this gas. One 
 square decimetre of such a surface, in the course of 
 six washings, has been known to absorb in four days 
 as much carbonic acid gas as will produce three- 
 quarters of a gram of calcium carbonate. This 
 quantity was found by Liebig to be several times as 
 great as that which is assimilated by the leaves and 
 roots of plants growing on an equal area, in an equal 
 space of time. It, therefore, seemed not unreasonable 
 to answer the above question in the affirmative. 
 
96 JUSTUS VON LIEBIG: 
 
 The evolution of carbon dioxide, which occurs at 
 night, seemed to Liebig to be a natural but secondary 
 incident. Since plants were supposed to absorb car- 
 bon dioxide during the day, but only to assimilate its 
 carbon in sunlight, he thought it likely that at 
 night some of the undecomposed gas would be re- 
 turned to the air, together with water vapour ; and he 
 considered that the nocturnal absorption of oxygen, 
 observed by Ingenhousz, was due to changes not at all 
 connected with the life of the plant, caused by the 
 action of the oxygen of the air on the organic sub- 
 stances composing its leaves, flowers, and fruit. In 
 this Liebig was doubtless wrong. The evolution of 
 of carbon dioxide and absorption of oxygen by plants 
 at night are now recognised as the characteristic ac- 
 companiments of the processes of cell-division and 
 cell-multiplication, by which their structure is in- 
 creased, and which take place, at any rate, chiefly 
 during night. 
 
 But on the main point he was right, for it is now 
 fully recognised that the oxygen absorbed and the 
 carbon dioxide set free at night, which misled so 
 many botanists and vegetable physiologists, are far 
 from being sufficient to counterbalance the carbon 
 dioxide taken up, and the oxygen simultaneously set 
 free during the hours of daylight, on which all plant 
 life is founded.* 
 
 One of the reasons which had led physiologists to 
 imagine that humus was particularly well suited for 
 
 * Davy showed the producing of oxygen from carbon dioxide by 
 plants very elegantly, hy growing grass under glass receivers standing 
 in water, and containing air supplied with small additional quantities 
 of carbonic acid gas from time to time. After the lapse of eight days, 
 the air in the receiver was found to be very decidedly enriched with 
 oxygen. 
 
HIS LIFE AND WORK. 97 
 
 nourishing plants was the fact that it differs from 
 wood chiefly in containing an excess of carbon. Like 
 wood, it contains carbon, together with hydrogen and 
 oxygen, in the proportions in which they combine to 
 form water, but, relatively to the proportion of carbon, 
 the proportion of water is less. Misled by this fact, 
 the physiologists concluded that this similarity of 
 composition between humus and the constituents of 
 plants must favour the assimilating of the former by 
 plants. Humus had only to unite with water in order 
 to form woody tibre. 
 
 In this, as Liebig pointed out, they went wrong, in 
 consequence of trusting that most dangerous guide 
 an argument from analogy. They had no right to as- 
 sume that the vital processes of plants are in this sense 
 similar to those of animals. The vital process of a 
 plant depends, as Liebig has taught us, on a change by 
 which it recovers carbon from a simple inorganic sub- 
 stance. Plants, unlike animals, are not fitted to assimi- 
 late the products of plant life. On the contrary as was, 
 indeed, already known when sugar, gum, or starch are 
 absorbed by a plant, these compounds do not nourish 
 it, for the very life of a plant consists in the elabora- 
 ting of these substances. Humus, therefore, and the 
 other products of life, can by no means be assumed 
 to be fit for the support of plant life. 
 
 Here Liebig drew a clear and broad distinction 
 between the vital functions of plants and animals, 
 \vhich was the corner-stone of his work in vegetable 
 and animal chemistry. 
 
 But, although humus is not the food of plants, 
 
 Liebig does not teach us to see in it mere dead, useless 
 
 matter. When a leaf, a twig, or a tree falls, it enters 
 
 on a new series of changes, in the course of which it 
 
 G 
 
98 JUSTUS VON LIEBIG: 
 
 will ultimately pass once more into the condition of 
 the simple oxidised compounds, which can afford 
 nutriment for a new generation of plants. Humus 
 represents one of the stages of this series of transfor- 
 mations. It is woody fibre in a state of decay. In 
 contact with air, as in well-tilled ground, it undergoes 
 steady if slow oxidation, and thus it becomes, according 
 to Liebig, the source of carbonic acid gas for very young 
 plants, the seedlings, to which it offers a supply of 
 this necessary substance before the development of 
 their roots and leaves enables them to take the latter 
 from the air. 
 
 And, besides this, the carbonic acid gas, by entering 
 into solution in the water which falls as rain, renders 
 this more capable of disintegrating minerals, and thus 
 promotes the processes by which the saline constitu- 
 ents of rocks become available for plants, Finally, 
 the oxidation of the humus ultimately liberates the 
 saline material of dead plants, so that they become 
 once more fit to serve for the sustenance of their 
 successors. 
 
 Humus, then, represents mainly certain stages in 
 the primitive processes which connect one genera- 
 tion of vegetables with those which succeed it, so far 
 as concerns those parts of vegetables which are not 
 utilised by man. 
 
 These conceptions concerning the relations of 
 plants and animals to one another, and to the air 
 they live in, which we thus owe to Liebig, have now so 
 completely taken their stand among the common- 
 places of science, that most likely hardly any of the 
 readers of this little book were unfamiliar with them 
 when they opened its pages ; and yet, probably, very 
 many, if not most, were unaware how recently they 
 
HIS LIFE AND WORK. 99 
 
 have become part of the equipment of mankind, by 
 what steps they have been reached, or who it was 
 that first traced out the few simple laws which 
 underlie the relations of the two kingdoms of organic 
 nature, by disentangling the intricacies of problems, 
 which, simple as Liebig has made them seem to us, 
 once appeared to be almost beyond solution. 
 
 Even in 1840, however, the most advanced chem- 
 ists and biologists were by no means unprepared 
 for such developments as those which we have just 
 considered, for, as we have seen, the work of several 
 then living investigators had already largely paved 
 the way for them. Indeed, only about a year later 
 Dumas, on behalf of himself and of Boussingault, 
 delivered a lecture at the conclusion of his course of 
 chemistry in the Medical School of Paris, entitled 
 " Essai de Statiqiw Chimique des Stres Organises" in 
 which they presented a resume of their chemical and 
 physiological researches, and propounded ideas closely 
 corresponding with those previously expressed by 
 Liebig, and in which an even broader view of the 
 relations of the vegetable and animal worlds was 
 brought forward and sustained. And before this, half 
 a century earlier, Lavoisier, the father of modern 
 chemistry, had anticipated the ideas which Liebig 
 was, as we have seen, the first to establish beyond the 
 possibility of further doubt. 
 
 " Plants," wrote Lavoisier in a document only dis- 
 covered after the publication of the first three volumes 
 of his collected works in 1862, " plants derive the 
 materials necessary for their formation from the air 
 Avhich surrounds them, from the water, and in general 
 from the mineral kingdom." 
 
 " Animals feed on plants, or other animals fed by 
 
100 JUSTUS VON LIEBIG : 
 
 plants, so that the substances composing them are, in 
 the last instance, always drawn from air and from the 
 mineral kingdom." 
 
 "On the other hand, fermentation, putrefaction, 
 and combustion continually restore to the air and the 
 mineral kingdom the principles borrowed from them 
 by plants and animals." 
 
 But whilst the discovery of this document must 
 add to our admiration for the immortal Lavoisier, 
 and sharpens, if that is possible, our regret for his 
 untimely end, it rather adds to, than detracts from, 
 the honour due to his illustrious successor that he 
 should have originated independently, and in so 
 nearly the same form, the brilliant speculations of his 
 great fore-runner ; and especially is this the case when 
 we remember that, aided by the advance of knowledge, 
 he succeeded in establishing the truth of these specu- 
 lations and in building upon them one of the pillars 
 of the natural sciences. 
 
 Plants do not consist of carbon only ; they contain 
 also, besides the elements of water, sulphur, nitrogen, 
 phosphorus, and mineral matter. With regard to the 
 sources of the hydrogen, the oxygen, and the sulphur 
 of plants, Liebig considered that the first-named 
 element, at any rate so far as that part of it which is 
 in the non-nitrogenous compounds is concerned, is 
 derived from water ; that the oxygen is provided 
 either by the water or by the carbonic acid gas ; and 
 that the sulphur has its origin in the sulphates which 
 are present in sufficient quantities in soils. 
 
 Besides the chief elements mentioned above, there 
 is another viz. nitrogen which exists in every part 
 of every plant, and which cannot be supposed to be 
 of less importance, although it is present in very 
 
HIS LIFE AND WORK. 101 
 
 much smaller quantities. The question in what form 
 does nature supply this nitrogen has for many years 
 been the subject of almost constant research and 
 discussion. 
 
 Nitrogen is, of all the elements, one of the least 
 apt to enter into direct combination, and hence it 
 has appeared improbable to chemists that the free 
 nitrogen of the air could be the source from which 
 plants gain their supply of it. 
 
 Liebig at first was of opinion that the source of 
 the nitrogen of plants lay in the ammonia produced 
 by the putrefaction of plants and animals, the remains 
 of one generation producing by decay what was re- 
 quired for another. A good deal of nitrogen is thus 
 returned to the soil in the form of ammonia in 
 manure ; some is yielded by the refuse of the crops, 
 but on a given farm, unless all the products are con- 
 sumed at home, there must be a loss in consequence 
 of the selling of certain products. In any event some 
 ammonia must also be lost during the process of decay 
 (we have all of us recognised the odour of smelling salts 
 in a badly-ventilated stable at one time or another) ; 
 hence, practically, the nitrogen returned to the soil 
 of a given farm by an agriculturist must always 
 be less than was contained in the crops grown. 
 Liebig therefore concluded that the ammonia brought 
 down by the rain probably constitutes a very im- 
 portant source of supply. This opinion was soon 
 challenged by Boussingault, who laid much more 
 weight on the importance of manures as a source of 
 nitrogen, and considered the supply obtained from 
 the air to be of less importance. 
 
 In 1843, in a new edition of his book, Liebig may 
 be said to have laid the foundation of the celebrated 
 
102 JUSTUS VON LIEBIG: 
 
 " mineral theory " of agriculture. He also returned to 
 the subject of nitrogen, and in his developed treatment 
 of it he pointed out that whilst nitrogen can only be 
 made to enter into combination with the other con- 
 stituents of plants by the most powerful chemical 
 means, and whilst this element is, moreover, frequently 
 emitted by plants, ammonia, on the other hand, is 
 remarkable, like water, for the number of the trans- 
 formations which it undergoes, and for the various and 
 opposite characters of the substances which it produces. 
 As carbonic acid gas is the last product of decay so far 
 as carbon is concerned, so is ammonia the form in 
 which the nitrogen of animal bodies is returned to 
 the air during putrefactive processes. The bodies of 
 a thousand million men and of thousands of millions 
 of animals are renewed every thirty years, and by 
 their decay their nitrogen is converted into ammonia 
 which escapes into the air, to be dissolved, for it is 
 eminently soluble, by water in the form of rain, and 
 so be carried back to the roots of plants, except in 
 regard to that part of it which is washed more directly 
 into the earth during decay, or is introduced fixed 
 in the form of manure by the cultivator of the soil. 
 
 Although Liebig by no means denied that nitrogen 
 may be supplied, with advantage, in the form of manure, 
 he was led by the evidence then available to conclude 
 that the ammonia in the air, like the carbonic acid 
 gas, is by itself sufficient to supply nitrogen to many 
 crops. This opinion was vehemently contested by his 
 opponents ; and partly because the data available 
 were incorrect, and partly in consequence of mis- 
 conceptions, it became the subject of a warm and pro- 
 longed discussion. Important as this discussion was 
 to agriculturists, its details >vould not be interesting to 
 
HIS LIFE AND WORK. 103 
 
 general readers ; but it may be mentioned that this 
 memorable dispute was only settled after Liebig's 
 death by the important discovery, made not very long 
 since by Professor Hellriegel and Dr. Wilfarth, that 
 the power of the leguminous plants to assimilate free 
 nitrogen* is dependent upon the presence of certain 
 minute organisms which flourish in and around their 
 roots, where they cause the production of tuber-like 
 formations which swarm with the micro-organisms 
 and abound in nitrogen. 
 
 But plants do not consist only of organic sub- 
 stances, and one of the numerous questions which 
 puzzled the early workers in agricultural chemistry 
 was the origin and function of the mineral parts of 
 plants. The later upholders of the humus theory, 
 though the} 7 could not absolutely deny the import- 
 ance of mineral matters to plants, held that they 
 were not exactly essential to their growth, but rather 
 mere stimulants useful perhaps but still stimulants, 
 and not a necessary part of their food. Some 
 of them, indeed, held that the minerals were pro- 
 duced in the plants by their vital forces ; and, astonish- 
 ing as it may seem, in 1800 the Berlin Academy 
 awarded a prize to an apothecary, one Schneder, for 
 an essay in which he claimed that he had proved 
 by actual experiment that plants produce mineral 
 matter by their vital forces, and, although this paper 
 was justly criticised by De Saussure, Schneder's 
 views, as late as 1807, received a distinct measure of 
 support from the English chemist, Dr. Thomson, who 
 wrote that Saussure's remarks are " by no means 
 sufficient to set aside the experiments of Schraeder." 
 Sir Humphry Davy, however, would by no means 
 
 * Established in recent years by Messrs. Lawes and Gilbert. 
 
104 JUSTUS VON LIEBIG : 
 
 accept his results, and indeed detected the source of 
 the error made by those inquirers who " adopted that 
 sublime generalisation of the ancients that matter is 
 the same in essence, and that different substances, con- 
 sidered as elements by chemists, are merely different 
 arrangements of the same indestructible particles." 
 
 But Liebig first seems to have fully understood the 
 importance of the mineral foods of plants, of which he 
 gives us many illustrations, from which the following 
 examples are taken. 
 
 Years ago it was the custom, in some parts of 
 Germany, to permit the poor people to remove the 
 leaves and twigs of trees from the forests, for use as 
 litter for their cattle. It was found, however, that 
 the trees suffered very much, far more, for example, 
 than from the removal of an equal mass of wood 
 alone. Why was this ? The analysis of the ashes of 
 twigs and leaves and of wood gives us the reason. The 
 ash of the former contains much more alkali than 
 that of the latter ; evidently the removal of the twigs 
 and leaves, by withdrawing alkali, robbed the trees 
 of a part of their food starved them, in fact. Had the 
 leaves been allowed to remain, they would presently, 
 by their decay, have restored their alkaline matter to 
 the soil ready for further use.* 
 
 The tobacco plant and the vine give ashes which 
 contain much lime. They do not grow well on soils 
 devoid of lime; but when lime is added to such 
 soils, the enriched soils become more fit for the growth 
 of tobacco and grapes. We cannot avoid the inference 
 that lime forms an essential part of the food of the 
 tobacco plant and of the vine. 
 
 * It must not be supposed that this is the sole cause of the injury 
 done. 
 
HIS LIFE AND WORK. 105 
 
 Again, at Bingen-on-the-Khine, the produce of the 
 vines was at one time greatly increased by the use of 
 nitrogenous manures, but, after a while, these were 
 found to lose their effect, and the condition of the 
 plants fell off so seriously that their possessor had 
 great reason to regret the experiment. By the 
 manure employed, as Liebig explained, the vines 
 were hastened in their growth, so that in a few 
 years they exhausted the soil of certain minerals, and 
 these being absent from the manures employed, the 
 plants were afterwards starved. Other vines on the 
 Rhine, when treated with manures rich in potash and 
 poor in nitrogen have lived for as much as a hundred 
 years. 
 
 By such facts as these, Liebig made it clear that the 
 mineral matter of the soil is as essential to the well- 
 being of plants as carbonic acid gas, water and 
 ammonia. 
 
 " Plants," he says, " live upon carbonic acid gas, 
 ammonia, water, phosphoric acid, sulphuric acid, 
 silicic acid, lime, magnesia, potash, and iron ; many 
 of them also require common salt. . . ." 
 
 " Manure, the excrements of the lower animals and 
 man, does not act on plant life through the direct 
 assimilation of its organic elements, but indirectly 
 through the products of its decomposition and putre- 
 faction. That is, by the transforming of its carbon 
 into carbonic acid gas, and of its nitrogen into am- 
 monia or nitric acid, Organic manure, which consists 
 of portions or debris of plants and animals, may be 
 replaced by the inorganic compounds into which it 
 breaks up in the ground." 
 
 From what has been said it follows, as Liebig 
 pointed out, that we must replenish the soil by adding 
 
106 JUSTUS VON LIEBIG : 
 
 whatever minerals have been withdrawn from it by 
 our crops if we desire to avoid exhausting it. 
 
 Since the supplies of carbon dioxide, of water, 
 and of ammonia are, for the most part, beyond 
 our control, and since, moreover, these are ample, it 
 appeared to him evident that the chief work of the 
 agriculturist, after keeping his soil in proper con- 
 dition by tilling it, is to prevent loss of mineral 
 matter, or when that is inevitable, to restore the 
 fertility of the soil by adding what is required. 
 
 There has been, as we have seen, much discussion 
 as to whether Liebig was right in supposing that the 
 air does really yield to the plants a sufficient supply 
 of nitrogen ; but this is, after all, a detail, and we 
 may say now, fifty years after the first publication of 
 Liebig's pioneering work, that nothing has been added 
 to, and nothing has been taken from, the fundamental 
 principles to which he then drew attention, and on 
 which he based his theory of a rational husbandry. 
 To-day Liebig's teachings are, in their main features, 
 universally received. But, in spite of his reputation, 
 they were not generally accepted by agriculturists 
 when he first announced them. Nor is this surprising. 
 The education of most agriculturists was then at 
 a low point. Science, or what passed for science, 
 had done very little for agriculture in the past. The 
 difficulties in the way of applying Liebig's ideas were 
 great, and at first, owing to imperfect data, serious 
 mistakes were made which led to many failures. 
 Hence it was only to be expected that practical men 
 would, for a time, consider them to be mere new- 
 fangled notions ; that they should regard them with 
 considerable suspicion, and cling to the ideas and 
 practices, which were many of them wise, handed 
 
HIS LIFE AND WORK. 107 
 
 clown by their forefathers. The publication of " The 
 Chemistry of Agriculture and Physiology " only he- 
 ralded, at first, a prolonged period of renewed experi- 
 menting and warm discussion. Liebig did not, however, 
 allow this to discourage him, but, for many years after- 
 wards, he devoted a large part of his life to studying 
 the practical application of his theory, to gaining the 
 serious attention of practical agriculturists, and, above 
 all, to bringing about such a just appreciation of his 
 views as should ensure their ultimately being intelli- 
 gently applied in practice. 
 
 In the first place, countless analyses of the ashes 
 of plants were made at Giessen by Liebig and his 
 assistants. These showed the presence of different 
 minerals in every species, that each species requires 
 from the ground the same class of salts, and hence 
 that it must sooner or later exhaust the supply of 
 these salts in a given plot, and render it unfit for the 
 growth of the species in question unless fresh supplies 
 are provided. 
 
 Liebig attempted to give the necessary supplies in 
 the form of " Mineral Manures," and soon set to work 
 to study practically the effect of mineral manures on 
 a large scale. In the year 1845, previous experiments 
 in a garden having proved unsatisfactory, he pur- 
 chased from the town of Giessen about ten acres of 
 barren land a sand pit, as he says, which surpassed 
 all the land in the neighbourhood in its barrenness 
 for ordinary cultivated crops ; in the year this land 
 hardly grew so much fodder as would have sufficed 
 for a single sheep. It consisted partly of sand, partly 
 of coarse quartz and pebbles, with strata of sand 
 and some loam. 
 
 Some of the soil was first tested by sowing it with 
 
108 JUSTUS VON LIEBIG: 
 
 seeds in pots after enrichment with some single 
 mineral manure, with the result that not one of the 
 plants raised got beyond flowering ; this showed that 
 the soil was bad enough for his purpose of testing the 
 value of minerals as manure. 
 
 A number of mineral manures were then prepared 
 for him according to prescriptions based on his analyses, 
 and these were spread over the land ; next he sowed 
 on different subdivisions of it wheat, rye, barley, clover, 
 potatoes, turnips, maize. In some cases he added 
 sawdust to the manure, and in one case he used stable 
 manure ; otherwise, no ammoniacal manure and no 
 mineral matter was employed, except that to one plot 
 he applied some forest soil and to another a mixture 
 of forest soil and mineral manure. Even in the first 
 year he had a harvest ; the best results were given by 
 those plots in which mineral manures were mixed 
 with forest soil or stableyard manure. This, as he 
 says, enabled him to correct his earlier ideas of the 
 functions of humus, which by its decay renders an 
 extra supply of carbonic acid gas to the plants that 
 is especially valuable at the early stages. Gradually, 
 without any further supply of manure except mineral 
 manure, the land so improved in productiveness that 
 in the fourth year his crops excited the wonder of all 
 who had known the original state of it. 
 
 In 1849 this little farm was purchased by his 
 gardener, who was then able to farm it with profit, 
 raising some cattle on it yearly and getting such 
 satisfactory crops of corn that in 1853 a neighbouring 
 farmer wrote : " With us the wheat crops are very 
 poor, but on the height (Liebig's plot) they have 
 harvested from three fuder of rye twelve simmer, 
 while I, from three fuder of the best rye, have only 
 
HIS LIFE AND WORK. 109 
 
 got five simmer. If you were to see it, you would be 
 astonished ; it is truly wonderful." 
 
 Of course, in order to maintain the fertility of this 
 plot it was necessary that the capital of minerals 
 introduced during the first four years should be care- 
 fully husbanded, and year by year returned to the 
 soil in the form of the manure produced on the farm, 
 and that little or none of it should be removed from 
 the farm by selling the crops. Under these conditions 
 those ten acres of land became, as it were, condensers 
 of carbon and nitrogen from the air. It was this 
 experiment, as well as the estimates of ammonia in 
 rain water, which led Liebig to form the opinion that 
 it was possible, by giving the soil proper physical 
 quality and composition, to bring about a state of 
 things in which sufficient ammonia to maintain its 
 fertility can be collected or condensed from the air. 
 
 The experiment was, of course, a costly one. It 
 rarely happens that an experiment is directly re- 
 munerative, but it served its purpose of testing the 
 agricultural doctrines of Liebig, and its success 
 enabled him, amid frequent difficulties, to work 
 patiently in other directions. 
 
 One great difficulty was, as Yogel says in his 
 memorial address, " the want of results " from the 
 mineral manures when they were applied in the 
 ordinary course of farming. The attempt to com- 
 pensate the soil for the withdrawal of crops by the 
 addition of suitable mineral matter seemed to fail, 
 because the form in which the salts were at first 
 supplied was the wrong one. It was supposed that 
 if soluble salts or solutions of them were applied to 
 the land, they would be ineffective because they 
 would soon be washed out of reach of the roots of the 
 
110 JUSTUS VON LIEBIG: 
 
 plants by the rain, and therefore no pains was spared 
 in attempting to render the mineral manures insoluble, 
 or, rather, so far insoluble, that the moisture in the 
 soil could only gradually absorb them for the use of 
 the plants. For this purpose suitable mixtures were 
 first melted in huge pots, and then ground to a powder 
 by means of mills, before they were put upon the land. 
 
 This plan was, however, a brilliant mistake which 
 a practical farmer might have avoided. For when 
 these mixtures were applied to the soil, they were 
 found to be almost without effect, or, at best, their 
 usefulness only showed itself after long years of 
 delay. The single ingredients when applied alone 
 acted, but the same substances mixed would not act. 
 There was evidently something wrong. 
 
 Liebig, convinced that the error must be in his 
 practice, that his hypothesis was a sound one, for- 
 tunately did not allow this failure to put an end to 
 his experiments, with the result that the failure led 
 him in the end to recognise the agricultural import- 
 ance of a certain quality of the earth which enables 
 it to protect, as it were, the plants from being robbed 
 of their mineral food by the rain as it percolates 
 through the soil. While Liebig was so busy trying to 
 make his manures insoluble, nature was ready to do 
 it for him in the most suitable manner. 
 
 It appears that earth is capable of withdrawing, to 
 some extent, soluble salts from their solutions, but the 
 importance of this fact which had been known for 
 some time in relation to the nourishment of plants 
 was overlooked until about 1850, when Liebig's atten- 
 tion was directed to it through some novel experiments 
 made in England by John Thomas Way on the 
 absorptive power of soils. Liebig at once recognised 
 
HIS LIFE AND WORK. Ill 
 
 their importance. Those who have ever attended a 
 course of lectures on chemistry will remember that 
 many coloured solutions, when filtered through a 
 layer of finely divided animal charcoal, lose their 
 colour, in consequence of the power possessed by the 
 finely divided carbon of withdrawing the coloured sub- 
 stances from the solutions. It is possible, at any rate, 
 in some cases, to recover the substances thus absorbed 
 by charcoal. Now, arable soil is in this respect very 
 much like charcoal. The dark drainings from a 
 manure heap, when filtered through a layer of good 
 soil, flow away without colour and without smell ; the 
 organic matter, the ammonia, and the salts which it 
 held in solution are all more or less completely with- 
 drawn from it by the soil The various substances do 
 not, however, appear to be destroyed, they are held 
 by the soil ready for the use of plants. The discoveries 
 made by Way led Liebig to himself perform a series 
 of experiments, in which he investigated this property 
 of "ground absorption," as it has been called, by which 
 the saline matters in the soil are at once protected 
 from the action of the rain and held ready for 
 absorption by the roots of plants. 
 
 Liebig's results, besides confirming the statements 
 of the English observer, enabled him to declare at 
 last what is the proper mode of applying the mineral 
 manures; they also helped him to understand the 
 root action of plants, and to explain clearly why it is 
 that the whole available quantity of a manure is not 
 taken up by the roots of the plants in a single season. 
 Why it is, for example, that if ten grains of a phos- 
 phate are taken up by a crop, many times that quantity 
 must be present in the soil It is because the roots of 
 a single plant do not touch all the soil about it, and 
 
112 JUSTUS VON LIEBIG: 
 
 because they only exhaust those parts with which 
 they actually come into contact. Liebig's excitement 
 when he recognised the effects of ground absorption was 
 very great. In his writings at this time he frankly 
 confessed the mistake into which he had fallen. This 
 new discovery, he declared, was like a new life to him, 
 explaining as it did all the processes of agriculture, 
 and why, whilst each single salt had succeeded as a 
 manure, combined they had failed. 
 
 Regret has sometimes been expressed that Liebig 
 devoted so much valuable time and effort to the 
 practical application of his mineral manures. It has 
 been questioned whether he would not have done 
 better if he had left to the practical farmers the duty 
 of applying the principles he himself had traced out 
 for their guidance. Had he done so, there can be no 
 doubt but that he would have saved himself much 
 annoyance and anxiety ; he might even have hastened 
 the success of his ideas, since it is possible that a 
 practical farmer might have avoided the mistake 
 which led Liebig to spend so much time in attempt- 
 ing to render his soluble salts insoluble. 
 
 But, on the other hand, if Liebig had stood aloof 
 from the testing of his opinions, science and agricul- 
 ture would alike have been the poorer science, 
 because in that case the full recognition of the import- 
 ance of ground absorption would almost certainly 
 have been delayed; agriculture, because in his 
 practical work Liebig gave the agricultural world a 
 splendid example of what experimental agriculture 
 ought to be, at a time when such an example was 
 sorely needed. Indeed, when we remember that 
 throughout his whole career Liebig was as greatly 
 distinguished for his enthusiastic services to educa- 
 
HIS LIFE AND WORK. 113 
 
 tion as for his work in pure and applied science, 
 and when we consider his keen sense of the urgent 
 need that farmers should play at follow my leader no 
 longer, but learn how to make their own experiments 
 on their own farms, I do not think we shall venture 
 too far if we assume that it was largely with the 
 object of showing them how this should be done that 
 he entered upon his labours in experimental agricul- 
 ture. However this may be, no one surely will 
 deny that the world is greatly the richer for the 
 outcome of this part of his many-sided activity. 
 
 Liebig showed on many occasions his desire 
 that farmers should think more for themselves. 
 In the preface to his " Natural Laws of Husban- 
 dry," published in 1863, he called attention to the 
 fact that no progress could be made so long as 
 agriculturists continued to allow themselves to be 
 guided merely by the facts observed in their own 
 neighbourhood, or at most by the system of some 
 recognised authority, and he elsewhere deplored the 
 frequent existence of such a state of mind as that 
 which had led a landowner to write to the eminent 
 agriculturist, Thaer : " If I receive a letter from you 
 this evening to fire my buildings, before night they 
 shall be in flames." 
 
 In the same book, writing on the subject of farm- 
 yard manure, Liebig returned to this subject, and 
 after showing that the rotation of crops which suits 
 one field may not suit another equally well, he said : " If 
 farmers would only make up their minds to acquire 
 by experiments on a small scale an accurate know- 
 ledge of the productive power of their land for certain 
 kinds or classes of plants, a few more experiments 
 would readily enable them to discover what nutritive 
 
114 JUSTUS VON LIEBIG: 
 
 substances the land contains in minimum proportion, 
 and what manuring agents ought to be applied to 
 ensure the production of a maximum crop." 
 
 " In matters of this kind," said he, " the farmer 
 must pursue his own course ... he must not 
 put the least faith in the assertion of any foolish 
 chemist who wants to prove to him analytically that 
 his field contains an inexhaustible store of this or 
 that nutritive substance." 
 
 One cannot read this book without perceiving 
 that Liebig's great desire was not merely to inculcate 
 correct rules of husbandry, nor to attempt to induce 
 the agriculturists to lean more on the chemists, but 
 to lead them to take an intelligent interest in their 
 own work, and to study the bearing of science on its 
 ever-varying details. 
 
 He found most agriculturists badly educated, 
 and either governed by tradition or the slaves of 
 particular leaders. They had lost the power of dis- 
 tinguishing opinions from facts. They had no idea 
 how to make an experiment. He hoped to help to 
 put an end to this by inducing the rising generation 
 to take an active part in the attempt to apply scientific 
 methods to their art. 
 
 Even those who most vehemently dissented from 
 some of Liebig's conclusions on special points have 
 always admitted in the warmest terms the magnitude 
 of the service he rendered to agriculture, by the 
 masterly review. of the then existing knowledge, and 
 by the sagacious generalisations which he brought 
 forward in his earlier works on agricultural chemistry. 
 And when he published his mature views in 1863, 
 in " The Natural Laws of Husbandry," his funda- 
 mental doctrines were already, in regard to most of 
 
HIS LIFE AND WORK. 115 
 
 their main features, generally accepted. It was then 
 no longer necessary that he should demonstrate the 
 errors of the humus theory, and, on the other hand, 
 the true value of humus was better understood. 
 In 1863 there was no need that he should insist 
 upon the necessit} 7 for mineral matter in the food 
 of plants, nor that he should direct attention to 
 that interdependence of plants and animals, which 
 alone has enabled countless generations of each class 
 to play their parts in the past history of our globe, and 
 which alone makes the continued existence of either 
 possible. By this time, also, the power of arable soil 
 to withdraw from solutions the food materials most 
 essential to plants was fully recognised, and correct 
 views of the action of roots accordingly had become 
 possible. In the new book, therefore, Liebig was able, 
 after many years of experimenting and reflecting, to 
 devote himself more especially to the task of explain- 
 ing how the new knowledge and corrected ideas 
 should be applied to the practical objects of agri- 
 culture. He could now attempt, in fact, to bridge 
 the gulf which had so long separated science from 
 practice. 
 
 First among the objects of the agriculturist must 
 be placed the maintaining of the permanent pro- 
 ductiveness of the soil To this subject Liebig 
 especially directed attention at this stage. 
 
 The following remarks will serve to introduce and 
 make clear the last part of Liebig's teachings in agri- 
 cultural chemistry to which it is possible to refer 
 viz. to the tendency of the system of farming adopted 
 by Western peoples to exhaust the soil, and so ulti- 
 mately to bring about their own extinction or dispersal 
 more or less completely. 
 
116 JUSTUS VON LIEBIG: 
 
 It must be understood that a fruitful soil consists 
 (1) of the arable surface soil, and (2) of the subsoil. 
 The former contains especially that part of the nutri- 
 ment of plants which is held there in consequence of 
 the absorbing power of humus * that part, in fact, 
 which is immediately available for nourishing the 
 plants. The fertility of a soil depends on its abund- 
 ance and on its suitability for the crops that are 
 to be grown. 
 
 The soil also affords another source of food, but in 
 this case the food is not immediately available. It is 
 stored up in the form of compounds which are not 
 soluble in water, and is only gradually brought into 
 the available condition by the process which we call 
 " weathering." This process takes place slowly. It is 
 accelerated by the mechanical operations of agricul- 
 ture, in the course of which the insoluble substances 
 are gradually disintegrated, so that the soluble parts 
 slowly pass into solution under the combined action 
 of air, water, and carbonic acid gas. As this saline 
 food is liberated, if the conditions are favourable, it 
 passes into what Liebig called a state of " Physical t 
 combination" with arable soil. It then becomes an 
 immediate source of supply for the use of plants. 
 
 Each soil has its own degree of power for absorb- 
 ing food, and this constitutes one of the differences 
 between soils from the agricultural point of view. 
 When manure is applied to the surface of many soils, 
 most of the food it conveys is absorbed by the first few 
 inches of the soil, and very little gets to the subsoil. 
 
 * Other constituents also contribute to ground absorption, 
 notably oxide of iron and certain silicates. 
 
 f Physical, because he believed that the salts retain each their 
 individual properties. 
 
HIS LIFE AND WORK. 117 
 
 Consequently, it is difficult to replenish an exhausted 
 subsoil by manure. It is advantageous to distribute 
 the manure by ploughing or digging, and the other 
 operations of the husbandman, because, as each 
 particle of the soil is limited in its absorbing power, 
 it is only by mixing the various parts that the whole 
 mass can be brought to the saturated state in which 
 it best serves to nourish plants. Evidently, also, the 
 working of the soil will tend to an equal distribu- 
 tion of the food which is gradually set free from the 
 insoluble components of the soil by weathering. 
 
 Food in the so-called state of " physical combina- 
 tion " is at once available for the use of plants, 
 because it is not held sufficiently strongly by the soil 
 to resist the absorbing power of their roots. But 
 it is retained firmly enough to prevent its rapid re- 
 moval by the rain water which percolates through 
 the soil. 
 
 The above facts enable us to understand why 
 rough uncultivated land will often sustain perennial 
 plants when it fails to afford satisfactory crops of the 
 summer plants grown by the farmer. The perennials 
 absorb their food at a slower rate than the others, but 
 they continue to absorb it for a longer period, hence, 
 they can take advantage to a certain extent of the 
 slow production of food by weathering ; they do not 
 altogether depend upon the amount of food in the 
 more readily available condition. But summer crops 
 must absorb quickly. They require far more food 
 in their short lives than the perennials need in an 
 equal time ; therefore, these only flourish on land which, 
 either in consequence of prolonged cultivation, or as 
 the result of natural processes, offers them an ample 
 supply of immediately available food such as is only 
 
118 JUSTUS VON LIEBIG: 
 
 to be found in land that is rich in " physically com- 
 bined " nutriment. 
 
 This, again, explains to us the great usefulness 
 of weeds in waste places. Weeds slowly accumulate 
 food from barren soils ; and if they are not removed, 
 by their decay after death they return this food to the 
 soil in such a form that it enters into the so-called 
 " physically combined state." In this way successive 
 generations of weeds gradually enrich the soil, and 
 prepare it for bearing more valuable crops. 
 
 The fact that the constituents of soils exist in the 
 two above-mentioned forms makes it very difficult to 
 ascertain the value of land by means of chemical 
 analysis. 
 
 By means of analysis the chemist can find out for 
 the farmer how much food of each kind his soil con- 
 tains how much potash, how much phosphoric acid, 
 and so on ; but when it comes to ascertaining what 
 proportion of the various components is in the 
 chemically combined state, and how much in the 
 " physically combined," or available state, analysis, in 
 Liebig's time, broke down, and I fear it must be added 
 that it breaks down still. In the early days, before the 
 distinction between the available and the unavailable 
 food in the soil had been made clear to us, this 
 circumstance caused great confusion, and opposed 
 a serious obstacle to progress, because the productive- 
 ness of soils was often found to be apparently quite 
 unconnected with their composition, which naturally 
 weakened the confidence of the farmers in the use- 
 fulness of science to their art. 
 
 We must now pass to the question of the ex- 
 haustion of the soil by the European methods of 
 cultivation, to which Liebig gave much consideration. 
 
HIS LIFE AND WORK. 119 
 
 The principal problem for agriculture is how to re- 
 place those substances which are taken from the soil 
 by crops, and which are not found in the atmosphere. 
 If the manure applied does not replace what is 
 taken by the crops, the fertility of a field, of a farm 
 or of a country will decrease; if, on the contrary, 
 more is given to the field than is taken away, its 
 fertility will increase. In a primitive state, a nation 
 of farmers who till their ground well and collect and 
 return to it every trace of inanurial matter produced 
 by those who consume its products, need not fear they 
 will exhaust their ground. The Japanese afford us an 
 example which is to the point. In Japan a teeming 
 population has been supported for centuries by means 
 of a simple agriculture, supplemented by fishing. 
 This consists in returning to the soil year by year 
 all that is taken from it for food, except only those 
 parts which pass into the air and so find their way 
 back to the earth by natural processes. 
 
 The only rational agriculture, according to Liebig, 
 would be a system in which, at least, everything that 
 is taken from the land should go back again. If we 
 can enrich our soils from without, and so add more 
 than we take, so much the better, provided that we 
 do really enrich them by adding all the necessary 
 elements of plant food in their proper proportions, and 
 do not merely stimulate the soil by adding one or two 
 elements only, for this cannot fail to ensure a premature 
 exhaustion of those components of the soil which we 
 do not add. 
 
 This ideal of Liebig's is perhaps reached in Japan 
 and by the Chinese, but not in Europe. In all 
 Western civilised countries a very great part, and 
 sometimes the greater part, of the food produced on 
 
120 JUSTUS VON LIEBIG: 
 
 the land is carried away to be consumed in the towns 
 or in distant countries. Thus the greater part of the 
 mineral matter removed by each crop is never re- 
 turned to the land. The carbon, the hydrogen, and 
 some of the nitrogen are indeed ultimately returned 
 through the atmosphere, but, by our systems of sewer- 
 age, the mineral matter and a part of the nitrogen 
 are often cast away and lost to us. Even the carbon 
 does not go back to the land to pass through the 
 humus stage, and so does not again play its whole 
 part in the processes which produce fresh supplies 
 of food. In China and Japan this terrible waste is 
 avoided, but not in the West. 
 
 An ingenious system for staving off the evil 
 day has made it possible for European farmers to con- 
 tinue to raise crops for a long while. By adopting a 
 certain rotation of crops, by feeding animals on the 
 land so as to return a good deal in the form of manure, 
 and by importing food-stuffs for their animals from 
 foreign countries, farmers have been able to raise 
 crops of corn, and also meat, and to sell them off their 
 farms without perceiving very obvious signs that the 
 land is being permanently exhausted. But what is it 
 that really happens ? 
 
 If we grow a crop of corn and remove it from the 
 farm, the surface soil will lose a certain portion of 
 the constituents that go to the formation of corn. 
 These must be returned in the form of manure; 
 and if this be done, the subsequent crops may reach 
 the level of those first obtained. To provide this 
 manure, it is the practice to grow such crops as 
 turnips, clover, and grass, and to feed cattle or sheep on 
 these on the farm. A part of what these cattle con- 
 sume in their food remains in their bodies, and this 
 
HIS LIFE AND WORK. 121 
 
 part, sooner or later, is removed from the land, and, 
 except perhaps so far as their bones are concerned, 
 like the <3orn crop, is mostly lost to it. Another and 
 very large portion of the nutritive substances drawn 
 from the land by these creatures is returned to it in 
 the form of manure, and this enables the arable sur- 
 face soil to again support corn crops. 
 
 But it is evident that the food constituents thus 
 supplied in the form of manure to the surface soil 
 were not created out of nothing by the fodder plants, 
 nor in the bodies of the animals. There is not the 
 slightest reason for supposing that this is possible. 
 The truth doubtless is, as Liebig insisted, that the 
 deep penetrating roots of the fodder plants enable 
 them to extract nourishment from the subsoil, which 
 is then partly carried away and consumed in the form 
 of milk, cheese, meat, and partly restored to the 
 surface soil in the manure provided by the cattle, to 
 be again presently withdrawn in the form of corn. 
 Only a small part of the nutritive substances returned 
 in this manner can ever reach the subsoil again. 
 Most of them will be absorbed by the upper layers, 
 whence they will soon be carried off in the subsequent 
 corn crops. 
 
 Under such a system as this, said Liebig, the corn 
 crops may be kept up, they may and even do increase, 
 and this increase may go on for a long while ; but, 
 unless we assume that the stock of nutritive matter 
 in the subsoil is infinite, there must be a limit. 
 Sooner or later the subsoil of each field, which is thus 
 drained of its phosphoric acid, potash, lime, etc., must 
 begin to lose its productive power for the fodder crops, 
 and then the nutritive substances taken away from 
 the arable soil in the form of corn can no longer be 
 
122 .JUSTUS VON LIEBIG: 
 
 returned to it from the stores which at first existed in 
 the subsoil. There are various devices for delaying 
 this dreadful consummation, but sooner or later, 
 according to the quality of the subsoil, exhaustion 
 must ensue, and then the corn crops will decline, and 
 continue to decline, unless all the mineral matter, etc., 
 which is taken from the soil be once more returned 
 to it. 
 
 Of course, the progress of the decline is slow, for the 
 store to be drawn upon at first is considerable, and 
 the results are only felt after many generations. They 
 may be deferred by gathering fallen leaves from the 
 woods, by breaking up new ground, by drawing on 
 outside sources of manure, such as guano, etc., but all 
 these methods are only palliatives. In face of the 
 fact that corn can only be grown if we replace at 
 short intervals what each crop has removed from the 
 surface soil, it is a crime against human society, urged 
 Liebig, to assume that the fodder plants and the sub- 
 soil are not subject to the same law, and that the 
 former will constantly find in the soil all the con- 
 ditions of their growth. 
 
 To continue to draw on the store of mineral 
 food in the soil of a farm without replacing it is 
 like drawing out money from the bank for daily 
 expenses and never troubling to earn any more to 
 replace it before it is all gone. Or rather, perhaps, as 
 we are wasting a store which, properly used, would 
 serve for the support of untold future generations, 
 the system which enables the farmer to sell successive 
 crops off his land without returning their equivalent 
 may be compared to a skilfully-contrived robbery, 
 by which the fathers rob their own children. 
 
 At present, owing to the opening up of virgin 
 
HIS LIFE AND WORK. 123 
 
 soils in various parts of the world, this question has 
 seemed to become less urgent, for the moment, than it 
 appeared to Lie big, so far as the general population is 
 concerned. This, however, is but a temporary state 
 of affairs ; at best it only gives us a breathing-time. 
 The question must be faced sooner or later. For the 
 agriculturist the problem how to bring back to the 
 land the nutritive matter taken from it to the towns 
 is not one whit less important now than when Liebig 
 wrote. At present we are gradually wasting a capital 
 which we ought to make increasingly valuable, and 
 which no human power can restore when once it is 
 dissipated. The problem is not insoluble. It has 
 been solved by races we are pleased to regard as 
 almost barbarians. Till we, too, attain a solution 
 suited to our conditions, we remain mere robbers and 
 wastrels. 
 
 The outcome of Liebig's work in agriculture must 
 by no means be measured solely by the new facts and 
 new views of facts contained in his writings. He was 
 not only the founder of a new school of agricultural 
 chemists, but so large a proportion of the last genera- 
 tion of agricultural chemists came directly or indirectly 
 from his school after he laid down the foundations of 
 his doctrine that, without disparaging the valuable 
 work done by others for example, by Boussingault in 
 France and by Lawes and Gilbert in England he 
 may almost be said to be the founder of agricultural 
 chemistry itself as we know it to-day. And it cannot 
 be doubted that we owe the existing machinery 
 for agricultural research and teaching very largely 
 indeed to the widespread interest he awakened in 
 scientific husbandry. 
 
 In his well-known address to the CTiemical Section 
 
124 JUSTUS VON LIEBIG: 
 
 of the British Association, in 1880, Dr. Gilbert called 
 attention to this fact, when he pointed out that it was 
 only after the publication of Liebig's first reports to 
 the Association that the Royal Agricultural Society 
 first appointed a consulting chemist, Dr. Lyon Playfair 
 (now Lord Playfair), Liebig's pupil, being the first 
 holder of the appointment. Abroad, both in Germany 
 and elsewhere, numerous " agricultural experimental 
 stations" have been established. These owe their 
 origin to the teachings and influence of Liebig. The 
 first of them was established at Mochern, near Leipzig, 
 in 1851-52. Twenty-five years afterwards there were 
 seventy-four such stations in Germany, sixteen in 
 Austria, ten in Italy, and altogether on the Continent 
 one hundred and twenty-two. At each of these was 
 a chemist, often with one or more assistants. The 
 officials of these institutions are charged with the 
 duty of examining and reporting on such substances 
 as the manures, food-stuffs, and seeds that come into 
 the market. Agricultural research has also been a 
 characteristic part of their work. Their activity in 
 this direction has covered a wide field. Whilst some 
 have occupied themselves with the study of soils, 
 manures, vegetable physiology, animal physiology, 
 feeding experiments, vine-culture, wine-making, 
 forestry, and milk-production, others have, according 
 to their localities, specially investigated such subjects 
 as fruit-culture, olive-culture, the utilising of bog and 
 peat land, or the producing of silk, spirits, etc. 
 
 Besides this work in Europe, a good deal is now 
 being done in the United States by the workers in 
 numerous agricultural stations. 
 
 In Great Britain much less has been done than 
 abroad. Neither the State, nor the great landowners 
 
HIS LIFE AND WORK. 125 
 
 as a class, have taken the lead in the matter with 
 the result that a few years ago the German Empire 
 already possessed above one hundred high schools, 
 middle schools, and lower schools, with a full pro- 
 vision of experimental stations attached to them, 
 besides more than a thousand others where the 
 principles of agriculture were taught to all classes; 
 whilst there were in England at the same time only 
 two agricultural colleges, one each hi Scotland and 
 Ireland, with a laboratory of agricultural chemistry 
 in London for higher students, and South Kensington 
 courses for those of a lower grade. Since that time 
 there is reason to hope that a happier state of things 
 has been started by the County Councils. But there 
 is still much lee-way to make up, and not only are 
 our farmers terribly below Liebig's ideal of a race of 
 husbandmen acquainted with the principles of their 
 art and capable of intelligently applying them, but 
 their opportunities of getting the scientific training 
 and knowledge, which alone can cure the present evil 
 state of things, are still terribly insufficient. 
 
126 JUSTUS VON LIEBIG: 
 
 CHAPTER VII. 
 
 PHYSIOLOGICAL CHEMISTRY. 
 
 Origin of Animal Heat Classification of the Components of 
 Food Plants elaborate the Nitrogenous Components for 
 Tissues of Animals Importance of Albumin in Food of 
 Animals Use of the Non-nitrogenous Components of Food 
 Liebig's Classification of Food-Stuffs into the "Plastic Foods" 
 and " Respiratory Foods " " Plants accumulate Force "- 
 Origin of Animal Fat Bischoff on Liebig's Contributions to 
 Physiology Relation of Nitrogenous and Non-nitrogenous Food 
 to Work Method of Studying Production of Urea in the 
 Organism Motions of the Juices in the Animal Body Re- 
 searches on Flesh, Creatine, Creatinine, Sarcosine, etc. Mineral 
 Matter in Flesh and Blood Chemistry of the Cooking of Flesh 
 Extract of Meat Vital Force Objects of Liebig in his 
 Physiological Work. 
 
 LIEBIG was very conscious of the vast importance 
 of physiological studies, and at one time he even 
 contemplated occupying himself with medicine ; and 
 though this idea was never carried out, a great part 
 of his life was devoted to efforts to advance medical 
 science through physiology. His first work on animal 
 chemistry constituted the second part of his Report 
 to the Chemical Section of the British Association, in 
 1842. In it he traced the applications of organic 
 chemistry to animal physiology and pathology. 
 
 Just as Lavoisier during the latter half of the 
 previous century had laid the foundation of what 
 may almost be called a new science, by the success 
 with which he applied to chemistry methods that 
 
HIS LIFE AND WORK. 127 
 
 had long before been employed in physics that 
 is, the use of weights and measures* so Liebig 
 aimed at applying the new and altered views that had 
 been introduced into organic chemistry to the elucida- 
 tion of the problems of physiology and pathology. 
 Ideas derived from chemistry had previously often 
 been successfully applied to the problems presented 
 by the medical art. But for some time before Liebig 
 directed his attention to physiology, physiologists 
 had concerned themselves especially perhaps almost 
 exclusively with the study of the forms of organised 
 bodies and of the phenomena of motion within them, 
 whilst the study of the uses and functions of the 
 different organs and of their mutual connection in the 
 animal body had, to a great extent, fallen into the 
 background. Their researches had given most valuable 
 results, but they yielded, as it seemed to Liebig, no 
 conclusions calculated to give a real insight into the 
 vital processes. And it appeared to him that the 
 greatest hope of advance in this direction was to be 
 found in once more making chemistry the handmaid 
 of physiology. 
 
 It was natural that Liebig, the great master of 
 organic chemistry, should apply himself to the task 
 of bringing the new organic chemistry into the service 
 of the sister science. 
 
 A hundred and fifty years ago, chemistry and 
 physics had comparatively little in common. A 
 hundred years afterwards it had become difficult 
 to draw a line between them ; Liebig looked for- 
 ward to the time when chemistry and physiology 
 
 * Lavoisier did not introduce the use of the balance into chemistry, 
 as has often been stated. No one, however, advanced chemistry by 
 the use of this instrument so much as he did. 
 
128 JUSTUS VON LIEBIG: 
 
 would similarly be so fused together, as it were, as 
 to make it difficult to clearly separate the one from 
 the other. " In the hands of the physiologist," he 
 says, " organic chemistry must become an intellectual 
 instrument by means of which he will be enabled 
 to trace the causes of phenomena invisible to the 
 bodily sight." It was the use of this new instrument 
 that he wished to illustrate by his " Organic Chem- 
 istry in its Applications to Physiology and Pathology." 
 The production of this book provoked at least as 
 much interest, and even more opposition, than 
 his writings on agriculture. This was probably due, 
 not only to what he said, but often very largely to 
 how he said it. For it must be confessed that Liebig 
 was severe upon what he regarded as the neglect by 
 physiologists of the quantitative methods which had 
 for some time been in use in chemistry. 
 
 Even before Liebig's great contributions to physio- 
 logical chemistry were published, there was evidence 
 that the reign of the empirical method in medicine was 
 coming to an end. The rapid progress of chemistry, 
 and especially of organic chemistry, could not fail to 
 attract the attention of physiologists, and it conveyed 
 to them the lesson that the true path of advance is 
 through the combined employment of experiment and 
 observation. 
 
 When it is observed that a lean goose weighing 
 four pounds gains in a few weeks three and a half 
 pounds of fat from the consumption of twenty-four 
 pounds of maize, the observation, taken by itself, 
 might well lead us to conclude that the twenty -four 
 pounds of maize contained in them three and a half 
 pounds of fatty matter. At the best it leaves the question 
 completely open whether a goose can produce any fat 
 
HIS LIFE AND WORK. 129 
 
 from the non-fatty substances in its food. But if we 
 appeal to an experiment, if we feed the goose on food 
 of known composition that is to say, if we analyse 
 samples of the food, and find that the amount of 
 fat in the food eaten is considerably less than that 
 which is accumulated by the goose, we shall 
 definitely settle the question. Similarly, by varying 
 the composition of the food administered to our 
 goose, we might hope to decide which of its com- 
 ponents was the source of the fat produced. By 
 similar experiments on other animals it would become 
 possible to decide the question whether or not fat can 
 be produced in the animal body from carbohydrates. 
 Here we have a simple illustration of the experi- 
 mental method as it may be applied to physiology. 
 
 During the decade which preceded the appearance 
 of Liebig's book, there had been, as said above, 
 distinct indications that some physiologists per- 
 ceived a new road opening before them. Johannes 
 Miiller and Tiedemann, for example, had already ex- 
 pressed themselves in support of the experimental 
 method, and in 1841 Bischoff had perceived, with 
 prophetic vision, that the direction which organic 
 chemistry was then taking was of the greatest im- 
 portance to physiology, and in this connection he had 
 already recognised the value of Liebig's work. More- 
 over, various useful steps had been taken. In spite 
 of the almost universal reluctance of investigators 
 before Liebig's time to abandon the idea that in 
 biological processes all manifestations of chemical and 
 physical action are in some mysterious way modified 
 and overruled by a special vital force, Wohler, as 
 we have seen, had weakened their hold on this idea 
 by producing, from mineral sources, urea, a most 
 
130 JUSTUS VON LIEBIO: 
 
 characteristic product of animal life. Prout had 
 observed the presence of hydrochloric acid in the 
 stomach, Gmelin and Tiedemann had investigated 
 the processes of digestion, Wohler had observed 
 that salts of organic acids in passing through the 
 animal body are converted into carbonates that is, 
 into the very substances which they would form if 
 consumed in combustion and many other isolated 
 but valuable contributions to animal chemistry had 
 been made. 
 
 Thus, when Liebig turned his mind to the study of 
 the chemistry of physiology, he found a mass of facts 
 ready to his hand, and, above all, a soil which was not 
 altogether unprepared to receive his ideas. Whilst, 
 in addition, his great authority as an organic chemist 
 made him secure of an audience which extended even 
 far beyond the ranks of those engaged in the study of 
 science and its applications. It was, therefore, under 
 auspicious conditions in every respect that he at- 
 tempted the task of elucidating the action of chemical 
 and physical laws in the life-processes of animals. 
 
 The most generally interesting and most wide- 
 reaching of all Liebig's teachings in physiological 
 chemistry are doubtless those which deal with the 
 relations of plants and animals to each other and to 
 their environment. These relationships have, how- 
 ever, already been partly discussed in the chapter on 
 agricultural chemistry ; and therefore, before returning 
 to them, it will be best to illustrate his physiological 
 work by examples of his treatment of some of the 
 more important questions which go to make up the 
 subject. I select for this purpose not simply those 
 questions on which his conclusions are still accepted 
 as true, nor those on which his opinions have become 
 
HIS LIFE AND WORK. 131 
 
 truisms, but rather those which will, on the whole, 
 convey the clearest idea of the scope and method of 
 his work to readers who are not already acquainted 
 with this branch of science. 
 
 One of the most interesting, and one of the most 
 important, questions which Liebig examined was that 
 of the origin of the heat of the animal body. 
 
 The temperature of every human being in a good 
 state of health may be said to be practically constant. 
 It only varies during the twenty-four hours from 
 36-5 to 37-5 C. (98 to 99 F.) ; and so it is with many 
 other animals, including most mammals and birds.* 
 The temperature varies a little in different parts of 
 the body; it is a little higher internally than ex- 
 ternally, for example. This uniformity of temperature 
 is maintained in spite of the fact that a constant loss 
 of heat occurs by radiation, evaporation, and the ex- 
 pulsion of warm air from the lungs in expiration, etc. 
 From the data given in' Professor HaUiburton's 
 "Chemical Physiology" I have calculated that a 
 human being in a state of rest loses enough 
 heat in twenty-four hours to melt, approximately, 
 sixty-six pounds' weight of ice. What is the source 
 of all this heat, which from one person, in the course 
 of a long life, would suffice to melt a small iceberg ? 
 By what processes is it generated ? 
 
 Long before Liebig's time Lavoisier perceived 
 the analogy between the processes of combustion 
 and respiration; for both of them air is required, 
 and by both of them carbonic acid gas is formed. 
 Lavoisier had suggested in a paper, published jointly 
 with Laplace, that the heat evolved by the animal 
 
 * Even the cold-blooded animals have a temperature slightly above 
 that of their environment. 
 
132 JUSTUS VON LIEBIG: 
 
 organism corresponds to the heat of combustion of 
 the carbon and hydrogen which are taken into the 
 body in the form of food, and they had made 
 some experiments on the subject. Subsequently 
 this heat was measured with the greatest degree 
 of exactness then attainable. The results showed 
 that whilst the explanation of Lavoisier and Laplace 
 would account for the greater part of the heat 
 given out by animals, it would not account for the 
 whole of it; about 10 or 11 per cent, remained, 
 which, it seemed, could not have come from the 
 Combustion of the food. 
 
 The mental attitudes of the older school of physio- 
 logists, and of Liebig and his successors, are well 
 illustrated by their respective methods of treating 
 the problem which was thus introduced, and which 
 urgently demanded an answer. 
 
 The older school accepted the result, and at once 
 proceeded to invent a theory to account for it. They 
 suggested new sources of heat, such as nervous 
 actions, or friction within the animal organism. 
 
 Liebig, on the other hand though then unaware, 
 it is said, of the principle of the conservation of 
 energy perceived the emptiness of these theories, 
 which only raised new difficulties. He, therefore, 
 carefully reconsidered the whole subject; and when 
 he found, after correcting the calculations in certain 
 respects, that a surplus there surely was, proceeded at 
 once to search for a possible source of error in the 
 experiments themselves. He found such a source of 
 error in the fact that the calorimeter * employed was 
 
 * A calorimeter is an instrument for measuring " quantities of 
 heat." A unit of heat may be taken to be that quantity which will 
 raise one gram of water from to 1 C. 
 
HIS LIFE AND WORK. 133 
 
 one in which the animals experimented on might 
 very conceivably have been cooled below their initial 
 temperature, so that the heat taken up and recorded 
 by the instrument might very possibly not have been 
 derived solely from the respiration of the animal ; 
 part of it might have been due to the cooling of its 
 body. In summing up his views on this subject, he 
 declared himself to be finally convinced that the whole 
 of the sensible heat produced in an animal body 
 could be accounted for by the processes of oxidation 
 which occur within the organism. 
 
 To-day the correctness of Liebig's decision is so 
 completely established that it may be regarded as 
 one of the truisms of science. Whilst we now 
 certainly admit that friction, nervous activity, etc., 
 may actually give rise to heat, we recognise that these 
 actions themselves are only intermediate steps, as it 
 were, between the chemical changes concerned and 
 the heat ultimately measured, and that, therefore, heat 
 from such sources could not lead to the production of 
 more heat by the life process of the animal than that 
 which corresponds to the chemical changes which 
 occur within it. 
 
 But Liebig's conclusion was by no means accepted 
 at once by his contemporaries. At first they rose up 
 against him, and said it was not true. The physicians 
 thought they had discovered special sources of heat 
 in the animal body. Even the chemists did not 
 support him. Berzelius wrote, near the end of a 
 well-known letter on the subject : " You will easily 
 see, my dear Liebig, that you are here standing 
 on hollow ground, and that whatever you build 
 must, in spite of your talent, sooner or later fall 
 to pieces." Afterwards, when it had become plain 
 
134 JUSTUS VON LIEBIG: 
 
 that Liebig was right, some of his opponents de- 
 clared that, though his conclusion was true, it was 
 not new, but only what Lavoisier and Laplace 
 . had said before him. This was true enough 
 in a sense, but it was unfair. For, as BischofF has 
 pointed out, Liebig by no means merely developed 
 the idea of his predecessors. His merit is that he 
 found false doctrines generally accepted, and over- 
 threw them almost single-handed in the face of the 
 strongest opposition. 
 
 In some of his most important contributions to 
 physiology, Liebig discussed the classification of foods, 
 and dealt generally with the chemistry of the food of 
 animals. 
 
 The components of plants on which animals feed 
 include, besides a small amount of mineral matter, 
 two great classes of organic compounds first, the 
 non-nitrogenous substances, such as the carbohydrates, 
 sugar, starch, and cellulose, together with fats ; secondly, 
 the nitrogenous compounds, of which the most im- 
 portant are the albuminoids. The gluten of flour, 
 vegetable albumin, and vegetable casein, which occurs 
 especially in the seeds of peas, beans, and similar 
 plants, belong to the nitrogenous group of substances. 
 
 Liebig's attention having been drawn, through the 
 writings of Mulder, to the similarity between the nitro- 
 genous constituents of plants and animals, and to the 
 probability that animals depend upon plants for these 
 substances, and are themselves unable to form them, 
 he and his pupils re-examined these substances. They 
 found that the nitrogenous compounds which form the 
 main part of the nitrogenous food of the graminivorous 
 animals are indeed composed of the same chemical 
 elements viz. carbon, hydrogen, oxygen, nitrogen, 
 
HIS LIFE AND WORK. 135 
 
 with a little sulphur and phosphorus, united in very 
 nearly the same proportions by weight as those which 
 are present in animal fibrin, and in the other albumin- 
 ous constituents of the blood. Their properties were 
 also examined, and, so far as could be then ascer- 
 tained, the two classes of substances were found to 
 correspond closely in their chief qualities. They were 
 not different substances with the same composition, 
 but actually identical, or nearly identical, com- 
 pounds. 
 
 Now, the graminivorous animals, as we all know, 
 form a link between the plants which they eat and 
 the flesh-eating animals in which class, for con- 
 venience, we may include man which eat them ; 
 consequently this discovery threw floods of light on 
 the mutual relations of the plant and animal kingdoms 
 of nature. 
 
 Since vegetable albumin and fibrin are so little 
 different from animal albumin and animal fibrin, it 
 seems to follow that the vegetables produce in their 
 organism, as it were, the blood of the animals. 
 
 It is well known that nitrogenous food-stuffs are 
 absolutely essential for the well-being of animals ; 
 the flesh-eating animals in consuming the flesh of 
 herb - eaters consume, strictly speaking, only the 
 vegetable principles which have first served for 
 nourishing the latter. Vegetable albumin and fibrin, 
 in short, according to Liebig, play a similar part in 
 nourishing the herb-eating animals as that which 
 the animal fibrin and albumin play in nourishing 
 the flesh-eaters. It follows that the growth and 
 development of all animals are dependent on their 
 receiving certain substances from the plants iden- 
 tical, or nearly identical, in composition and in their 
 
136 JUSTUS VON LIEBIG: 
 
 general properties with the chief constituents of the 
 blood of the animals themselves. 
 
 In this sense, Liebig taught that the animal 
 organism gives to the blood only its form, that it 
 is incapable of creating blood except out of substances 
 which already contain its chief constituents, in the 
 form of compounds closely allied to those which 
 occur in the blood itself. Liebig did not mean that 
 the animal organism cannot produce new compounds 
 from its food, for it. was well known that an extensive 
 variety of compounds do result from the life processes 
 of animals, but that the organism depends for its 
 starting-point on certain substances in the blood 
 which are so much like the albuminoids of plants 
 that at Liebig's time they were practically indistin- 
 guishable, so that as far as the albuminoids are 
 concerned, the development of the animal begins 
 where the life of the plant ends. From this point of 
 view, the plant holds an intermediate position between 
 the mineral and animal worlds. The animal is in- 
 capable of assimilating the compounds stored up in 
 inorganic nature. To render them fit for the 
 support of the animal, they must undergo a course 
 of preparation in the plant, in which process the 
 simple stable molecules of the mineral substances are 
 converted into more complex and less stable molecules 
 of a higher order, from which may afterwards be built 
 up the yet more complex and yet more instable sub- 
 stances which are capable of doing service in forming 
 and maintaining the life of the animal. 
 
 The main pillar of this great generalisation, which 
 we owe to the work of Liebig and Mulder, consists in 
 the fact just quoted viz. in the identity, or, to be 
 more correct, in the close chemical similarity between 
 
HIS LIFE AND WORK. 137 
 
 the nitrogenous compounds found in plants and those 
 which occur in the blood of animals. 
 
 Liebig illustrated the importance of the albumin- 
 ous substances to animal life by calling attention to 
 the changes in which a chick develops from an egg. 
 Both the white of egg and the yolk are largely 
 composed of albuminoids. An egg, after impregna- 
 tion, if maintained at a suitable temperature, with 
 the aid of the oxygen of the air, which finds ready 
 access through the porous shell, gradually develops 
 all the parts of the animal body feathers, claws, 
 membranes, fibrin, blood-vessels, nerves, and so on. 
 In the process, all the albumin disappears. Evidently 
 albumin is the foundation, he said, of the whole series 
 of peculiar tissues which constitute those organs 
 which are the seat of vital actions. The elements of 
 these organs, which now possess form and vitality, 
 were originally elements of albumin. 
 
 The results of examining other alimentary sub- 
 stances always told the same tale ; he found albumin 
 or, at least, those bodies closely allied to it, which I 
 have called albuminoids present in every food, which 
 by itself suffices to support animal life. 
 
 What we call " meat " that is, the muscle of 
 herbivora is very largely composed of solid albu- 
 minoids. 
 
 If we examine milk, the food prepared in the 
 body of the mother for the nourishment of her 
 young, we shall find in it, besides a kind of sugar, 
 fat, and a little albumin, a substance called casein. 
 Now casein is nearly identical in composition with 
 the albuminous constituents of blood fibrin and 
 albumin, and also analogous in its nature to the 
 vegetable casein found in peas and beans. The 
 
138 JUSTUS VON LIEBIG: 
 
 young animal, therefore, receives in this form the 
 albuminous material, which provides it with the 
 fundamental constituents of its blood, from which 
 its flesh, bones, and nerves must be elaborated. 
 
 Again, eggs, which are so rich in albuminous 
 matter, in a form very ^favourable to its complete 
 absorption, will, even by themselves, support life ; and 
 cheese, which contains both the fat and the casein of 
 milk, would doubtless do still better if it were equally 
 digestible ; but a diet of fat, let us say of butter or 
 of starch, or of butter and starch together, soon leads 
 to starvation. This we can well understand aided by 
 the light which Liebig has thrown on the subject. Fat 
 and starch are non-albuminous substances; albuminous 
 material must be provided in our food to secure the 
 production of blood, and therefore blood cannot be 
 formed from a diet consisting of fat and starch alone. 
 That is why these foods will not by themselves 
 support life. 
 
 Albuminous material, then, must be present in the 
 food of animals; they cannot produce it for them- 
 selves, though they may modify it ; they depend upon 
 the plants for a supply of it, and those parts of plants 
 which contain a good proportion of albuminous matter 
 will, if digestible, generally be found more nutritive 
 than those which contain very little. 
 
 In the " Familiar Letters on Chemistry " (p. 350) 
 Liebig summed up his views in 1851 in the following 
 oft-quoted passage : 
 
 " How admirably simple, after we have acquired a 
 knowledge of this relation between plants and animals, 
 appears to us the process of formation of the animal 
 body, the origin of its blood and of its organs ! The 
 vegetable substances which serve for the production 
 
HIS LIFE AND WORK. 139 
 
 of blood contain already the chief constituent of 
 blood ready formed, with all its elements. The nutri- 
 tive power of vegetable food is directly proportional 
 to the amount of these sanguigenous compounds in it ; 
 and, in consuming such food, the herbivorous animal 
 receives the very same substances which, in flesh, 
 support the life of the carnivora. 
 
 " From carbonic acid, water, and ammonia that is, 
 from the constituents of the atmosphere with the 
 addition of sulphur and of certain constituents of the 
 crust of the earth, plants produce the blood of animals ; 
 for the carnivora consume, in the blood and flesh of 
 the herbivora, strictly speaking, only the vegetable 
 substances on which the latter have fed. These nitro- 
 genised and sulphurised vegetable products, the albu- 
 minous or sanguigenous bodies, assume, in the stomach 
 of the herbivora, the same form and properties as the 
 fibrin of flesh and animal albumin do in the stomach 
 of the carnivora. Animal food contains the nutritive 
 constituents of plants stored up in a concentrated 
 form. 
 
 " A comprehensive natural law connects the de- 
 velopment of the organs of an animal, their growth 
 and increase in bulk, with the reception of certain 
 substances, essentially identical with the chief con- 
 stituents of its blood. It is obvious that the 
 animal organism produces its blood only in regard 
 to the form of that fluid, and that nature has denied 
 to it the power of creating blood out of any other 
 substances, save such as are identical in all essential 
 points with albumin, the chief constituent of the 
 blood. 
 
 " The animal body is a higher organism, the de- 
 velopment of which begins with those substances 
 
140 JUSTUS VON LIEBIG: 
 
 with the production of which the life of those vege- 
 tables ends which are commonly used for food. The 
 various kinds of grain and of plants used for fodder 
 die as soon as they have produced seeds. Even in 
 perennial plants a period of their existence terminates 
 with the production of their fruit. In the infinite series 
 of organic products which begins with the inorganic 
 food of plants, and extends to the most complex con- 
 stituents of the nervous system and brain of animals, 
 the highest in the scale, we see no blank, no interrup- 
 tion. The nutritive part of the food of animals, that 
 from which the chief material of their blood is 
 formed, is the last product of the productive energy of 
 vegetables." 
 
 The vegetable matters which the graminivorous 
 animals consume are not, however, entirely, or even 
 chiefly, composed of albuminous substances, and in 
 some of them the proportion of albuminoids is very 
 small. 
 
 The non-nitrogenous components of vegetables 
 may be divided into mineral matter the carbo- 
 hydrates which occur in relatively large quantities 
 almost invariably, and fats, which are usually .present 
 in small or decidedly moderate proportions. It is 
 found that a large part of these organic materials can 
 be absorbed and made useful by animals ; the fibre 
 of vegetables is to a great extent rejected, however, 
 especially the coarser parts, but such components 
 as starch and sugar are readily absorbed, and are 
 very useful. The carbohydrates and the fats contain 
 only carbon, hydrogen, and oxygen ; they contain no 
 nitrogen, no sulphur, no phosphorus. 
 
 The food of the carnivora also contains a certain 
 amount of non-albuminous matter. Flesh contains a 
 
HIS LIFE AND WORK. 141 
 
 variable quantity of fat, and niilk also contains fat 
 (from which comes butter), and with it a cry st alii sable 
 substance called sugar of milk, which belongs to the 
 class of compounds called the carbohydrates. (See 
 p. 85.) 
 
 The relative proportions in which the albuminoids, 
 fat, and sugar occur in milk are not constant, and 
 similarly the proportions which albuminoids bear 
 to the other constituents of flesh and of vegetables 
 vary considerably in different specimens. Thus oats, 
 on an average, contain about 65 per cent, of carbo- 
 hydrates and fibre, and about 12 per cent, of 
 albuminoids, but a given sample of oats would pro- 
 bably be found to have rather more or rather less 
 than this proportion of albuminoids or of carbo- 
 hydrates. 
 
 These facts lead us to Liebig's celebrated classifi- 
 cation of the food of men and animals. " The food 
 of men and animals," he said, " consists of two 
 classes of substances essentially different in their 
 compositioa" 
 
 " The one class, consisting of nitrogenous sub- 
 stances, albumin, etc., serves in the formation of blood 
 and in building up the various organs of the body ; it 
 is called the plastic food. The other, consisting of 
 non-nitrogenous substances, the fatty bodies, and the 
 so-called carbohydrates, resembles ordinary fuel, serv- 
 ing as it does for the generation of heat ; it is de- 
 signated by the term respiratory food. . . . We 
 heat our body, exactly as we heat a stove, with fuel 
 which, containing the same elements as wood and 
 coal, differs essentially from the latter substance in 
 being soluble in the juices of the body." 
 
 In order to establish this classification of foods, he 
 
142 JUSTUS VON LIEBIG: 
 
 pointed out that the food of all animals contains, as 
 we have seen, besides the plastic or sanguigenous 
 constituents from which the blood and the organs are 
 derived, a certain amount of the substances which con- 
 tain only carbon, hydrogen, and ox}^gen. By making 
 various mixtures of articles of food, a diet can be 
 obtained which, so far as its composition is concerned, 
 will suit the needs of any given man or animal, and in 
 making such mixtures a healthy man is guided by an 
 instinct which prescribes for him the best proportions 
 in which to mix the plastic and non-nitrogenous 
 materials for his diet. These proportions may be 
 altered within certain limits, which again vary Avith 
 the individual, his mode of life, state of health, etc., 
 and may even be varied beyond those limits, under 
 compulsion, without at once involving the death 
 of the individual; but such alterations are never 
 made without more or less injury to the bodily 
 and mental powers of the individual concerned. In 
 no case can life be for long maintained if the pro- 
 portion of the nitrogenous constituents be reduced 
 below a certain fixed minimum. And though, on the 
 other hand, it is possible to maintain life on a purely 
 albuminous diet, it is always disadvantageous to do so. 
 Guided by what was then known of the composi- 
 tion of the body and of the food-stuffs, Liebig con- 
 cluded that it was demonstrated that different foods are 
 exceedingly unequal in their ability to influence the pro- 
 ducing and restoring of the powers which enable men 
 and animals to do work or produce manifestations of 
 energy through the nervous system : that wheat sur- 
 passes rye, that rye surpasses potatoes, and that flesh 
 /- surpasses all other foods in respect to the production of 
 L. these effects. Now, the proportion of the plastic to the 
 
 |><xx^*~e 
 
 *+JSUU* 
 
HIS LIFE AND WORK. 143 
 
 non-nitrogenous materials (taking Liebig's data) in 
 the case of potatoes is smaller than in the case 
 of rye, that in the case of rye is smaller than 
 that in the case of wheat, and that in the case of 
 wheat again is very much smaller than that in beef 
 or mutton, from which it seemed obvious that the 
 plastic constituents of food must be the proximate 
 cause of the power to do work and of the ner- 
 vous manifestations in the animal organism. " This 
 conclusion is confirmed by the fact that all the effects 
 which animals produce by their brains or by their 
 muscles are determined by the organised structure 
 of their bodies, and that the unorganised parts, such 
 as fats, are unable to change their relative positions 
 by any inherent power of their own. 
 
 But if the effects producible by a man or beast, 
 whether by the voluntary or involuntary motions of 
 his muscles or by the organs of sense, depend upon 
 the producing and restoring of the organised tissues, 
 if these organised tissues are built up and renewed 
 by albuminoid material derived from the blood, and 
 if finally the albuminoids of the blood find their origin 
 in the albuminoid constituents of plants, it seems to 
 foUow that in these albuminoid constituents of plants 
 we have the source of all those effects which animals 
 produce by means of their organs of sense, thought, 
 or motion. 
 
 Liebig believed that in this relation of the animal 
 kingdom to the vegetable kingdom he had found a 
 new and wonderful revelation. " Plants, which serve 
 as food to animals, are the producers of the plastic 
 constituents of food, and 'hence are accumulators of 
 force." * 
 
 * We should now say " energy." 
 
144 JUSTUS VON LIEBIG: 
 
 By becoming organised parts of the animal body 
 the plastic constituents of plants determine, ac- 
 cording to Liebig, the continuance of all vital 
 phenomena. 
 
 But what, then, is the function of the non-nitro- 
 genous food ? Liebig found in this the source of 
 animal heat. 
 
 The animal body is not, he said, merely a source 
 of mechanical power and of vital actions ; it is also an 
 apparatus for producing heat ; sufficient heat is given 
 off in a year by an adult man to raise about twenty- 
 three thousand pounds of water from freezing-point to 
 boiling-point. This heat is the outcome of the com- 
 bination of the oxygen of the air, taken up in respira- 
 tion, with the constituents of the food in the body, and 
 the amount of heat produced is approximately in 
 proportion to the oxygen consumed. On comparing 
 the consumption of plastic material with the oxygen 
 taken up by the lungs, Liebig found that the com- 
 bustible elements of the former were insufficient to 
 convert all the oxygen absorbed into carbonic acid 
 gas and water that a horse, for example, takes up 
 about five times as much oxygen as is wanted for the 
 complete combustion of the albuminoid substances in 
 the food. For this and other reasons he concluded 
 that the non-nitrogenous substances in the food 
 supply the blood with materials which surpass the 
 albuminoid compounds in their tendency to undergo 
 oxidation, and thus protect, as it were, the albuminoids 
 from the destructive action of the oxygen. This is 
 the function, according to Liebig, of the non-nitrogen- 
 ous part of food, 
 
 " Sugar, starch, and fat," he says, " serve to pro- 
 tect the organised tissues ; and, in consequence of the 
 
HIS LIFE AND WORK. 145 
 
 combination of their elements with oxygen, they keep 
 up the temperature of the body." 
 
 " The sulphurised and nitrogenous constituents of 
 food determine the continuance of the manifestations 
 of force ; the non-nitrogenous serve to produce heat. 
 The former are the builders of organs and organised 
 structures and the producers of force, the latter 
 support the respiratory process; they are the 
 materials for respiration." 
 
 The two quotations given above are taken from 
 the "Familiar Letters" (1851). But the same idea, 
 very clearly expressed, is to be found in Part I. of 
 his second " Report to the British Association," pre- 
 sented in 1842. 
 
 This beautiful conception quickly won its way, 
 and for a long time it was generally accepted. It 
 was, in a word, a great success; but, like all other 
 classifications, it was imperfect. It is difficult to draw 
 the line which will exactly separate the plastic foods 
 from those which only support respiration, and 
 physiology now teaches us that all varieties of food 
 are both assimilable and respiratory. But the value 
 of a conception of this kind does not depend on its 
 actual and final truthfulness : it depends on its use- 
 fulness. A theory, as Liebig always taught, is only 
 a tool. It is only valuable if we can employ it for 
 the purpose of extending the bounds of knowledge. 
 Sooner or later its work, like that of other tools, will 
 be done, and it will be cast aside. Liebig's theory of 
 the plastic and respiratory foods, though now, at 
 last, no longer serviceable, in its day did yeoman's 
 service. Even to-day it is found convenient to retain 
 Liebig's classification of foods into the nitrogenous and 
 non-nitrogenous groups ; for it is still true, and always 
 J 
 
146 JUSTUS VON LIEBIG: 
 
 will be, that the members of the two groups exhibit 
 marked differences in their effect on the animal 
 organism. The nitrogenous constituents of food 
 alone are by themselves capable, though not usually 
 advantageously, of sustaining animal life. They alone 
 contain all the elements that are required both for 
 building up the tissues and for supporting the re- 
 spiratory processes. Trie non-nitrogenous consti- 
 tuents, on the other hand, by themselves are quite 
 incapable of sustaining life. They do not contain the 
 necessary elements for forming the organised tissues ; 
 they only supply those elements which by their oxida- 
 tion are capable of serving as sources of heat and 
 work. In a modified form this is the distinction 
 that was drawn by Liebig, and on which he founded 
 his theory that the use of the non-nitrogenous parts 
 of food is to protect, as it were, the nitrogenous 
 constituents. Before Liebig, although physiologists 
 had experimented with various simple foods, and had 
 learned whether this, that, and the other substances 
 were suitable for the use of animals, their results had 
 led to no general conclusions. They had not recog- 
 nised, as Liebig did, that all foods fall into two large 
 and fundamentally distinct classes. It was this, 
 perhaps, which led Dr. Theodor von Bischoff to 
 declare, in 1874, that though the objections which 
 had been raised on physiological grounds against 
 Liebig's classification were undoubtedly correct, yet 
 the truth of his views as a whole would always 
 remain, and that it was impossible to deny the great 
 merit they possess of pointing out in the briefest way 
 the essential differences which separate the several 
 varieties of food. 
 
 It will be remembered that Liebig's views on the 
 
HIS LIFE AND WOKK. 147 
 
 relations of plants and animals led him to declare that 
 through their nitrogenous constituents the plants 
 act as accumulators of energy. This brilliant idea 
 was also in the main correct. The advance of know- 
 ledge has shown us that plants do indeed store up 
 energy not, however, only in the processes by which 
 their nitrogenous parts are formed, but, speaking 
 broadly, by their life processes as a whole, in which, 
 as we now believe, energy derived from the sun is 
 made use of for separating the carbon and oxygen 
 from the carbonic acid gas of the air. This energy 
 remains, stored up as it were, in the new substances 
 formed and in the liberated oxygen, and again becomes 
 available on the recombination of the carbon and 
 oxygen either in our bodies or under other conditions. 
 
 Another conclusion of Liebig's which led to a 
 most animated discussion, was his doctrine of the 
 origin and functions of fat in the bodies of animals. 
 
 By a careful investigation of the conditions under 
 which fat appears in animals he was led to conclude, 
 positively, that fat is actually elaborated by the 
 organism ; that it is not all taken in as fat in the 
 food, but is partly produced from the carbohydrates 
 which the animal finds ready prepared for him 
 by the plants. The hypotheses by which he 
 sought to explain this phenomenon are no longer 
 acceptable, but the fundamental fact of the pro- 
 duction of fat by animals has been abundantly 
 confirmed. When Liebig first announced his con- 
 viction that fat is produced in the bodies of 
 animals from the carbohydrates, his decision was 
 vigorously criticised by some of the leading French 
 chemists, who were already committed to the oppo- 
 site view. About a year before Liebig's Report was 
 
148 JUSTUS VON LIEBIG : 
 
 published viz. on August 20th, 1841 Dumas and 
 Boussingault, in their celebrated lecture on the 
 " Statique Chimique des Etres Organises," had 
 declared that plants produce fat, and that animals 
 consume it ; and these two especially threw them- 
 selves into the debate on this subject, which soon 
 became lively, and was not quicky concluded. Liebig 
 stated that the food consumed by the herb-eating 
 animals does not contain anything like as much fat 
 as that which they store up in their bodies. His 
 opponents, on the other hand, held that the animals 
 receive all their fat from the plants. Curiously enough, 
 Liebig was shown to be right, largely by the re- 
 searches of those who opposed him. This sometimes 
 happens in scientific disputes, and had already occurred 
 sufficiently often to Liebig to lead him to say on one 
 occasion, " My mill has ever received its best supply 
 of water from my opponents." Their experiments 
 showed, indeed, that the vegetable food of animals 
 contains more fat than had previously been supposed, 
 but that in test cases the amount is quite insufficient 
 to account for the quantities that are deposited in the 
 bodies of pigs and geese when they are given a vege- 
 table diet. The question was finally settled when it 
 was proved that bees produce wax, which Brodie has 
 shown to be a fat, when they are fed exclusively upon 
 sugar. 
 
 The work of the French chemists yielded, how- 
 ever, a further interesting result. It showed that this 
 producing of fat from the carbohydrates was largely 
 dependent on the co-operation of nitrogenous foods. 
 This Liebig admitted, and also that under certain 
 conditions the nitrogenous materials themselves 
 might become the source of supplies of fatty matter, 
 
HIS LIFE AND WORK. 149 
 
 At a later stage some modern physiologists re- 
 opened this question, and, going beyond what Liebig 
 was able to admit concerning the producing of fat 
 from the albuminoids, assumed that all the fat 
 produced in the animal comes from this source. 
 Liebig, without denying that this view might be right, 
 held that all that could be said with certainty was 
 that in the case of the graminivora the albuminoids 
 and carbohydrates work together to produce fat ; 
 and the results of subsequent experiments have, it 
 may fairly be said, vindicated his opinion on this 
 subject. 
 
 So much has been said about the classification of 
 foods into nitrogenous and non-nitrogenous, that it 
 has become important to remind the reader that 
 neither nitrogenous food alone, nor non-nitrogenous 
 food alone, nor both together, will support life. 
 Organic foods that is to say, those which contain 
 carbon, hydrogen, oxygen, nitrogen, sulphur, and 
 phosphorus do not by themselves include all the 
 materials present in the animal body ; and it is said 
 that if a diet composed of purely organic materials be 
 offered day after day to an animal, he will, after a 
 time, refuse to take it that not even the tortures 
 of starvation will induce him to do so. 
 
 If the body of a dead animal, or a portion from 
 almost any part of the body of an animal, be burnt as 
 completely as possible, there will always remain a 
 more or less voluminous residue of ash. This ash or 
 mineral matter is as essential to the life of an animal 
 as the organic matter, and it must, therefore, be pro- 
 vided in his food. At one time the importance of 
 mineral matter both in the food of plants and animals 
 was to a great extent overlooked, and we have seen 
 
150 JUSTUS VON LIEBIG: 
 
 that even comparatively lately it was supposed that 
 organisms could produce mineral matter for them- 
 selves. The importance of mineral matter for the 
 bones of the higher animals was of course recognised, 
 but that was all. 
 
 Liebig, as has been seen in the earlier sections of 
 this book, was pre-eminent among vegetable physio- 
 logists in regard to the importance he ascribed to 
 the mineral food of plants ; naturally, therefore, he 
 did not overlook its vital significance in the animal 
 economy. When we have said, however, that mineral 
 food is of vital importance to animals, that neverthe- 
 less it does not serve as .an important source of energy, 
 and have added a few facts, such as that the bones 
 contain phosphate of calcium, that bile contains much 
 sodium, chiefly in combination with certain peculiar 
 acids, we have told almost all that is known. In fact, 
 this subject, important as it is, is still imperfectly 
 understood, and only merits this brief mention, in 
 order that the preceding pages shall not mislead 
 unscientific readers by creating the idea that it is 
 only the organic constituents of foods which are 
 important. 
 
 In the second part of his second report to the British 
 Association Liebig attempted to trace the changes 
 that occur during the processes which convert the 
 constituents of food into blood, those of blood into 
 tissues, and, finally, these into the secretions and 
 excretions. 
 
 The value and interest of most of this splendid 
 review of physiology, from the standpoint of the 
 chemist, has now necessarily to a great extent passed 
 away, in consequence of the transformation which the 
 subject has undergone in the hands of a host of modern 
 
HIS LIFE AND WORK. 151 
 
 workers. Liebig himself, there can be no doubt, fore- 
 sa\v that to a great extent his conclusions in this 
 department would in time be modified. He admitted 
 frankly that some of his results had surprised him- 
 self no less than they would surprise others ; that they 
 had excited in his own mind the same doubts which 
 others would entertain; that he gave them because 
 he was convinced that the method by which they 
 were obtained was the only one from which an insight 
 into organic processes could be hoped for. He felt 
 that they deserved attention hi so far as they 
 pointed to the road which chemists must follow if 
 they would be really of service to physiology and 
 pathology. 
 
 By showing how to apply chemistry to physiology, 
 Liebig insured, first, the advance of physiology, 
 and, secondly, the overthrow of many of the details 
 of his own contributions to its advancement. This 
 second effect of his work may be illustrated by an 
 example drawn from his treatment of the meta- 
 morphoses of the tissues alluded to above. 
 
 The basis of many of Liebig's early conceptions con- 
 cerning the nature of the tissues was the supposed 
 existence of the substance called protein, which had 
 been obtained by the physiologist Mulder from albu- 
 minoids by the action of caustic potash. When 
 the progress of physiological investigation showed, 
 as it subsequently did, that the protein of Mulder was, 
 so to speak, an artificial substance, a mere product 
 of the laboratory and no true component of the 
 tissues at all, all those hypotheses of Liebig's which 
 were based upon the protein theory fell to the ground. 
 But the very fact that they did not survive, but were 
 overthrown in this way to make room for truer con- 
 
152 JUSTUS VON LIEJBIC: 
 
 eeptions, was itself a testimony of the highest kind to 
 the immense value of his method, since the very 
 progress which led to the overthrow of these particular 
 conclusions was itself, in no small degree, the outcome 
 of the method which Liebig had given to physiology. 
 
 It is worth while to lay emphasis on this matter, 
 for, important as Liebig's contributions to the advance 
 of physiology were, they are to-day to a great 
 extent forgotten. This was the case even twenty 
 years ago, when Theodor von Bischoff said on the 
 subject : " I do not think I am mistaken if I hold the 
 opinion that there are not many among the younger 
 physiologists and medical men who know, or have 
 even a distant notion how great or, rather, I should 
 say, how immense the influence of Liebig's researches, 
 writings, and teachings has been, and is still, not only 
 on physiology and medicine, but on organic science 
 at large. The majority enjoy the advantages gained, 
 and rejoice in the progress, without being aware of 
 their author. They consider as self-evident the facts 
 established by Liebig and the methods and principles 
 of research diffused by his teaching. They believe it 
 could not be otherwise, and care but little for him to 
 whom science, and with science they themselves, are 
 indebted for their present position." 
 
 The man Avho wrote these words was not only 
 himself an eminent biologist, but he had known Liebig 
 and his work for thirty years. He was one who knew 
 the condition in which Liebig found physiology and 
 how he left it. On the whole, it seems that 
 what Liebig especially did for physiology was this : 
 He taught the physiologists of his time how to 
 frame many questions, and how to search for their 
 answers. 
 
HIS LIFE AND WORK. 153 
 
 In the third section of this Report Liebig dealt 
 with the phenomena of motion in the animal body. 
 He recognised a connection between the oxygen 
 absorbed and the work done, and he perceived that 
 an increased transformation of muscular fibre deter- 
 mines the doing of a greater amount of work. He 
 perceived, in short, an intimate relation in the 
 animal body between absorption of oxygen, change 
 of matter, and manifestations of energy. But he was 
 too earty in this field, and, besides, the problem was 
 one for a physicist rather than for a chemist. The 
 thought he gave to the subject was, however, by no 
 means unproductive. In connection with it he at- 
 tacked a question of the greatest practical importance 
 in regard to the feeding of animals for work viz. 
 whether an animal when at work requires its food 
 to be enriched with the nitrogenous elements, 
 or whether the increased demands are rather upon 
 the non-nitrogenous constituents. The answer to 
 this question for some time seemed doubtful, but 
 it now appears to be pretty clearly proved that, 
 contrary to Liebig's opinion, under ordinary circum- 
 stances the enlarged requirements of the animal are 
 chiefly for the non-nitrogenous foods. 
 
 The final solution of the question has only been 
 arrived at by following a line of investigation sug- 
 gested by Liebig's writings. 
 
 Practically speaking, we may say that just as the 
 carbon of the food is ejected from the animal system 
 through the lungs in the form of carbonic acid gas 
 after its work is done, so the nitrogen is mainly 
 though not, indeed, solely ejected by the higher 
 animals in the form of a single substance, urea. It 
 follows from this that if we can in any way collect day 
 
154 JUSTUS VON LIEBIG: 
 
 by day the urea which passes from an animal and 
 weigh it, we shall have an approximate measure of 
 the amount of nitrogenous tissue used up during 
 separate periods. Further, if we divide the life of a 
 given animal into periods of work and periods of 
 rest, and compare the quantities of urea thrown oft 
 during those periods respectively, we shall be able 
 to tell whether increased demands have been made 
 on the nitrogenous matters of the tissues during 
 the working periods ; because in that case we shall 
 find that more urea is produced during or imme- 
 diately after work. 
 
 In order to trace the influence of work on the 
 tissues by periodically measuring the urea produced 
 it is necessary to devise some means of readily 
 separating the urea from the substances which 
 accompany it, and then to measure it. This was 
 accomplished by Liebig by taking advantage of the 
 fact, that if a solution of corrosive sublimate be added 
 to urea in solution, these two combine and separate 
 as a precipitate of definite composition. The quantity 
 of the urea in a solution may be determined either 
 by collecting and weighing the precipitate which is 
 troublesome or, more simply, as Liebig showed, by 
 adding a solution of corrosive sublimate of known 
 strength to the solution of urea, so long as any pre- 
 cipitation occurs, and then calculating from the 
 amount of sublimate employed how much urea was 
 in the solution to be tested. 
 
 This process, though superseded of late years, has 
 been of immense value, not only to the student 
 of pure science in the study of the transformations 
 of the tissues in the body, but also in the hands of 
 physicians, by enabling them to watch the action 
 
HIS LIFE AND WORK. 155 
 
 of the organ (the kidney) by means of which urea 
 is, under normal conditions, absorbed from the 
 blood, and so removed from the system, and to 
 detect and treat certain diseases. Liebig and his 
 pupils also enriched physiology by a chemical 
 examination of blood, by a study of the nature of 
 bile, and by comparative work on the elimination 
 of the waste products of man and the carnivora. 
 Jointly with Wohler he conducted a research on 
 uric acid, which is justly regarded as a model of 
 experimental inquiry. In these contributions to 
 science, however, though Liebig showed his wonder- 
 ful powers as an investigator, he was not touching 
 any of the fundamental questions, and we must not, 
 therefore, dwell upon their details, but pass on to 
 his inquiry into "the motion of the juices in the 
 animal body," * in which he suggested some further 
 extensions of the applications of chemical and physical 
 laws to physiology. 
 
 Nourishment produced from the food is, we know, 
 carried to every part of the body by the blood. It 
 travels through tubes some of them large, some 
 of them very small indeed which extend by their 
 ramifications throughout the whole organism. But 
 these tubes necessarily have walls, though some of 
 them are very thin, and the tissue which they supply, 
 however well it may be provided with blood-vessels, 
 cannot all of it, or even most of it, be in actual 
 contact with a blood-vessel. 
 
 The question, then, is How does the nourishment 
 find its way from the blood, through the walls of the 
 vessels and cells, to the points where it is wanted ? 
 And, again, how do the products of the never-ceasing 
 
 * " Chemistry of Food," by Dr. Gregory, published 1848. 
 
156 JUSTUS VON LIEBIG : 
 
 changes that go on in the tissues find their way 
 through the same obstacles and into the blood 
 which is to carry them away when their work is 
 done ? 
 
 If we take a bag, or closed tube made of parchment, 
 of bladder or of any similar membrane, and put in it 
 alcohol or a solution of common salt, and if we then put 
 the closed end of this under distilled water in a basin, 
 we shall soon find that the water in the outer vessel is 
 no longer pure. It will contain alcohol, or salt, as the 
 case may be ; for these things, and many others, when 
 dissolved, have the power of passing through such 
 membranes. The name osmose has been given to the 
 process by which this transference of solids in solution 
 through membranes takes place; and it has been 
 supposed that nourishment is conveyed to the living 
 tissues by a process of this kind. 
 
 From the results of a number of experiments, some 
 made by himself, others by other investigators, Liebig 
 formed the opinion, that in the organism the causes 
 which determine the motions of the juices are far 
 more powerful than that to which the name osmose 
 has been given. That the passage of the digested 
 food into the blood, the passage of the nutrient fluid 
 outwards from the blood, and its motion towards the 
 parts where its constituents acquire vitality, involve 
 something more than a simple law of mixture. He 
 concluded that the membranes probably exert an 
 important influence on the secretions. The following 
 quotation will perhaps indicate the position he took 
 as well as a more detailed account of his work : 
 
 " Since," he says, " the chemical nature and the 
 mechanical character of membranes and skins exert 
 the greatest influence on the distribution of the fluids 
 
HIS LIFE AND WORK. 157 
 
 in the animal body, the relations of each membrane 
 presenting any peculiarity of structure, or of the 
 different glands and systems of vessels, deserve to be 
 investigated by careful experiment ; and it might 
 very likely be found that in the secretion of the milk, 
 the bile, the urine, the sweat, etc., the membranes and 
 cell-walls play a far more important part than we are 
 inclined to ascribe to them ; that, besides their 
 physical properties, they possess certain chemical 
 properties by which they are enabled to produce 
 decompositions and combinations, true analyses; and 
 if this were ascertained, the influence of chemical 
 agents, of remedies, and of poisons on those properties 
 would be at once explained." 
 
 It is nearly fifty years since these words were 
 written, and the subject is still, owing to its difficulty, 
 not far from where Liebig left it, except that the 
 modern physiologist would remind us that osmose in 
 a living membrane is probably very different from 
 that in a dead membrane. But is not this just 
 what Liebig intended to point out when he suggested 
 that the membranes and cell-walls play a specially 
 important part in the phenomena in question ? 
 Sooner or later Liebig's suggestions will probably 
 bear fruit, and, indeed, an interesting step in the 
 direction indicated by the above remarks has been 
 made by Professor Waymouth Reid within the last 
 few years. Professor Reid, by means of experiments 
 with a frog's skin, has found that in the living 
 membrane there is a distinct absorptive force which 
 makes osmose take place more readily from without 
 inwards than in the reverse direction. 
 
 A year before the publication of this account of 
 his experiments on osmose Liebig produced, in his 
 
158 JUSTUS VON LIEBIG: 
 
 researches on the chemistry of food, an account of his 
 investigation of the constituents of flesh. 
 
 The introductory part of this little book illustrates 
 rather forcibly Liebig's power of giving " hard knocks." 
 In his own work he had often shown the good use 
 that may be made of an exact knowledge of the chemi- 
 cal composition of substances. But he now found it 
 necessary to remind the physiologists that the function 
 of chemistry in regard to physiology is by no means 
 exhausted when a substance is analysed and neatly 
 labelled with a formula more or less correct, especially 
 as the most exact analyses cannot by themselves 
 always lead us to true formulae. 
 
 It seems that the discovery of a close similarity 
 in composition between casein, albumin, and fibrin, 
 from animal and vegetable sources, which appeared 
 to throw much light on the processes of digestion 
 and nutrition, had, in Liebig's opinion, created an 
 undue sense of the importance of the mere chemical 
 composition of organic substances, with the result, as 
 he said, that for some time a great many analyses had 
 been made, but not much real progress. This led 
 him to give a new example, drawn from one of his 
 own most brilliant investigations, of the true manner 
 in which organic chemistry should be applied to the 
 investigation of physiological problems. 
 
 There are two reasons why the mere analyses of 
 the substances which occur in animal bodies cannot 
 by themselves considerably advance physiology. First, 
 that an organic analysis is, as he reminded his 
 readers, a means of acquiring knowledge, but is not 
 that knowledge itself. The results of an analysis, 
 however perfect it may be, do not give the least 
 information as to the arrangement of the elements 
 
HIS LIFE AND WORK. 159 
 
 in the molecules of the substances, nor as to how 
 they will behave themselves under the influence 
 of chemical agencies ; and till we know this 
 we cannot possibly form any definite views of the 
 part a given compound will play in the vital pro- 
 cesses. 
 
 Secondly, it is only when the number of atoms in 
 the molecules of a compound is small that an analysis 
 can be trusted even to lead to a correct formula ; and 
 as the molecules of the substances dealt with in 
 physiological chemistry often are large, small in- 
 evitable errors in their analyses may, and do, lead to 
 very serious misconceptions. 
 
 There is, however, a method, which was well 
 known to a master like Liebig, by which we can 
 both check the result of an analysis and simul- 
 taneously gain some knowledge of the arrangement 
 of the atoms in the molecules of an organic substance. 
 This method is simple enough in principle. It may 
 be described as an application of the maxim " divide 
 to conquer." It consists in this, that when the com- 
 position of a substance has been found as accurately 
 as is possible by analysis, and its most probable 
 formula has been provisionally fixed upon, portions of 
 it are treated with reagents, so that, if possible, its 
 molecules shall be broken up into two or more 
 substances with less complex molecules. These more 
 simple compounds are then in their turn submitted to 
 analysis, and the results so arrived at are compared 
 with those given by the substance from which they 
 were derived. 
 
 For each organic substance there may be several 
 formulae which agree almost equally well with the 
 results of its analysis, but probably only one of 
 
1 60 JUSTUS VON LIEBIG : 
 
 these will also agree with the formula deduced 
 from the results of the analyses of the substances 
 derived from it. 
 
 The importance of this method of control had not, 
 it is true, been entirely overlooked by workers in the 
 field of physiology; but it is subject to sources of 
 error, which had not been properly guarded against. 
 In order that this method shall guide the investigator 
 to true results, the decomposition products must 
 every one of them be fully and thoroughly investi- 
 gated. If one of them be missed, for example, an 
 error results which may mislead the investigator and 
 his followers, with most deplorable results. 
 
 Above all, hypothetical compounds must not be 
 assumed to exist among the products oi the decom- 
 position in order to support preconceived ideas, or to 
 save trouble. 
 
 These are words which might well be written in 
 letters of gold over the door of every laboratory for 
 chemical research, and Liebig, by insisting on the 
 importance of the principle thus laid down, did a 
 distinct service to the sciences which it was ever his 
 object to advance. 
 
 In the second section of the book Liebig dealt 
 with his discovery of the formula of creatine, and 
 with his other important discoveries concerning the 
 components of flesh. His method in this work 
 afforded to the chemist and physiologist an admirable 
 illustration of the principles laid down in the preceding 
 paragraphs; but the subject is of too technical a 
 character to be generally interesting in spite of the 
 important results he obtained. 
 
 In the last section of his "Chemistry of Food" 
 Liebig dealt with the chemistry of cookery, so far as 
 
HIS LIFE AND WORK. 161 
 
 concerns the use of flesh. It is obvious, he pointed out, 
 that if the flesh employed for food is to become again 
 flesh in the animal body, none of the constituents of 
 raw flesh ought to be withdrawn from it in its pre- 
 paration for the table. If any essential constituent be 
 removed from a piece of flesh, that piece of flesh will 
 no longer have the power of resuming in the body the 
 form and quality on which its properties in a living 
 organism depend. But when flesh is boiled in water, 
 more or less of its saline constituents are removed, 
 together with a certain amount of its organic con- 
 stituents. It follows that boiled meat eaten without 
 the liquor in which it was boiled is less well adapted 
 for nourishing an animal, in proportion to the amount 
 of matter that has been extracted from it in pro- 
 portion, that is to say, to the length of time during 
 which the process of boiling is continued and to the 
 quantity of water employed. 
 
 It is possible to extract from finely divided meat, 
 by the action of cold water, a great deal of albumin- 
 ous matter the proportion varies with the age of the 
 animal and other circumstances hence it is not wise 
 to steep fresh meat for long in cold or warm water. 
 On the other hand, washed muscular fibre becomes 
 hardened by boiling it in water, consequently the 
 tenderness of boiled meat depends largely upon the 
 cook's success in depositing the soluble albuminous 
 matter coagulated among the fibres, since this 
 prevents to some extent the contracting or hardening 
 mentioned above. The method to be recommended, 
 therefore, for boiling meat, especially when the water 
 in which it is boiled will not be consumed, is to 
 plunge the cold meat into boiling water, then after a 
 few minutes to cool the water to from 76 degrees to 
 K 
 
162 JUSTUS VON LIEBIG: 
 
 72 degrees,* and to maintain this temperature for 
 a sufficient time. 
 
 It is easy to understand that the effect of the 
 brief immersion in boiling water is to form an 
 outer case of hardened flesh, which protects the 
 mass beneath from the solvent action of the water 
 afterwards. Then the long - continued moderate 
 heating gradually warms the whole to a sufficient 
 temperature to coagulate the albumin in the juice 
 of the meat without unduly hardening it. The 
 colour of underdone meat, as Liebig explained, is 
 due to the mass having been insufficiently heated. 
 The albumin of meat juice congeals at about 60 C.,t 
 but the colouring matter of the blood requires a 
 higher temperature. That is why the meat should 
 be heated to at least 72 degrees. Finally, a rather lower 
 temperature may be employed for white meat, such 
 as the flesh of fowls, as this contains little blood. 
 This last fact explains why such meat is more quickly 
 cooked than beef and mutton. 
 
 The superior flavour of roast meat, as will now 
 be seen, is largely due to the fact that in cooking 
 it nothing has been washed out of it; besides this, 
 the retained albumin reduces hardening of the fibres 
 to the utmost possible extent. But even in roasting, 
 the escape of the juices should be retarded by heating 
 as strongly as possible at first ; the juice then hardens 
 on the outside and produces a protecting surface, 
 which prevents subsequent loss. 
 
 In order to prepare soup we must, as far as 
 possible, reverse all the above proceedings, and con- 
 duct the operations so as to extract as much as 
 
 * 165 to 158 degrees Fahr. 
 f 140 degrees Fahr. 
 
HIS LIFE AND WORK. 163 
 
 possible from the meat. For this purpose, therefore, 
 Liebig advised that the meat should be very finely 
 divided, and then be brought into cold water and 
 heated very gradually to the boiling-point, at which 
 it should be kept for a few minutes. 
 
 Soup made in this way may not form a jelly on 
 cooling, but it will be very rich in the most valuable 
 of the soluble parts of meat. Housewives have always, 
 and probably do still, attach great importance to the 
 tendency of soup to form a jelly. Liebig insisted 
 that the importance of this quality is much over- 
 rated. 
 
 As the high temperature Liebig employed at the 
 end of his process may coagulate albumin, a modifica- 
 tion of it has been suggested in which the finely 
 divided meat is placed in cold water and then gradu- 
 ally heated as before, but not to so high a tempera- 
 ture. Thus we avoid coagulating the albuminous 
 substances dissolved at the lower temperature, which 
 are consequently retained in solution. 
 
 A few words must now be said about Liebig's 
 celebrated extract of meat. 
 
 By boiling down extract of lean flesh, made by 
 Liebig's process, at a low temperature in a water-bath, 
 or by one of the other methods of low-temperature 
 evaporation which are at the command of chemists, we 
 obtain " Liebig's extract of meat." In 1847 Liebig 
 did not lay so much stress on the usefulness of this 
 product as he did afterwards. In fact, at that time 
 he only recommended it as capable of making strong, 
 well-flavoured soup, and as likely to prove a valuable 
 restorative to wounded soldiers. 
 
 In his celebrated "Familiar Letters on Chemistry" 
 Liebig again dealt with this subject, and recommended 
 
164 JUSTUS VON LIEBIG: 
 
 soup in the strongest terms as the medicine of the 
 convalescent. He here pointed out that in South 
 America, in Mexico, and in Australia, where animals are 
 chiefly valuable for their skins and wool, there might 
 be made from meat of only nominal value * immense 
 quantities of the best extract, which might acquire 
 great importance for potato-eating populations, and 
 replace soup in the hospitals. In these letters 
 he also discussed more fully the question of the 
 dietetic value of gelatine, and of soups containing 
 it, quoting the report of a commission of the French 
 Academy, with Magendie at its head, to show that 
 the value of a soup is very little increased by the 
 addition to it of gelatine, since numerous experiments 
 have proved that by itself gelatine has little or no 
 dietetic value at any rate, that it cannot be made to 
 act as a substitute for a true plastic food, in spite 
 of the nitrogen which it contains. 
 
 The extraordinary and rapid rise into popularity 
 of the condensed extract of meat was remarkable, 
 and it was made more wonderful by the fact that 
 for a long time a most marked want of agreement 
 existed as to its real value. Whilst some people 
 regarded it as highly unwholesome, or, indeed, almost 
 a poison, owing to the large proportion of potash salts 
 in it, others some of them, however, from interested 
 motives, it is to be feared went so far as to recom- 
 mend it as a really sufficient- substitute for meat ; 
 and, more wonderful still, a great many others 
 believed them. These last, however, were, perhaps, 
 to be excused. Ignorant of the most elementary 
 
 * Lean beef in Australia about this time was worth from one 
 halfpenny the pound to nothing ; great quantities were thrown 
 away after boiling down for the fat. 
 
HIS LIFE AND WORK. 165 
 
 principles of science as the general population was 
 then, -and still is, and familiar as they were with 
 the striking, and to them quite unintelligible, results 
 that often follow the administering of apparently trivial 
 quantities of active chemicals, such as quinine or 
 arsenic, it is not, after all, really surprising that many 
 people were ready to believe almost anything they 
 were told about this wonderful invention of the great 
 German chemist. 
 
 Liebig himself was not likely to be, and never was, 
 under any mistake about the value of extract of meat. 
 At first it would appear from his chemistry of food 
 that he regarded it as a valuable restorative, and as 
 a substitute for soup. Afterwards he perceived that 
 it might have a wider application, and become the 
 means of bringing to Europe some part of all the food 
 that was wasting at a distance literally for the want 
 of anyone to eat it. But he meant the extract to 
 be eaten with liberal additions of bread, peas, lentils 
 with food, that is, which contains a good supply of those 
 nitrogenised constituents in which the meat extract 
 was, from the process of its manufacture, intentionally 
 and of necessity wanting. It is still difficult to under- 
 stand its exact action ; but the tendency is to regard 
 it as a useful stimulant, ranking, perhaps, with tea 
 and coffee, but, of course, with its own peculiar 
 virtues. Bischoff has pointed out that the salts in 
 extract of meat are probably just those which are 
 most wanted for the producing of flesh, though, of 
 course, salt must be added ; but as nitrogenous matters 
 of vegetable origin are inferior, for the food of car- 
 nivora, to those obtained by means of flesh, he con- 
 cluded that a diet in which the vegetable albuminoids 
 replace those of flesh must be inferior to flesh food, 
 
166 JUSTUS VON LIEBIG: 
 
 even when supplemented by meat extract. Liebig, 
 however, as Bisehoff has said, by no means overlooked 
 this, and never recommended the extract as equiva- 
 lent to flesh, but only as a substitute for it when 
 taken with suitable accompaniments. 
 
 In concluding his contribution to domestic chemis- 
 try so far as the treatment of flesh is concerned 
 Liebig turned to the processes of salting and preserving 
 meat. By collecting the brine from a given piece of 
 flesh which had been pickled, and measuring it, he 
 found that it corresponded to about a third of the 
 juice of the given flesh; and on subjecting this brine 
 to analysis Liebig was interested to discover that it 
 contained the constituents of soup, which had been 
 extracted to a greater extent than can be done in the 
 ordinary way, even by boiling water. Not only the 
 salts, but a large quantity of albumin, together with 
 creatinine, etc., were found in it. These striking obser- 
 vations were of the greatest practical interest. They 
 explained why salt meat affords a less valuable food 
 than the same meat unsalted ; why, in the days 
 when salt meat had to be largely employed for food 
 during the winter, our countrymen, in spite of the 
 other conditions of their lives being in many ways 
 favourable, were scourged by various diseases from 
 which we ourselves are comparatively free ; and why, 
 even in modern times, toilers on the sea, when they 
 are driven to subsist for a long while on salt meat, 
 often suffer for it severely. In the course of his 
 experiments on salting meat Liebig made the further 
 interesting observation ' that the presence in the salt 
 of calcium or magnesium compounds, in some 
 degree protects flesh from its extractive power. 
 It seems as though the salts of these metals, on 
 
HIS LIFE AND WORK. 167 
 
 meeting with the soluble phosphates of the flesh, 
 produce insoluble substances by interacting with 
 them, viz. phosphate of calcium, or of magnesium, 
 which cannot be so freely removed from the flesh 
 as the more soluble alkaline phosphates themselves ; 
 and he suggested that meat pickled with salt 
 containing these metals would, if eaten with a 
 sufficiency of the esculent vegetables, which are rich 
 in potash, offer a very superior diet to ordinary salted 
 meat. 
 
 One of the most interesting problems connected 
 with physiology is the question whether the life 
 processes of living organisms are determined by the 
 action of a so-called " vital force." 
 
 It is not possible, when we think upon the wonderful 
 results that flowed from Liebig's endeavours to apply 
 the methods of organic chemistry to physiology, to 
 avoid asking ourselves the question Did he, then, 
 consider that in the laws of chemistry and physics we 
 have a key which may ultimately unlock the chamber 
 which hides the mystery of the origin of life ? Did 
 he suppose that the laws of chemistry and physics 
 were the sole laws of physiology ? We know that he, 
 like Davy and some others, rejected altogether the 
 idea that there exists a peculiar vital force which is 
 able to override the laws of chemistry as, for example, 
 by the creation of mineral substances from the inor- 
 ganic components of their food and which presides 
 unassisted over the phenomena exhibited by organised 
 beings. But did he go so far as to reject the 
 very idea of the existence of any vital force, as 
 some thinkers have been inclined to do ? It would 
 be easy to find passages in his writings which seem 
 to imply that this was the case. "It is evident," he 
 
168 JUSTUS VON LIEBIG: 
 
 says in Chapter XX. of his Familiar Letters,* " that 
 physiology has two foundations, and that by the fusion 
 of physiological physics, the foundation of which is 
 anatomy, with physiological chemistry, which rests on 
 animal chemistry, a new science must arise, a true 
 physiology, which will stand in the same relation to 
 the physiology of the present day as modern chemistry 
 does to that of the eighteenth century." 
 
 Then, after showing how chemistry had in the past 
 absorbed and made part of itself branches of physics, 
 with the result that the latter had determined the whole 
 character of modern chemistry, he goes on to say that 
 " Exactly in the same way the more accurate know- 
 ledge of vital phenomena will establish the conviction 
 that a number of physiological properties depend on 
 chemical composition ; and physiology, when it shall 
 have taken up animal chemistry as a part of itself, 
 will possess the means of investigating this relation of 
 dependence. It will then be able to find a juster 
 expression for physiological phenomena." 
 
 But elsewhere he makes it plain that by such 
 expressions as these he did not by any means mean 
 to imply that in chemistry and physics we have the 
 whole foundation of physiology. Thus later on he 
 writes : 
 
 " It is certain that a number of effects observed in 
 the living body are determined by chemico-physical 
 causes, but it is going much too far to conclude from 
 this that all the forces which act in the organism are 
 identical with those which govern dead matter .... 
 Those who adopt such an opinion have lost sight of the 
 first and simplest rule of physico-chemical inquiry, 
 which directs us to prove that an effect which we ascribe 
 
 * Third edition. 
 
HIS LIFE AND WORK. 169 
 
 to a cause is really due to that cause, and no other." 
 And he closes this chapter, after showing how little we 
 know even of the known forces and their relations, 
 by saying that this imperfect state of knowledge of 
 the essence and effects of known natural forces 
 " explains how it comes to pass that at this moment 
 we are unable to solve the question in reference to 
 the existence of a peculiar force or influence acting in 
 the living organism, by the method of exclusion or 
 elimination." 
 
 Bischoff,who was Liebig's intimate friend and corre- 
 spondent, has shown us by what he has said on this 
 subject that Liebig himself did not deny that there 
 may be a distinct vital force which determines the 
 production of organised structures, and works in them 
 with the chemical and physical forces. Vital force for 
 Liebig was most nearly like chemical force and related 
 to organised structures, somewhat as chemical force 
 is related to chemical compounds. 
 
 The tendency of physiologists and pathologists, for 
 a long period before Liebig joined in their work, had 
 been to attempt to investigate the most complex 
 phenomena of life before they were acquainted with 
 the simplest. In his efforts to apply organic chemistry 
 to physiology Liebig threw himself with characteristic 
 enthusiasm into the company of those who were be- 
 ginning to realise the inevitable sterility of such a 
 method. It was, as has been said, his object and 
 theirs to introduce into physiology the method which- 
 in a few years had revolutionised some branches 
 of chemistry the method, that is to say, of pro- 
 ceeding from the simple to the complex. It seemed 
 to him that chemistry and physiology had reached 
 such a stage that a fusion at the boundaries of these 
 
170 JUSTUS VON LIEBIG: 
 
 two sciences might be looked forward to as one of 
 the most striking results of scientific progress that 
 would be attained in the future. Physiology, he 
 thought, could no longer dispense with chemistry in 
 the study of vital phenomena, and chemistry was 
 prepared largely by. Liebig's own work, it must be 
 added to enter into the service of physiology. 
 
 These, then, were his objects as a student of 
 physiology to make the methods of physics and 
 chemistry the methods of this subject also, and to 
 bring about a union of chemistry and physiology that 
 should result in a new and a greater physiology, which 
 should embrace these two great branches of knowledge 
 which had been too long separated. 
 
 The measure of his success can be best expressed 
 by quoting the following extract from the writings of 
 Bischoff, who knew Liebig, and who knew physiology 
 both before Liebig set his hand to his self-imposed 
 task and after his labours were over. Nearly at the 
 end of his address on the influence of Liebig on the 
 development of physiology, he said : " However great 
 and immortal may be the services of Aristotle, Galen, 
 Vesalius, Harvey, Linmeus, Haller, Cuvier, Charles 
 Bell, J. Miiller, and others to physiology ; however 
 greatly they and others may have enriched our know- 
 ledge in the province of organic nature, I can find no 
 proof in the history of science that any single one of 
 these has so influenced and altered the views arid 
 methods of physiology, and also consequently of 
 medicine, by facts and methods as Liebig. I regard 
 Liebig as the one who has brought organic natural 
 science into the pathway of exact investigation." 
 
 And now let us return for a moment to Liebig's 
 great generalisations on the relations of plants and 
 
HIS LIFE AND WORK. 171 
 
 animals to each other and to mother earth. It will 
 not be necessary to dwell for long on this topic, for 
 most readers by this time will have discovered for 
 themselves the main features of the wonderful cycle 
 of changes which Liebig's mind first made plain to us. 
 They have learnt from him how plants absorb from 
 the air and from the earth simple inorganic substances 
 carbonic acid gas, water, and ammonia, with a few 
 mineral salts ; how by the help of sunlight these are 
 so altered that the oxygen of the carbonic acid gas is 
 returned in the elemental state to the air, whilst the 
 rest are retained, and go to form first the various 
 parts of vegetables, afterwards the blood, the flesh, the 
 bones, and the nerves of animals ; and how, either 
 in the bodies of the herb-eating animals, or in those 
 of the flesh-eaters, the carbon in the course of time 
 again enters into combination with oxygen, which the 
 animals absorb from the air by their lungs, and is 
 returned to the air, except so far as concerns that 
 portion which is retained in their tissues by young 
 growing animals. How in the same way the nitrogen 
 of ammonia, or of nitrates, and the mineral matters are 
 also thrown off, at a rate approximately equal to that 
 of their consumption, and returned, the salts to the 
 earth, the ammonia to the air or earth ; and how 
 finally, on the death of the animal, every trace of its 
 components return by the processes of decay to 
 their original condition of lifeless inorganic matter, 
 ready to nourish new generations of plants and 
 animals, provided only that man does not as, alas, he 
 too often does disturb the balance of nature by 
 casting into the sea that which should be used for the 
 enrichment of the land. 
 
 Looking at the matter from another point of view, 
 
172 JUSTUS VON LIEBIG: 
 
 we have seen, with Liebig's aid, how the plants are, 
 as he expressed it, " the accumulators of force ; " 
 how these, utilising some of the energy, as we now 
 call it, which reaches them from the sun, reduce car- 
 bonic acid gas, so that its carbon enters presently 
 the bodies of animals in such forms that it can after- 
 wards liberate, on its re-union with the oxygen, sup- 
 plies of energy which are available for doing work 
 in its various forms, or for maintaining the necessary 
 temperature of the organism. In short, if we have not 
 learned from Liebig quite all that science can teach 
 us to-day on this subject, it is no fault of Liebig's, 
 but due to the fact that the " Physicists " were not 
 quite ready for the chemists when Liebig attacked this 
 question. 
 
HIS LIFE AND WORK. 173 
 
 CHAPTER VIII. 
 
 EDUCATIONAL AND OTHER WORK. 
 
 Influence on the Methods of Education Giessen Laboratory 
 Liebig as a Teacher Founding of other Chemical Schools 
 His Attempts to carry results of his Studies to those outside 
 the Universities Interest in Technical Education What is 
 Technical Education Supposed "Spontaneous Combustibility" 
 of Human Bodies Literary Work. 
 
 IT might reasonably be supposed by anyone who had 
 become acquainted for the first time with so much of 
 Liebig's work in science as it has been possible to 
 allude to in the foregoing pages, that surely this must 
 be the sum of it. That he must have spent his days 
 and nights in his laboratory and his study ; that as a 
 teacher he must have been content to deliver his 
 university lectures, and have devoted all the time 
 not occupied by them to meditating, experimenting, 
 and writing. But this was by no means the case. 
 Liebig made a great departure in the teaching of 
 chemistry. And we may even venture to call him 
 the earliest of " extension teachers," since he had the 
 happy idea of conveying to the general public some 
 notion of the progress of his science in his university 
 and elsewhere, by means of a series of letters which 
 he addressed to the Augsbwrger Allgemcinc Zeituny. 
 In these letters he gave, from time to time, the 
 results of such of his inquiries as he could render 
 intelligible to general readers. 
 
 A very prince of extension teachers, surely ! Can 
 
174 JUSTUS VON LIEBIG: 
 
 it be doubted that these wonderful letters " Liebig's 
 Familiar Letters on Chemistry " had much to do 
 with the present intelligent attitude of the German 
 " practical man " to science, which has contrasted so 
 strangely with that of his average English brother for 
 many years past, much to the material disadvantage 
 of the latter, it is to be feared. 
 
 We have already learnt that on his return to 
 Germany from Paris Liebig obtained, through the 
 interest of Humboldt, an appointment first as extra- 
 ordinary, and soon afterwards as ordinary, professor of 
 chemistry in the little German University of Giessen. 
 Here he stayed for twenty- eight years. Speaking of 
 his life there, he said afterwards, "It was as if 
 Providence had led me to the little university. At a 
 larger university, or in a larger town, my energies 
 would have been divided and dissipated, and it would 
 have been much more difficult, and perhaps im- 
 possible, to reach the goal at which I aimed ; but at 
 Giessen everything was concentrated in work, and in 
 this I took passionate pleasure." 
 
 A few years before Liebig went to Giessen, it 
 was, as he has told us, " a very wretched time for 
 chemistry in Germany." Only the pharmacists had 
 preserved any remains of practical instruction, and in 
 order to get any real training it was necessary for 
 Germans to go to Paris or to Stockholm. Even in 
 those places the student found no public laboratories, 
 but had to rely on such interest as he could command 
 to gain admission to the workroom of some chemist 
 of distinction, which, after all, was sometimes only 
 a garret or a cellar. Liebig, as we have seen, be- 
 came the pupil and friend of Gay-Lussac, and in 
 his laboratory formed the idea of effecting a reform 
 
HIS LIFE AND WORK. 175 
 
 at home. Soon after receiving his appointment, he 
 took his first step by opening his first or, rather, the 
 first public laboratory for the teaching of practical 
 chemistry in Germany. Pupils soon began to stream 
 in, so that the teaching of analysis and other parts 
 of practical chemistry had to be organised on a 
 systematic plan. This necessity led to the intro- 
 duction of a system of teaching elementary and 
 advanced practical chemistry on which all his suc- 
 cessors have founded their methods. 
 
 Actual teaching in the ordinary sense in practical 
 work was only given in the Giessen laboratory to be- 
 ginners ; this work was put into the hands of skilful 
 assistants. Liebig himself took charge of the special 
 pupils ; their progress largely depended on themselves. 
 Their master gave the task, and supervised the doing 
 of it. There was no actual instruction. Liebig received 
 every morning from each pupil a report upon what he 
 had done on the previous day, as well as his views 
 upon the work. He approved or criticised this, but 
 every student was obliged to follow his own course. 
 Owing to constant intercourse each student partici- 
 pated in the work of all the others ; every one was a 
 learner, every one was a teacher. Twice a week Liebig 
 gave a review of the most important questions of the 
 day a report of his own work, combined with that of 
 the students and with the researches of other chemists. 
 
 Work went on from daybreak till nightfall. There 
 was no amusement and no dissipation to be had in 
 Giessen, and the only complaint, which was a frequent 
 one, was that of Aubel (the laboratory man), who 
 could not get the workers to depart in the evening, 
 when he wanted to clean the laboratory. 
 
 In his practical teaching Liebig laid great stress 
 
176 JUSTUS VON LIEBIG: 
 
 on the producing of chemical preparations on the 
 students preparing, that is to say, pure substances in 
 good quantity from crude materials. The importance 
 of this was, even in Liebig's time, often overlooked, 
 and it was, he tells us, more common to find a man 
 who could make a good analysis than to find one 
 who could produce a pure preparation in the most 
 judicious way. There is no better way of making 
 oneself acquainted with the properties of a substance 
 than by first producing it from the raw material, then 
 converting it into its compounds, and so becoming 
 acquainted with them. By the study of ordinary 
 analysis one does not learn how to use the important 
 methods of crystallisation, fractional distillation, nor 
 acquire any considerable experience in the proper 
 use of solvents. In short, one does not, as Liebig 
 said, become a chemist. 
 
 Unfortunately, owing to the readiness with which 
 analysis lends itself to competitive examinations, the 
 importance of this side of experimental chemistry has 
 for many years been greatly exaggerated by many 
 English teachers. There are signs, however, that 
 sounder views are now reasserting themselves in the 
 minds of teachers. 
 
 It was during the early }^ears at Giessen that 
 Liebig busied himself with the task of improving 
 the methods of organic analysis ; and in a few years, 
 when this work was completed, the Giessen laboratory 
 must have presented a busy scene. Talented young 
 men were now streaming in, not only from Ger- 
 many, but from all parts of Europe and from the 
 United States of America ; and Liebig had working 
 under his supervision a band of twenty or more skilful 
 and indefatigable young chemists, by whom thousands 
 
HIS LIFE AND WORK. 
 
 of experiments and analyses were carried out every 
 year. From their results the master gained an 
 abundance of the facts and materials out of which he 
 developed the wonderful conceptions with which he 
 enriched chemistry, physiology, and agriculture. 
 
 Liebig's system of teaching the methods of re- 
 search to advanced students gives a " character " to 
 scientific education in Germany to this day. Students, 
 when they come back from Germany, tell us that in 
 German universities the lectures and elementary 
 practical instruction are, if anything, inferior, cer- 
 tainly not superior, to the teaching in our own schools 
 and universities, and that the well-taught Englishman 
 when he goes to a German university is certainly not 
 at a disadvantage in this respect. What he finds 
 there, and what it is difficult to find at home, in spite 
 of the great and valuable contributions of our country- 
 men to science, is an opportunity for soaking himself 
 with the spirit and methods of research by joining 
 a numerous band of young and devoted workers, 
 with whom he will be in constant intercourse from 
 eight in the morning to six at night, hearing and 
 seeing nothing for a year or two but original experi- 
 ments all conducted under the guidance of a master 
 mind. 
 
 All this, the preliminary courses, on which we 
 at home may pride ourselves, the great centres of 
 organised scientific investigation which are the glory 
 of Germany, we owe to the early labours of Liebig 
 in his little laboratory at Giessen ! Through his 
 work the Giessen University became one of the 
 forces of the world. 
 
 When we remember that a very few years be- 
 fore Liebig went to Giessen he could not find in 
 
178 JUSTUS VON LIEBIG : 
 
 all Germany any one who could teach him 
 chemistry, and that, as a lad, he had followed a 
 professor from Bonn to Erlangen, in order that 
 the professor might carry out a promise to analyse 
 some minerals with him, only to find that, unfortu- 
 nately, his guide did not himself know how to do it, 
 we can hardly sufficiently admire the energy and 
 insight which, in a few short years, enabled Liebig to 
 build up a complete system of public instruction, 
 which in ite main features still seems to us to come 
 very near to being a council of perfection. 
 
 In this, the first one might almost say, the ideal 
 school of chemistry, Liebig gathered round him the 
 elite of the ambitious and able young students in this 
 science from the whole civilised world, so that an 
 immense number of the chemists of to-day owe their 
 education, either directly or indirectly, to him. From 
 England students flocked to him, so that English 
 chemistry especially came under his influence, the 
 more so that Hofmann, his brilliant pupil, afterwards 
 directed the first chemical laboratory opened in Eng- 
 land for giving public instruction in the science. 
 
 This is not the place for introducing a long list 
 of the names of Liebig's eminent pupils. Such a list 
 would only be interesting to a limited number of 
 readers. It will be sufficient, therefore, to say that 
 Liebig's English pupils have shown his influence not 
 only by their work in pure chemistry, as teachers and 
 as investigators, but that they will be found equally 
 eminent among the ranks of those who have devoted 
 themselves to the applications of chemistry in the 
 various departments of industry. 
 
 Liebig as a lecturer was not, it seems, exactly 
 eloquent, but his personality was magnetic, and he 
 
HIS LIFE AND WORK. 179 
 
 could say what he wanted. He had a greater gift 
 than eloquence. He could stimulate the reflective 
 powers of his pupils and make them think. This 
 was the secret of his success and theirs. 
 
 Hofmann says of him that " whoever has had the 
 good fortune of attending his lectures, will not easily 
 forget the deep impression of his peculiar style of 
 eloquence. Liebig," he says, " was not exactly a fluent 
 speaker, but there was an earnestness, an enthusiasm 
 in all he said, which irresistibly carried away the 
 hearer. Nor was it so much the actual knowledge he 
 imparted which produced this effect as the wonderful 
 manner in which he called forth the reflective powers 
 of even the least gifted of his pupils. And what a 
 boon it was, after having been stifled by an oppressive 
 load of facts, to drink the pure breath of science, such 
 as flowed from Liebig's lips. What a delight, after 
 having perhaps received from others a sackful of 
 dry leaves, suddenly, in Liebig's lectures, to see the 
 living, growing tree." 
 
 It was not, however, really in lecturing that Liebig 
 excelled, but in the laboratory. Here he was followed 
 not only because he was admired, but because he 
 was loved still more. His pupils felt a pleasure in 
 submitting themselves to his fascinating control. 
 And Hofmann tells us that if anything could 
 exceed their wonder at the amount of work he did 
 himself with his own hands, it was the wonder they 
 felt at the mountain of toil which he got them to go 
 through themselves ; that, if anything exceeded their 
 pride in his friendship and approval, it was their 
 delight to know that they helped him, and that 
 whilst they were receiving his lessons they Avere also 
 performing his work. The aid that he obtained in 
 
180 JUSTUS VON LIEBIG: 
 
 this way was doubtless considerable, but he never 
 failed to give full credit for such aid, and, indeed, so 
 far was he from underrating it, that in his generosity 
 he often gave a pupil the whole credit for a success 
 obtained by experiments suggested by himself and 
 based upon his own previous trials and discoveries. 
 
 In his autobiographical fragment, from which I 
 have made so many extracts, Liebig, when speaking 
 of the wonderful chapter of accidents by which he 
 made the acquaintance of Gay-Lussac, says that "since 
 he found favour as a boy in the sight of Thenard, 
 Humboldt, Dulong, and Gay-Lussac, it had frequently 
 been his experience that marked talent awakens in all 
 men," he believes without exception, " an irrepressible 
 desire to bring about its development." Whether he 
 goes too far in saying this or not, there can be no 
 doubt that in Liebig's own character this instinct 
 existed in a most marked degree. 
 
 The founding of Liebig's School of Chemistry at 
 Giessen formed an epoch in the history both of 
 science and of education. Almost from the day when 
 this school was inaugurated, chemistry and the 
 branches of science connected with chemistry have 
 advanced by leaps and bounds. It is impossible not 
 to find one of the most potent causes of this advance 
 in the work that was done at Giessen by Liebig. The 
 great success of this laboratory quickly led to the 
 building of others, and soon every university at least 
 possessed such a laboratory. At first they were modest 
 places enough, many ot them, till Hofmann on his 
 recall from England to Germany was provided with 
 something very like a palace at Bonn. It is custom- 
 ary, in Germany, to provide the professor with a 
 residence in the laboratory, and so fine were the apart- 
 
HIS LIFE AND WORK. 181 
 
 ments provided at Bonn that the King of Prussia, 
 when he opened the building, is said to have re- 
 marked : " I should like to live here myself." Soon 
 afterwards a second palatial building, also for Hof- 
 mann, was built at Berlin, and thus an era of palatial 
 laboratories was inaugurated. Other countries, in- 
 cluding England, followed suit more or less quickly, 
 and now, as we ah 1 know, not only the universities, but 
 all our chief towns, are provided, some with one, and 
 some with more than one, laboratory, which, though 
 not aLvays nor, indeed, often a palace, is efficient 
 and fully equipped for teaching and frequently for 
 chemical research. Indeed, few towns of even the 
 second and third rank are now quite without some 
 provision for the practical study of the subject, 
 whilst our leading schools have also, in the great 
 majority of cases, provided themselves with similar 
 equipment. Much doubtless still remains to be done, 
 but the progress made is already very great, and it 
 may all be traced to the inspiration of Liebig. 
 
 And what is the result ? Will the work done in 
 these lecture-rooms and laboratories be worth their 
 cost ? In seeking to answer this question, we must not 
 forget that the value of these places of public instruc- 
 tion in chemistry does not consist simply in the fact 
 that they provide a certain number of workshops, 
 more or less magnificent, more or less well provided, 
 for the use of those who desire to study pure 
 chemistry. They provide also practical instruction 
 for that much larger and equally important class 
 who purpose to devote their lives to the service of 
 their fellows in medicine, in the various and ever- 
 growing chemical industries, and in a variety of other 
 directions too numerous to mention. And, again, we 
 
182 JUSTUS VON LIEBIG 
 
 must remember that chemical laboratories did not for 
 long stand alone ; presently other such work-places, 
 physical laboratories, biological laboratories, engineer- 
 ing laboratories, rose beside them, ready to do for 
 physics, biology, and engineering what the chemi- 
 cal laboratories were doing for chemistry. The 
 result, neglecting altogether the immediate material 
 gains, which though difficult to calculate must be 
 enormous, is briefly this that civilised mankind is 
 rapidly learning the importance of facts and the value 
 of the experimental method by which facts are gained, 
 and is realising better than for many centuries the 
 dignity and the intellectual value of intelligently- 
 directed handwork. 
 
 So long as the recognised method of solving every 
 problem was to sit down and think about it, it was 
 not unnatural that those who worked with their hands 
 should be looked upon as necessarily inferior to the 
 head-workers. But the influences developed during 
 the last fifty years are steadily changing the whole 
 aspect of affairs, and it cannot but be that before long 
 the intelligent hand-worker in every department will 
 receive a fuller measure of esteem than has ever 
 before, as far as we know, been awarded to him. 
 
 One of the most beautiful and fascinating experi- 
 ments of the chemist is to prepare a very clear, 
 strong, hot solution of a salt more soluble in hot than 
 in cold water, and, after protecting this from the 
 approach of dust, to let it stand till cool. If the ex- 
 periment be perfectly made, such a solution will re- 
 main a solution it maybe indefinitely, at any rate for 
 a long while ; no crystals of the solid will make their 
 appearance, though there is far more of it in the 
 liquid than the water present can normally retain in 
 
HLS LIFE AND ^VOHK. 183 
 
 the liquid state. But if the most tiny, invisible frag- 
 ment of the solid salt be allowed to touch the solution 
 at any point, a change will at once set in, crystals will 
 appear at that point, will quickly become visible, and 
 will spread with ever-increasing rapidity, till in a few 
 minutes or maybe, in a few moments what was 
 before fluid will become a solid mass, as if by magic. 
 
 It was somewhat in this sense that the founding 
 of Liebig's laboratory at Giessen was the cause of 
 the movement which is now rapidly introducing 
 what is sometimes called the scientific method in so 
 many spheres of human activity. All the necessary 
 elements for the change existed before Liebig went 
 to Giessen. His fruitful idea acted like the crystal. 
 It provided the necessary nucleus. 
 
 I have already spoken of Liebig as a very early 
 exponent of the idea which is at the bottom of the 
 " University Extension Movement." He endeavoured 
 to bring the learning accumulated in the university 
 to the people at large in two ways first, by his 
 celebrated letters on chemistry ; secondly, by bringing 
 to the university selected persons from the provinces 
 to attend his courses of lectures, in order that on 
 returning they might carry a little knowledge and 
 some interest back with them into the provinces, 
 and, maybe, sow the seeds which should produce 
 greater things afterwards. This he made possible 
 by inducing the Government, after his call to Munich 
 in 1852, to grant stipends to a number of school- 
 masters from different districts to enable them to 
 defray the expenses of the visit to the city. He 
 hoped in this way to point out the road to mental 
 culture to those who had the task of educating the 
 country populations. 
 
184 JUSTUS VON LIEBIG: 
 
 Speaking of the results of this experiment, in 
 1861, he said in the course of an academical address : 
 "It must be confessed that even among those who 
 have not had the chance of attending our gymnasia, or 
 our modern and technical schools, the opinion is rooted 
 that a little more knowledge is of use even to the 
 most common hand- worker ; that some knowledge of 
 botany to the gardener, some knowledge of chemistry 
 to the soap-boiler, the tanner, or the dyer, is essential 
 to the carrying on of their businesses ; that a gar- 
 dener is not a worse gardener for knowing a little 
 of plant life; that a baker, because he knows what 
 bread, meal, and salt are; or a soap-maker, because 
 he truly knows the nature of fat, ashes, and lye, 
 what are their good qualities, and how they may 
 be recognised by tests that these workmen will 
 not be any the worse makers of bread or soap 
 because they know this. Even the most simple 
 citizen thinks it a gain if his neighbour, who 
 may be a magistrate, has some knowledge of 
 the principles on which the rules of health are 
 established." Then, after discussing some defects in 
 the German agricultural colleges as they then were, 
 he went on to say : " The agriculturist and techno- 
 logist must learn that he lives in a cruel time, when 
 existence becomes more difficult every day for the 
 incompetent and weak, and will soon perhaps be im- 
 possible." His remarks on this occasion are said to 
 have given great offence. Fortunately for Germany, 
 however, the lesson was by no means altogether 
 thrown away, in spite of the opposition it aroused. 
 
 As has already been explained, Liebig's famous 
 "Familiar Letters on Chemistry" were originally a 
 series of letters published in an important South 
 
HIS LIFE AND WORK. 185 
 
 German newspaper. They were afterwards collected, 
 and on the occasion of preparing a third edition in 
 1851, Liebig took the opportunity of adding a number 
 of letters on the origin and development of chemistry. 
 
 The first letter in this edition is especially inter- 
 esting; in it Liebig lays down, with the greatest clear- 
 ness, the practical relations of the sciences to one 
 another, and that of the sciences to the arts with 
 which they are respectively connected, and indicates 
 the manner in which each of these must approach 
 another for help and guidance ; he shows the con- 
 ditions which must be fulfilled, for example, before 
 chemistry can help to solve the problems of physio- 
 logy or contribute to the development of a chemical 
 industry. 
 
 In many respects chemistry is, as he here points out, 
 like mathematics. The latter teaches us in its applica- 
 tion how to measure land, to erect buildings, to raise 
 weights. In short, it is an instrument which confers 
 the most obvious advantages on those who know how 
 to employ it. On the other hand, mathematics, by 
 means of a language of its own, teaches us how to 
 draw logical conclusions according to definite rules, 
 and to express them, in a simple manner. We all know 
 that mechanicians, physicists, and astronomers use 
 mathematics ; that it is an indispensable instrument 
 for the ends they strive to reach. In order to make 
 use of mathematics, they must become so expert 
 in the subject that its management almost becomes 
 a mechanical habit. But notwithstanding the value 
 of this instrument, it is not the mere instrument which 
 does the work. Besides having the necessary fami- 
 liarity with the processes of mathematics, the mechani- 
 cian or astronomer must be able to propound questions 
 
186 JUSTUS VON LIEBIG- 
 
 to himself, and to test the truth of the answers given 
 by mathematics. It is only if he has these further 
 powers that he is qualified to undertake to inves- 
 tigate nature. Mathematics, the instrument, can only 
 help astronomy, physics, and mechanics when it is in 
 the hands of some one who can, so to speak, propound 
 problems for it to be used upon. Besides a mastery 
 of mathematics, the astronomer, the physicist, and 
 the mechanician must possess acute observing power 
 and imagination. These two last afford, so to say, the 
 motive power. Of course the mathematics are equally 
 essential, but they are not a cause, they are an instru- 
 ment ; mathematical work, as we know, may actually 
 be to some extent done by machines, and we must 
 not confuse the instrument with the cause in this 
 case, just as we must not ascribe to a steam-engine 
 results which have really been brought about by the 
 human intellect. 
 
 And so it is with chemistry and the experimental 
 sciences. These also putting aside their value as a 
 mental training, and in other ways which do not 
 concern us at present these also are instruments ; and 
 discoveries in chemistry, in physics, and so on, like 
 discoveries in mathematics, are simply steps towards 
 the perfecting of the instrument. Chemistry and the 
 other sciences, like mathematics, each of them speaks 
 a language of its own; and if we would apply any 
 one of them to the service of man in this or that 
 department, we must be thoroughly acquainted with 
 that language. But here again, as in mathematics, 
 these physical sciences do not make discoveries for 
 us. If we apply chemistry to a chemical problem 
 set out in the right way, chemistry will find us the 
 answer to that problem, provided, of course, that we 
 
HIS LIFE AND WORK. 187 
 
 do not go beyond the powers of the instrument for the 
 time being. But the question must be clearly and 
 definitely put. If the ideas of the inquirer are not 
 clear as to the problem to be solved, chemistry can 
 give him no help. The inquirer, in short, must 
 understand the instrument as a whole ; it is not 
 enough that he shall have seen a wheel here and a 
 wheel there ; he must also have sagacity and observing 
 power ; he must be able to propound definite questions, 
 and understand how to test the truth of the answers 
 obtained by yet other questions when that is 
 necessary. 
 
 " Can men," asked Liebig, " who do not apprehend 
 the nature of scientific investigation in a philosophical 
 spirit, and who cannot interpret the language of 
 phenomena can such men be expected to derive the 
 least advantage from the discoveries of chemistry or 
 physiology ; and can they be deemed capable of 
 making the most insignificant application of those 
 discoveries to practical purposes ? " 
 
 These were the considerations which led Liebig to 
 take great interest in technical education ; which led 
 him to praise the apothecary, who, after learning the 
 elements of his art in the shop, resorted to the 
 university for instruction in the science on which it 
 rests; and to blame those physiologists and agri- 
 culturists who had objected that chemistry could 
 and did give them no answer to their questions, when 
 they had not learned the language of the instrument, 
 and hence were unable to put the questions to which 
 they desired to have answers. 
 
 Has not Liebig, by his teachings on this point, 
 answered beforehand very plainly a question which we 
 often hear nowadays What is technical education ? 
 
188 JUSTUS VON LIEBIG: 
 
 Does it mean the teaching of the arts, or does it 
 mean teaching the sciences on which those arts are 
 based ? And again, as regards these sciences, may a 
 technical student learn only just so much of them as 
 directly applies to his art; or must he, like other 
 students, aim at the best all-round knowledge of them 
 that he can attain ? 
 
 Of course it is obvious that if a youth is to 
 become a baker he must learn baking in the bake- 
 house ; that if he would be an industrial chemist he 
 must sooner or later enter a chemical works ; that he 
 who would be a farmer must spend years upon a 
 farm. 
 
 For the rest, what Liebig wrote in 1851 still 
 conveys the whole truth in 1894 
 
 " Hitherto," said Liebig, " scarcely any demand 
 has been made upon the science of chemistry* by 
 arts, manufactures, or physiology, which has not been 
 responded to. Every question clearly and definitely 
 put has been satisfactorily answered. Only when the 
 inquirer had no precise idea of the problem to be 
 solved has he remained unsatisfied." 
 
 Clear and precise problems can only be put in 
 chemistry and the other experimental sciences, as in 
 mathematics, by those who are well acquainted with 
 their method and language, and who have learned 
 how to set such problems. 
 
 It follows that he who, "in these cruel times," 
 would take advantage of the resources of science in 
 his art, must study the sciences on which it depends, 
 so far as he can, in the same way as other students. 
 That is to say, he must learn the " go " of each instru- 
 
 * He was then speaking for chemistry only, but his words are not 
 less true of other sciences. 
 
HIS LIFE AND WORK. 189 
 
 ment as a whole ; he must not be satisfied with a peep 
 at this part and a peep at that part. Above all, his 
 scientific training must cultivate his powers of obser- 
 vation and his powers of putting questions to nature 
 to the utmost possible extent. The needs of the 
 technical student, in short, do not differ essentially 
 at the early stages from the needs of those who will 
 afterwards devote themselves to pure science. 
 
 Liebig on several occasions did good service by 
 helping to dispel popular errors. This was notably 
 the case in regard to the " spontaneous combustion " 
 question. 
 
 In one of those delightful discourses, the " Familiar 
 Letters," Liebig treated of the at one time much dis- 
 cussed question, Is it possible for a human body 
 to ignite spontaneously ? The notion that a living 
 human body is capable of spontaneously catching 
 lire appears to be comparatively modern. The 
 earliest recorded example which Liebig could dis- 
 cover was said to have occurred in 1725. A 
 woman of Rheims was found half a yard from a fire- 
 place burnt to death ; very little was left of her. Her 
 husband had an attractive servant girl, and murder 
 was suspected. But at the trial learned experts de- 
 clared spontaneous combustion to be possible, and the 
 husband was pronounced innocent. 
 
 This idea, therefore, arose before Lavoisier had 
 explained the nature of combustion, and when men 
 were not in a position to understand such pheno- 
 mena. It persisted, however, long after Lavoisier had 
 explained the chemistry of fire. And Liebig himself 
 on one occasion was called as a witness, when this delu- 
 sion had been set up in defence of a man-servant who 
 was accused of the murder of his mistress, the Countess 
 
190 JUSTUS VON LIEBIG: 
 
 of Gorlitz, in Darmstadt, From 1725 till Liebig in- 
 vestigated the subject that is, during rather more 
 than a century some forty-eight cases of supposed 
 spontaneous combustion had been recorded ; and so 
 firmly fixed was the belief in the possibility of the 
 phenomenon, that even after Liebig had demonstrated 
 the complete absence of any real evidence in sup- 
 port of its truth, and after it had been finally aban- 
 doned by men of science, and had ceased to be ad- 
 mitted in the law courts, it retained a sufficient hold 
 to be still made use of in fiction ; and Charles Dickens, 
 indeed, published a defence of one form of the doctrine 
 in the preface to an edition of " Bleak House." 
 
 It need scarcely be said that Liebig was not the 
 first to feel doubtful as to the possibility of this extra- 
 ordinary phenomenon. Others before him had rejected 
 the popular idea as improbable and incredible ; but 
 till Liebig's arguments at length exploded the belief 
 it remained floating in the heads of some doctors and 
 lawyers, and it was accepted by them as at least an 
 open question. 
 
 Liebig found, on investigating the history of the 
 recorded cases, that on most occasions the victims 
 were intoxicated ; that the supposed spontaneous 
 combustion occurred usually where the rooms are 
 heated by open fires or pans of glowing charcoal, 
 and were rare in countries like Germany, where 
 closed stoves are employed ; that they occurred in 
 winter ; that no trustworthy witness had ever yet 
 seen the phenomenon occur ; that the physicians who 
 collected the data had never really known the history 
 of the cases they attempted to explain ; and that 
 the amount of combustible matter on the spot was 
 never ascertained. When he came to look into one of 
 
HlS LIFE AND WORK. 191 
 
 certain cases which were specially reserved, as seeming 
 to be established by Dr. Franck, he found, omitting 
 many corroborative circumstances, that, in this case, 
 before the combustion took place, there was a lamp full 
 of oil in the room, and afterwards the lamp was empty 
 and the wick burnt up ; that the body was only burnt 
 where the clothes were also burnt ; finally, that while 
 the clothes were in many parts burnt to ashes, the 
 skin Avas only detached, hanging in shreds ; from 
 which it would seem to follow, if spontaneous com- 
 bustion had occurred, that the kindling of the clothes 
 was caused by the skin, which yet itself did not burn. 
 A great many writers in Liebig's time, perhaps 
 the majority, did not, however, suppose that in 
 these forty-five or forty-eight cases the victims had 
 really ignited spontaneously had caught fire, that 
 is to say without the aid of an external flame. 
 The most commonly-accepted view was rather that, 
 though the healthy human body is very difficult to 
 burn, the flesh and skin may become much more 
 combustible through diseased conditions produced, 
 for example, by the excessive use of strong spirits ; 
 that they may become not merely like a block of 
 wood, which easily goes out when kindled, but more 
 like, let us say, tinder, which, when once ignited, 
 continues to burn. The supporters of the theory 
 said that the fact of spontaneous combustion in this 
 sense was not refuted by all that science then knew ; 
 that circumstances and facts were quite concordant 
 with this view ; and they asked whether, considering 
 how many phenomena there are which science has 
 not yet explained, it was fair or decent to reject the 
 testimony of so many upright men who have avowed 
 their belief ia spontaneous combustion, and to class 
 
192 JUSTUS VON LIEBIG f 
 
 them with liars or blockheads because we do not 
 agree with them. 
 
 Liebig pointed out, in reply to this, that nobody 
 denied the facts stated viz. that a number of 
 people had been burnt ; what was denied was the 
 hypothesis by which the facts were accounted for. 
 It was plain that every kind of assertion could be 
 supported on such grounds as these. And, moreover, 
 that the opinion that a man can burn of himself was 
 obviously not founded on a knowledge of the exact 
 circumstances of death, but, on the contrary, was 
 based upon ignorance of these circumstances. 
 
 A man or woman is found in a room burned 
 to death. The experts, after an inquiry, cannot dis- 
 cover how the fire originated, or how it was propagated 
 in the body ; therefore, because at intervals during 
 a century or more in the past similar unaccount- 
 able deaths from fire have occurred, and have been 
 ascribed to spontaneous combustion, we are asked to 
 ascribe these deaths also to the same cause. And this 
 in spite of the fact that our predecessors appear to 
 have been quite as ignorant as ourselves of the facts 
 of the respective cases, were not even present at the 
 time of the occurrences, and that their explanation 
 is not only without any independent support of a 
 definite kind, but also contradicts all that we know 
 about the combustibility of animal matter. 
 
 Before it can be admitted that a given death by 
 fire was due to the body having acquired a morbid 
 state, in which it becomes as combustible as straw, 
 we must, said Liebig, "not only prove that it is 
 possible for a piece of flesh to become thus com- 
 bustible, but we must prove that such combustion, 
 when it occurred, has proceeded from the flesh 
 
HIS LIFE AND WORK. 193 
 
 outwards ... it must be shown that a morbid 
 state, such as is assumed, actually exists ; and further, 
 that the persons who were burned were in that 
 morbid condition." 
 
 " Xow, nothing of the kind has ever been done ; 
 no doctor has ever observed a condition of the 
 human body in which it was readily combustible; 
 and no one knows any signs by which we could 
 recognise such a condition." 
 
 In concluding this letter Liebig made a valuable 
 contribution to medical jurisprudence, which, though 
 it related only to the question of spontaneous com- 
 bustion, upheld a really important principle. 
 
 The physician, he said, " who is called on for 
 a judgment in such cases can only say, if he act 
 according to duty and conscience, in what state the 
 body was found ; whether the injury from burning 
 took place before or after death ; whether death was 
 caused by fire alone, or before the action of the fire 
 by other causes, such as wounds, strangulation, a blow 
 on the head, etc. In no case is it permitted him to 
 explain anything he has not seen by cases which he 
 has also not seen, or by a theory which he cannot 
 understand." Those who disregard these limitations 
 protect criminals, and delay or prevent the adminis- 
 tering of justice. 
 
 The comparative ease with which Liebig over- 
 threw this popular fallacy throws an interesting 
 sidelight on the progress of civilisation in Europe. 
 It had now become not only safe to attack a popular 
 error, but one might even hope to destroy it. This was 
 not so a little earlier. We are told that when Kepler 
 the astronomer went to Tubingen, in the sixteenth 
 century, to save his mother from the stake, he 
 
 M 
 
194 JUSTUS VON LIEBIG : 
 
 succeeded not by proving that there are no such people 
 as witches, but only by showing that she possessed none 
 of the characteristic signs essential to a witch. 
 
 This was not the only occasion on which Liebig 
 set himself to correct popular misconceptions. Thus 
 when, after the discovery of electro-magnetism, his less 
 instructed countrymen were pleasing themselves with 
 the idea that electricity would replace steam for putting 
 machinery in motion, and that, because zinc is used for 
 generating electricity, and is plentiful in Germany, 
 soon Germany would become the chief seat of the 
 manufactures in place of England, Liebig was quick to 
 point out to them that such expectations, though 
 attractive to the human mind, are quite fallacious, 
 unless they are the outcome of exact comparisons and 
 calculations. " Out of nothing," said Liebig, " no kind 
 of force can arise." From a pound of coal we get about 
 six times as much force as from a pound of zinc when 
 we burn them, and hence, even if it were true that the 
 latter produces four times as much force in a galvanic 
 pile as it does when we burn it, the coal would still be 
 the more economical substance in practice. Besides, 
 coal is wanted to reduce zinc from its ores, and it 
 is probable that, if the coal required for smelting the 
 zinc were burned under a stearn-engine, we should 
 obtain much more force than all that the zinc reduced 
 by means of it could yield us. 
 
 These are a few illustrations drawn from this re- 
 markable book, into which Liebig put so much of his 
 very best work and thought in a form in which they 
 could help every intelligent person who would read 
 them. They are, as Liebig meant them to be, a great 
 deal more than mere popular expositions. In these 
 letters science is freed from technicalities, but Liebig 
 
HIS LIFE AND WORK. 195 
 
 did not allow himself to attenuate the science in 
 getting rid of the technicalities. 
 
 It has been necessary in the preceding pages to 
 refer to several of Liebig's most important books, and 
 it has been mentioned that the mere list of his 
 memoirs to scientific journals occupies twenty- three 
 columns of the Royal Society's catalogue, and in- 
 cludes 318 separate papers; but this does not give 
 anything like a sufficient idea of his immense con- 
 tributions to, and influence on, scientific literature. 
 In 1832 he founded one of the most celebrated of 
 scientific periodicals, known generally as the Annalen, 
 in the editing of which he had for a long while the 
 assistance of his friend Friedrich Wohler and of Her- 
 mann Kopp, the historian of chemistry. It may be 
 imagined what this alone meant when it is said that 
 no less than 165 volumes of the book had appeared 
 at the time of Liebig's death in 1873. In the 
 Annalen are to be found all the researches carried out 
 by Liebig and by his pupils at Giessen, from the time 
 of their first publication. Jointly with Wohler again, 
 and with the co-operation of Poggendorff, he was 
 author of a dictionary of pure and applied chemistry, 
 which was for long a most valuable resource, and on 
 which a later dictionary by Fehling was founded. 
 Besides this he gave chemists the benefit of his 
 unexampled mastery of organic chemistry in his 
 handbook on that subject, the origin of which did as 
 much credit to his heart as its execution did to his 
 head. It appears that shortly after the death of an 
 early friend, P. L. Geiger, a new edition of his once 
 celebrated book on pharmacy "Geiger's Handbook 
 of Pharmacy " was called for, and that Liebig, with 
 characteristic generosity, undertook to revise the 
 
196 JUSTUS VON LIEBiG : 
 
 chemical part of it in the interest of his dead 
 friend's widow. Soon, however, revision was found 
 to be impossible; so great were the strides which 
 chemistry had made in the interval since Geiger 
 had published his last volume, that Liebig had to 
 give up the idea of revising the work, and pro- 
 ceeded to re-write it, with the result that the new 
 " Handbook of Organic Chemistry " soon appeared. 
 This book had a wonderful success ; what had previ- 
 ously been a maze of incoherent information be- 
 came in this volume for the first time an articulated 
 science, it was soon translated into French and 
 appeared also in England as a part of Turner's 
 " Chemistry " in the later editions of that work. 
 
 On the death of Berzelius, he was called by 
 the unanimous voice of chemistry to continue the 
 Annual Reports on the progress of chemical science 
 which Berzelius had for many years drawn up. 
 When it is mentioned that for the compiling of 
 these reports the work of a small army of in- 
 vestigators, published in perhaps a hundred jour- 
 nals, and in several languages, had to be read and 
 sifted, it will be understood what a labour this work 
 must have involved, and how great a service Liebig 
 did his fellow chemists by undertaking it. And 
 even when all this is told, there remain his various 
 discourses on subjects of popular interest, his re- 
 ports (1) On the State of Chemistry in Austria, 
 and (2) On the State of Chemistry in Prussia, 
 which exercised a most marked influence on the 
 progress of scientific education in Germany ; and 
 some minor books and pamphlets, enough by them- 
 selves to have made a great reputation. 
 
HIS LIFE AND WORK. 197 
 
 CHAPTER IX. 
 
 CHARACTER AND LATER YEARS. 
 
 Dominant Characteristics of Liebig Address to Bavarian Academy 
 after Franco- Prussian War Relations with English Men of 
 Science Letter to Faraday A Testimonial from England 
 Liebig and his pupils Munich and his later years. 
 
 THE dominant characteristics of Liebig were his 
 intense desire for truth, his unselfishness, the com- 
 plete absence from his mind of any tincture of the 
 partisan, and his unfailing vivacity. 
 
 His scientific disputes were, from the novelty of 
 many of his ideas, not, unnaturally, rather numerous. 
 In scientific discussions he too often forgot the man 
 in his desire to rend and destroy the error. In his 
 ardour he doubtless sometimes forgot that his oppo- 
 nents, like himself, were animated, in most cases at 
 least, by a desire to promote the discovery of truth 
 and the overthrow of error. But there was so much 
 to admire in Liebig, so much to love when you 
 knew him and he was so ready to admire what he 
 saw to be true even in the work of an opponent that 
 when the battle was once fought out reconciliation 
 was in most cases easy; and we find, for example, 
 that, in spite of the vivacity of his encounters with 
 Dumas, these two great men were more than once 
 to be found working together. And again, that hi 
 spite of the discussions with Laurent and Gerhardt,^in 
 which Liebig took an active part, and in which the 
 two former chemists had to suffer treatment which 
 
198 JUSTUS VON LIEBIG: 
 
 has since been described as " quite painful to think 
 of," Gerhardt described his meeting with Liebig at 
 Munich some time afterwards to Hofmann with 
 " glowing delight." 
 
 Nor must we forget, when reading Liebig's most 
 lively passages, that they were not written yesterday. 
 Neither in pugilistic encounters, nor in scientific 
 disputes, had it then become the custom to fight with 
 the gloves on ; and, besides, his antagonists were by 
 no means unable or unwilling to give very hard 
 knocks themselves when an opportunity arose. 
 
 Liebig was one of the first eminent Germans after 
 the Franco-Prussian War to hold out the olive branch. 
 At a moment when the irritation on both sides was 
 still keen, he attempted, in an address to the Bavarian 
 Academy, to soothe the feelings of the moment by an 
 eloquent appeal to the traditions of the glorious past. 
 
 " This," said Liebig, " is, perhaps, a fit opportunity 
 for declaring, on the part of our Academy, that a 
 hatred of race between the German and Latin nations 
 does not exist. 
 
 " We look on the heavy affliction which in former 
 times the French nation has caused to Germany as 
 on an illness, the pains of which are utterly forgotten 
 with the return of health. 
 
 " The peculiar nature of the German, his know- 
 ledge of languages, his appreciation of other nationali- 
 ties, compel him to do justice to foreigners, so much 
 so as occasionally to become unjust to himself; and 
 thus we cannot possibly underrate the debt of 
 gratitude we owe to the great philosophers, mathe- 
 maticians, and natural inquirers of France, who, in 
 so many departments, have been our masters and 
 exemplars 
 
HIS LIFE AND WORK. 199 
 
 " Warm sympathy for all that is noble and great, 
 and disinterested hospitality, are among the finest 
 features of the French character. It will be on 
 the neutral ground of science that the best minds 
 of the two nations must meet in endeavouring 
 to reach the high goal common to both, that 
 these sentiments will be again kindled into life 
 and activity; and thus the feeling of fraternity in 
 science, which can never be entirely extinguished, will 
 gradually contribute to mitigate the bitterness with 
 which the deeply wounded national pride of the 
 French is filled by the consequences of the war they 
 have forced upon us." 
 
 The last few words might have been different, 
 but they were said in 1871. The rest formed a 
 characteristic tribute to the nation who had fostered 
 and helped to inspire Liebig himself half a century 
 before. 
 
 Liebig's relations with the English men of science 
 were of the most cordial character. After his first 
 visit to England, in 1837, he wrote on his return 
 that he had seen astonishing things. At that tune 
 Thomas Graham alone, among the chemists, made a 
 very strong impression on him, and he noted with 
 regret that the opportunities for learning chemistry 
 were then bad. English students went to Germany 
 in large numbers to become his pupils. Two of the 
 most important of his works were presented to the 
 British Association, as we have seen, and most of his 
 other books were translated into our language; con- 
 sequently his influence on the progress of chemistry 
 in this country has been very great. In the course 
 of his repeated visits he became acquainted with 
 Faraday, for whom he conceived feelings of the most 
 
200 JUSTUS VON L1EBIG: 
 
 profound admiration, respect, and friendship. The 
 following letter from Liebig to Faraday is interesting 
 alike for the light it throws on the relations between 
 these two great men, and for the delightful picture, 
 mere sketch though it be, which it contains of 
 Liebig's home life in Giessen : 
 
 " GIESSEN, December 19th, 1844. 
 
 "DEAR FARADAY, I intended to have written you long 
 ago of my safe arrival, and that I had found my wife and 
 children well. The opening of my winter course, and a mass of 
 work which had accumulated during my absence, have hitherto 
 prevented my fulfilling my intentions. Now, however, that I 
 have a few days of rest during the Christmas holidays, I will 
 not let the opportunity slip of wishing you, with my whole heart, 
 a merry Christmas aud a happy new year. Often do my thoughts 
 wander back to the period which I spent in England ; among the 
 many pleasant hours of which the remembrance of those passed 
 with you and your amiable wife is to me always the dearest and 
 most agreeable. With the purest pleasure I bring to mind my walk 
 with her in the Botanical Garden, at York, when I was afforded 
 a glance of the richness of her mind : what a rare treasure you 
 possess in her ! The breakfast in the little house with Snow- 
 Harris and Graham, and our being together at Bishopthorpe, 
 are still fresh in my memory. I wish it were only my good 
 fortune to see and talk with you ofteuer, and to exchange ideas 
 with you. 
 
 "Nature has bestowed on you a wonderfully active mind, 
 which takes a lively share in everything that relates to science. 
 Many years ago your works imparted to me the highest regard 
 for you, which has continually increased as I grew up in years 
 and ripened in judgment ; and now that I have had the pleasure 
 of making your personal acquaintance, and seeing that in your 
 character as a man you stand as high as you do in science, a 
 feeling of the greatest affection and esteem has been added 
 to my admiration. You may hence conceive how grateful I 
 am for the proof of friendship which you have given me. 
 
 " I have every reason to be satisfied with my journey in 
 Great Britain : rare proofs of recognition have indeed been given 
 
HIS LIFE AND WORK. 201 
 
 me. What struck me most in England was the perception that 
 only those works which have a practical tendency awake atten- 
 tion and command respect, while the purely scientific works, 
 which possess far greater merit, are almost unknown. And yet 
 the latter are the proper aud true source from which the others 
 flow. Practice alone can never lead to the discovery of a truth 
 or a principle. In Germany it is quite the contrary. Here, in 
 the eyes of scientific men, no value, or, at least, but a trifling one, 
 is placed on the practical results. The enrichment of science 
 is alone considered worthy of attention. I do not mean to say 
 that this is better ; for both nations the golden medium would 
 certainly be a real good fortune. 
 
 " The meeting at York, which was very interesting to me from 
 the acquaintance of so many celebrated men, did not satisfy me 
 in a scientific point of view. It was properly a feast given to 
 the geologists, the other sciences serving only to decorate the 
 table. The direction, too, taken by the geologists appeared to 
 me singular, for in most of them, even the greatest, I found only 
 an empirical knowledge of stones and rocks, of some petrifacts 
 and a few plants, but no science. Without a thorough knowledge 
 of physics and chemistry, even without mineralogy, a man can 
 be a great geologist in England. 
 
 "I saw a great value laid on the presence of petrefactions 
 and plants in fossils, whilst they either do not know or con- 
 sider at all the chemical elements of the fossils those very 
 elements which made them what they are. 
 
 " This letter has already grown too long, and truly I fear to 
 weary your patience. I cannot, however, deny myself the pleasure 
 of expressing a sincere wish to see you and your wife here in 
 Giessen next summer. Did you know how quietly we live at 
 our German universities you would certainly expect from your 
 visit only benefit to your health. Except scientific pursuits we 
 have no other excitements of the mind. We take walks in our 
 beautiful green woods, and in the evening drink tea at the neigh- 
 bouring old castles. This is our recreation. I beg of you, dear 
 Faraday, to listen to niy request. I pray your dear wife to 
 assist me in trying to make you decide on this journey. My 
 wife unites with me in begging this, it would give her the 
 greatest pleasure to make the personal acquaintance of you and 
 your lady. 
 
202 JUSTUS VON LIEBIG: 
 
 " Farewell, dear Faraday, preserve to ine your friendly 
 favour, and believe me, with all sincerity, to be 
 
 " Tours very truly, 
 
 "DR. JTJST. LIEBIG." 
 
 One cannot help seeing in this letter a charming 
 simplicity of mind which reminds one of Faraday 
 himself. Different as these two men were, in many 
 respects, they resembled one another in this, and 
 in a certain modesty, which was never out of sight 
 in the case of Faraday, and showed itself in Liebig, 
 too, upon occasion as, for example, when, as a young 
 man, he wrote to Wohler, in 1830, regretting that they 
 had never yet met, and expressed a fear that Wohler 
 would find out later his (Liebig's) real poverty of 
 acquired knowledge and which led him to say so 
 little about the various scientific distinctions which 
 were liberally showered upon him, even before he 
 reached middle life, that frequently many of his in- 
 timate friends were unaware that he had received 
 them. Nor was this silence due to want of appre- 
 ciation. This is shown by the correspondence given 
 below, which took place on the occasion of his accept- 
 ing the call to Munich, when Liebig's English friends 
 united, under the presidency of Graham, to recognise 
 the occasion by presenting a testimonial to him : 
 
 " LONDON, July, 1854. 
 
 " SIR, Your retirement from the Chair of Chemistry in the 
 University of Giessen has appeared to many in this country 
 a fitting occasion for the public acknowledgment of your 
 eminent scientific services. Accordingly a numerous body of 
 your friends aud admirers have united to present to you a 
 Testimonial, commemorative of their profound and unalterable 
 regard. In the list of subscribers hereto annexed, you will 
 recognise, with those of your pupils and personal friendSj 
 the names of many other gentlemen eminent in science, in 
 
HIS LIFE AND WORK. 203 
 
 social position, and in the practical arts of life, who were 
 anxious to join in this jnst tribute to your merit. 
 
 "In presenting to you this Testimonial, the subscribers 
 desire to express their sense of the benefits which your genius 
 and labours have conferred upon mankind in adding to the 
 world's stock of positive knowledge. These benefits are limited 
 to no one people or time ; but it is felt that Englishmen may, with 
 propriety, take the lead upon this occasion, as the impulse which 
 you have given to chemical science has been experienced 
 especially in England. More students from this country than 
 from any other laud beyond the bounds of Germany, have 
 worked in the laboratory of Giessen, and have derived incal- 
 culable benefit from the instruction there imparted, and from 
 the noble example there presented to them of an elevated philo- 
 sophical and scientific life. In England, also, have the applica- 
 tions which you have made of chemical science to the cultivation 
 of the soil been peculiarly appreciated and adopted. 
 
 " Your discoveries in practical agriculture have enriched the 
 laud, aiid with you originated the method of scientific inquiry 
 which is here pursued on an extended scale by numerous investi- 
 gators, and which is rapidly changing the features of the most 
 ancient and important of human arts. 
 
 " We earnestly hope that your life, which has been devoted to 
 the highest aims to which man can aspire, may be prolonged to 
 many years of happiness and honour. 
 
 " Signed on behalf of the subscribers, 
 
 "THOMAS GRAHAM. 
 
 " To Baron Liebig." 
 
 Liebig's reply : 
 
 "MUNICH, July 20th, 1854. 
 
 ' SIR, The man of science generally knows of no other 
 reward for the time he has devoted to the discovery of trnth 
 and to the investigation of the laws of Nature's powers, than 
 the mental satisfaction which springs from the consciousness of 
 having, to the best of his ability, contributed his part towards 
 the advancement of human happiness and human welfare ; for 
 toils like his, attended as they are with so many difficulties and 
 sacrifices, and with such mental effort, and fatigue, cannot be 
 priced in the market or sold cannot be performed to order, 
 
204 JUSTUS VON LIEBIG: 
 
 or turned into money. If he has been fortunate enough to have 
 gained by his successes the acknowledgment and esteem of his 
 contemporaries, he has obtained the highest object of his ambition. 
 
 " If I have laboured for the period of almost a human life in 
 promoting the progress of chemistry, and in making its principles 
 subservient and useful to other branches of knowledge, more 
 especially to the industrial arts and to agriculture. I gratefully 
 acknowledge that I have received in return all that a man could 
 justly aim at. My satisfaction in this respect is not a little 
 enhanced when I look back to the number of zealous and able 
 men in whose education I have been enabled to assist, and who are 
 now occupying, in various countries, a distinguished position in 
 the front rank of Science, and are, with splendid success, culti- 
 vating and extending her domains teaching, diffusing, and 
 successfully applying those principles of investigation which con- 
 stitute the true foundations of scientific progress. It is with 
 pride that I am able to add that in these my former pupils 
 I have gained an equal number of warm friends, who, I am 
 sure, look back with pleasure to the time when we combined 
 our powers in one common aim and effort. 
 
 " And now, in addition to all that a benevolent destiny had 
 already granted me in measure above many, I receive from my 
 friends in England, in this gift of friendship, in this testimonial 
 of honour, a token and a proof of their recognition and approbation 
 of my labours. 
 
 " When I reflect that whatever of good a man accomplishes 
 flows from an inner impulse of which he is often but imperfectly 
 conscious, and that a higher power has a part in all his labours 
 and usefulness giving to them their life-germ and their capacity 
 of growth, I must own that in receiving this noble Present I am 
 blessed far beyond my deserts. 
 
 "I feel myself in the highest degree honoured and most 
 deeply touched by this substantial and permanent expression of 
 the kind feelings of my friends in England. Convey to them 
 all my best and warmest thanks. This Gift of Honour possesses 
 for me inestimable value, and will remain a lasting memorial in 
 my family. " DR. JUSTUS VON LIEBIG. 
 
 "To Thomas Graham, Esq." 
 
 In his private life Liebig's character showed the 
 
HIS LIFE AND WORK. 205 
 
 same simplicity and nobility that distinguished him 
 in his relations with Faraday, Wohler, and Dumas. 
 His intercourse with his pupils was marked, they tell 
 us, by a dignified composure combined with a marked 
 simplicity and kindness which could not fail to en- 
 courage the most timid beginner ; whilst he gave to 
 the assiduous worker a degree of helpful sympathy 
 which led him to make any sacrifice, and lasted long 
 after the period of personal intercourse. Nor did he 
 reserve his interest and support for his own pupils 
 alone. His generous instincts, which led him to help 
 these so unstintedly, impelled him also to give counsel 
 or assistance Avherever he recognised that it was 
 deserved. Thus, when Sir Henry Roscoe was a 
 candidate for the professorship of chemistry at 
 Owens College Liebig who had become acquainted 
 with Mr. Roscoe on the occasion of one of his visits 
 to England hearing of his candidature at a moment 
 when he was about to depart on a journey, when the 
 carriage was, indeed, standing at his door, at once 
 delayed his departure hi order to write a few 
 words in support of the young Englishman, of whom 
 he had formed a high opinion. 
 
 One more illustration of the generous disposition 
 which prompted Liebig to help, whenever he saw that 
 help was needed, whether by a friend or by a stranger, 
 will be welcomed, as it shows so well his genuine 
 goodness of heart. Shortly after his call to Munich 
 (1853), Liebig, in company with Hofmann and two 
 other friends, was making, as Hofmann has told us, 
 an excursion among the mountains of the Tyrol. 
 
 One morning, in the course of their walk they 
 came upon an old soldier, wasted by fatigue and en- 
 feebled by disease, a pitiable object, travelling slowly 
 
206 JUSTUS VON LIEBIG : 
 
 along. As they came up to him he accosted them 
 with a piteous tale, asking their aid. Liebig, whose 
 purse was always as readily opened as his heart, and 
 his friends soon made up a little stock of money for 
 this poor wayfarer, which doubtless seemed to him 
 a small fortune. They, passing on, soon reached the 
 next village where they were to rest and dine, leaving 
 the traveller to follow at his slower pace, and presently 
 he, too, was observed to enter the inn. Their own 
 meal finished, his benefactors settled down to a siesta 
 before continuing their journey, soothed by the re- 
 flection that for once the poor soldier also had the 
 means to procure himself a substantial meal. 
 
 Half an hour later Hofmann awoke. But where 
 was Liebig ? There were his two fellows fast asleep. 
 But where was their leader ? Surprised by his dis- 
 appearance, Hofmann at once got up and inquired 
 of the landlord what had become of his friend, " the 
 elderly, spare man of the party." The landlord told him 
 that a little while before the gentleman had inquired 
 for a pharmacy ; and on being told that there was 
 none nearer than that in the next village over the 
 hill, he had set out in that direction. By no means 
 without anxiety Hofmann forthwith set out along 
 the road Liebig had taken, and presently saw his 
 figure on the brow of the hill. Hurrying forward, 
 he soon met him, and then learned that he had 
 noticed in the soldier symptoms of low fever, such 
 as quinine was certain to cure, and had been to the 
 nearest pharmacy to get some. 
 
 On his arrival he had found the pharmacy closed, 
 and the apothecary away, but his wife had given 
 Liebig the run of the place and allowed him to take 
 what he liked and fix his own price. Fortunately 
 
HIS LIFE AND WORK. 207 
 
 quinine was in the stock, and so his object was secured, 
 and soon the wayfarer was provided with a box of 
 powders which were probably sufficient for his cure. 
 
 The incident was simple enough. But how many 
 of us at fifty would have undertaken this extra tramp, 
 after a midday dinner and a long walk, with another 
 walk before us, in order to afford succour to a wayside 
 beggar ? Could even those whom Liebig belaboured in 
 the arena of scientific controversy help loving such a 
 man when they came to know him, or fail to forgive 
 him if in his ardour in the support of what he con- 
 sidered to be true, he sometimes exceeded the bounds 
 of courtesy in scientific warfare ? 
 
 Liebig was probably never what we should call a 
 robust man, though he possessed undoubtedly very 
 remarkable powers of sustained work of the most 
 exhausting kind. In his letters to Wohler complaints 
 of ill-health were certainly frequent ; but from the 
 amount of work he got through, it seems likely that 
 he suffered rather from occasional malaise than from 
 actual illness. For many years it seems pretty certain 
 that he was always somewhat overworked, for though 
 he was frequent in his injunctions to his friend 
 Wohler to " throw writing to the devil," he himself 
 never failed to respond to any call that was made 
 upon him to do work of the literary kind. And we 
 know, too, from Wohler's letters that the discussions 
 in which he shared were no play to him, but, while 
 they lasted, grim reality, and a source of considerable 
 strain, which told upon his physical health. We must 
 not regret this, however. Our science would have 
 been less forward to-day if Liebig had not thrown 
 his whole soul into those battles of the giants which 
 distinguished the middle of the present century. 
 
208 JUSTUS VON LIEBIG: 
 
 After twenty-eight years at Giessen, Liebig was 
 called to Munich in 1852. And there, in spite of an 
 invitation to Berlin in 1865, he worked till the end. 
 At Munich Liebig made no fresh great departures, but 
 he continued to work, following the lines he had 
 already laid down. The change from quiet Giessen to 
 Munich seems to have been in every way grateful to 
 him. When he arrived there, he found the new 
 laboratory was barely roofed in, he was without 
 materials and apparatus, and there were new assistants 
 who had to be trained to his ways ; but, in spite of all 
 difficulties, he was soon able to start his first course of 
 lectures, which was attended by two hundred and 
 fifty students, and at Christmas he declared, in a 
 letter to Wohler, that " he had made a good exchange." 
 
 Liebig was persona grata at the Court of King 
 Ludwig, and at an early day was called upon to 
 lecture before his Majesty and some members of the 
 Royal Family. There was an explosion ; it looked like 
 a bad one. and Liebig, as he recovered from the effects, 
 was horrified to see blood streaming from the faces of 
 Queen Theresa and Prince Leopold. Fortunately their 
 injuries were trivial ; and their agitation soon subsided 
 when they found that Liebig, who, of course, had been 
 in the thick of it, was also not seriously damaged. 
 
 The advantages gained by the change to Munich 
 were twofold. First, there was the wider social life, in 
 which Liebig found much pleasure. Secondly, he was 
 no longer called upon to personally superintend the 
 work of large classes of students in practical chemistry. 
 And, besides, the greater leisure which he found in 
 his new post made it possible for him to enjoy, more 
 freely than before, the pleasures to be found in a 
 cultivated society, and to indulge a marked taste for 
 
HIS LIFE AND WORK. 209 
 
 general literature, which we first hear of in connec- 
 tion with the correspondence between Liebig and his 
 friend Platen. 
 
 As the years rolled by, Liebig, in spite of his pro- 
 found interest in his work in applied science, now and 
 then looked back with regret on the early days at 
 Giessen, when he laboured chiefly in pure science. 
 When discouraged, as he sometimes was, by the mis- 
 conceptions of his opponents, or by the slowness with 
 which many physiologists and agriculturists learned 
 what he had to teach them, he was now and then 
 tempted to envy Wohler, who had not plunged with 
 him into these troubled waters, but remained to 
 the end constant in his devotion to the study of 
 chemistry. It was in such a mood that in 1857 he 
 wrote to Wohler 
 
 " Your letters of the 5th and 14th affect me like a 
 story of the old times ; there is the old fire and youth, 
 and years which are gone and tones which have 
 sounded rise up and transport me to the early days of 
 our happy collaboration. You have kept your single- 
 mindedness, and have enjoyed ever-renewed happi- 
 ness ; but I seem to myself like a renegade, a deserter, 
 who has given up his religion and has nothing in 
 exchange. I have left the paths of science, and, in 
 my efforts to be of some use to agriculture and 
 physiology, I am rolling the stone of Sisyphus, 
 which ever falls back on my own head, and I despair 
 sometimes of the possibility of getting it on a firm 
 foundation. . . ." 
 
 Again, in the same year, he wrote : " I admire you 
 and your beautiful researches. How happy you are 
 in your province ! You are older than I, and yet I 
 am much less bright than you. Yon seem to me in 
 
 N 
 
210 JUSTUS VON LIEBIG: 
 
 your work like the man in the Indian story, from 
 whose mouth bunches of roses fell whenever he 
 laughed ; I, with the agriculturists, am condemned by 
 fate to bear water to the cask of the Danaids. All 
 I do is in vain. I trouble myself and consume my 
 best powers without gaining any result ; I have not 
 won a single voice for me and my principles by the 
 Chemical Letters." 
 
 It is needless to say that these expressions were 
 but the outcome of passing moods, and that, upheld 
 by a real inward conviction, Liebig never ceased to 
 declare what he believed to be the true lessons that 
 chemistry offered to agriculture and physiology. Nor 
 was he, as time went on, without abundant evidence 
 that his labours were appreciated; for, besides the 
 ordinary honours, which were literally showered upon 
 him, within a single year (1865) a call to Berlin would 
 have been his at a word, he was invited to visit 
 England at the cost of Parliament to give evidence 
 on the utilising of town sewage, and received a vote 
 of thanks from the Corporation of London for the 
 assistance he had given in regard to this difficult 
 subject: though it is to be feared that, in this case, 
 he must have been disappointed with the immediate 
 result of his efforts, for we know he hoped that England, 
 with its vast wealth, was about to set an example 
 to the other countries of Europe, and to teach them 
 how to make use of the vast stores of invaluable plant- 
 food which, under the name of sewage, is daily cast 
 irrevocably away by the great cities. 
 
 In 1867, after much hesitation, Liebig consented 
 to go to Paris as one of the Presidents of the great 
 Exhibition. In spite of his hesitation, this visit proved 
 a rich and great joy to him, as he told Wohler 
 
HIS LIFE AND WORK. 211 
 
 afterwards. The renewed intercourse with such 
 veterans as his old friends Deville, Fremy, Wurtz, 
 IVligot, and Chevreul was delightful. During 
 his visit he dined with Napoleon, whom he com- 
 mended afterwards as one who "could not only 
 talk, but listen." In exercising his function as a presi- 
 dent, he was much struck by the want of order in 
 the French committees, where the members did not 
 address the president, as in Germany and England, 
 but each other. This made his work difficult some- 
 times, but his " vice " helped him out, and so he got on 
 pretty well. It is interesting to notice that in this 
 year Liebig, for the first time, dismissed an assistant. 
 When he wrote at the time to ask Wb'hler to find 
 him a new one he paid a handsome compliment to 
 the pharmacists, saying that he would prefer " a 
 pharmacist, who is accustomed to order, cleanliness, 
 and has a sense of duty." 
 
 A large proportion of the numerous letters which 
 passed between Liebig and Wohler,and of those between 
 Liebig and Berzelius, have been published since Lie- 
 big's death the former under the editorship of the 
 late Professor Hofmann, who was assisted by Frau- 
 lein Emilie Wb'hler; the latter under that of Justus 
 Carriere. The earlier letters dealt very largely with 
 their scientific work, and will afford a happy hunting- 
 ground for future historians of chemistry, whilst his 
 correspondence with Reuning (1854-73) will be not 
 less interesting to agriculturists. His later letters 
 to Wohler, especially those written from Munich, and 
 some of the others were, however, much less ex- 
 clusively scientific. These throw a clear beam of light, 
 by which one gets some pleasant glimpses of the 
 simple and happy home-lives of these two men, 
 
212 JUSTUS VON LIEBIG: 
 
 and of their deep affection for each other. Very 
 many of the quotations already given are taken 
 from the letters of the Munich period. Individually 
 these letters are, most of them, very simple. Many 
 relate to little gifts of bock or cigars from one to the 
 other, or consist of discussions of plans for holidays, 
 or of short descriptions of past excursions, and such 
 like matters. In one we find Wohler's daughter 
 Fanny at Munich taking care of Liebig, and rendering 
 " invaluable" services by helping him to manufacture 
 bread on chemical principles. Several give us little 
 domestic details of this kind. Thus one shows us 
 something of the family anxiety for the fate of 
 Dr. Georg Liebig, who was in India during the 
 Mutiny, but who escaped, and not long afterwards 
 came home, married, and settled in the Fatherland. 
 From another we learn that all, or nearly all, 
 Liebig's grandchildren were the subjects of successful 
 experiments with his "infant's food," which is still 
 often found valuable by the doctors. In a few a 
 very few, happily we hear of visits from the Angel 
 of Death. The frequent references to his health make 
 it clear that in the later years Liebig's work was often 
 done in the face of great difficulties. More than 
 once his yearning for the sight of his old friend's 
 presence shows itself, and we find him giving voice 
 to the wish that his few remaining years could be 
 passed in Wohler's company. Early in 1861, when 
 he was sleeping badly, he wrote : " If only I had the 
 old wish to work, the old power to overcome 
 difficulties ! Write to me more often. Your chatter 
 gives me a cheerful day. When Pfeuffer comes to me, 
 he says to me frequently, ' Ah, you have had a letter 
 from Wohler ! ' " 
 
HIS LIFE AND WORK. 213 
 
 In 1870 Wohler, for the last time, proposed 
 a new joint research. But Liebig refused; he de- 
 clared himself unequal to what Wohler proposed. 
 " With my last," he says, " I concluded my course." 
 During 1871 his health was very poor, and it is plain 
 that he then sometimes felt the end was not very far 
 off. On the last day of this year he wrote to Wohler 
 the following touching words 
 
 " I cannot allow the year to pass without a sign of 
 my continued existence, We shall not for long be 
 able to send each other these happy wishes for the 
 new year ; but even when we are dead, and have long 
 been dust, may the bonds which have united us ever 
 keep us both in the remembrance of men as a not too 
 frequent example of two men who were faithful, 
 without envy and ill-feelings; who wrestled and 
 strove in the same arena, and were ever firmly knit 
 in friendship." 
 
 But though Liebig felt that he must now abandon 
 the hope of doing much more himself, his generous, 
 yet patriotic, address to the Bavarian Academy after 
 the close of the Franco- Prussian War shows us that 
 even in 1871 he still retained in a high degree his 
 old vigour and fire, and that, if he was sometimes 
 saddened by the thought that his own work was so 
 nearly done, he was still full of hope for Germany 
 and mankind. 
 
 During the winter of 1872 he continued to lecture 
 as usual ; in January, 1873, he even wrote that his 
 lectures were a refreshment to him ; and in the 
 following month he was well enough to be trying 
 experiments on the making of cyanogen and on the 
 feeding of pigs. But the end was now close at 
 hand. 
 
214 JUSTUS VON LIEBIG: 
 
 Liebig's last letter to Wohler was written on April 
 3rd of this year, and the last letter he received from 
 Wohler was written on the 7th day of the same 
 month. Liebig wrote : 
 
 " MUNICH, 3rd April, 1873. 
 
 " I intended to have written to you yesterday, but I had a bad 
 night, without any sleep, and the whole day I lay tired and 
 exhausted upon the sofa. I thought of you your good sleep, 
 your good appetite, the normal activity of all your functions. 
 "Would it be possible to perish in old age merely through 
 sleeplessness, without sickness ? It is the vegetative life, the 
 recuperation in the night. When this fails, the lamp is 
 gradually extinguished. I was not well a single day in 
 Wiesbaden, and feared to sojourn in the low ground; also many 
 other things did not agree with me there. I should not be 
 averse to go to Hanau to your brother-in-law. We might 
 af&rwards pass a few more days in the Bavarian Mountains. 
 My plan is to take leave from Easter onward, and to do nothing 
 for the half year. I have a great wish to go to Yienna ; from 
 thence to Magdeburg, to Rimpau ; then to Hamburg and Kiel, 
 to Meyer's. This is to hope that you will come with me. What 
 do you say ? I shall not go to a cure. It was no use to me last 
 year; and in Wildbad, for instance, walking through the streets 
 to the bath is not agreeable. 
 
 " We have heard with the greatest grief of the death of your 
 sister-in-law in Berlin : she has been, I hear, suffering for some 
 time. It is certainly my friend, General Hartmaun, who is 
 dead, the same who was with us in Reichenhall. He went to a 
 funeral in winter, and thence, as in so many cases, contracted 
 hi* fatal illness. 
 
 " With heartiest greetings to Fanny, 
 
 " Your faithful 
 " J. VON LIEBIG." 
 
 Wohler was not well enough to attempt to travel, 
 however, and on the 7th he replied as follows. This 
 was the last letter he wrote to Liebig : 
 
HIS LIFE AND WORK. 215 
 
 " WIESBADEN, April 7th, 1873. 
 
 " My best thanks for your letter of the 3rd. It is now a year 
 since we have seen each other, and it would distress me very 
 much if it should not come to pass that we meet this Easter. 
 But I fear extremely that I must resign myself to this. I could 
 not come to you this time, since I have first to consider the full 
 restoration of my strength, which I hope to attain in this mild 
 climate by a completely quiet life. I still hope that you will 
 accept the pressing invitation of my brother-in-law, and that we 
 mny meet in Hanau ; whereto your remark, ' I am not averse 
 to come there,' encourages me. You certainly seem inclined 
 for travel, but I am much surprised that you wish to go to 
 Yieuna, and to rush into this disagreeable, unsatisf}-ing, excit- 
 ing spectacle, the Exhibition. 
 
 " A f CAV days ago Buff with his wife took us by surprise ; 
 they stayed here two nights, and then journeyed to Crefeld, to 
 their son Henry. 
 
 " In answer to my questions, Lipsius writes that the eagles on 
 the captured French flags really consist of gilded aluminium, 
 a metal that was first produced in Berlin in 1827. Sic erunt 
 fata." 
 
 The visit to Hanau never took place. Liebig and 
 AY <>hler met no more, for Liebig died at Munich on 
 April 18th, 1873. 
 
 . -\^V\MiAA^^r^ ^ 
 
 vi 
 
INDEX. 
 
 Acids compounds of hydrogen, 
 48 
 
 Agricultural education and re- 
 search, 124 
 
 Agriculture, Japanese, 119 
 
 " , Perfect," 81 
 
 Agriculturists : Liebig's desire 
 they should think for them- 
 selves, 113 
 
 Air the source of carbon for 
 vegetables, 91 
 
 , Relation of plants and 
 
 animals to, 98 
 
 Alcohol, Nature of, 50 
 
 Aldehyde, Discovery of, 62 
 
 Ammonium thiocyanate, Action 
 of heat on, 44 
 
 Amygdalin, Nature of, 34 
 
 Animal bodies, Motion in, 153 
 
 body, Motion of juices in, 
 
 155 
 
 heat, 131 
 
 Animals need nitrogen, 138 
 
 Atomic theory, Brief account 
 of, 32, 49 
 
 Autobiography, 10, 12 
 
 Bavarian Academy, Address to, 
 198 
 
 Benzoyl, 33 
 
 Berzelius : description of his 
 laboratory, 26 ; electro- 
 chemical theory of, 56 
 
 Bischoff, Memorial address by, 
 
 10 
 
 Bonn, State of science in, 14 
 Boussingault, Work of, 86 
 Bromine, Failure to discover, 
 
 45 
 
 Carbohydrates, 85 
 
 Carriere on Liebig and Platen, 
 
 10 
 
 Chemical equivalents, 49 
 Chloral, Discovery of, 41 
 Chlorinated vinegar, 59 
 Chloroform, Discovery of, 41 
 Compound radicles, 33 
 Cookery, Chemistry of, 160 
 
 Davy, Lectures of, on agricul- 
 ture, 86 
 
 Dualism, 56 
 
 Dumas, Early life of, 51 ; cor- 
 respondence with Liebig, 51 ; 
 and Boussingault on relations 
 of plants and animals, 99 ; 
 meeting with Humboldt, 53 
 
 Early life of Liebig, 1 1 
 Education, Liebig's influence 
 
 on methods of, 173 
 English men of science, Liebig's 
 
 relations with, 199 
 Englishmen, Memorial from, to 
 
 Liebig, 202 
 
218 
 
 INDEX. 
 
 Enzymes, 78 
 Erlangen, Studies at, 15 
 Erlenmeyer, Memorial address 
 
 by, 10 
 Ether, Nature of, 50 
 
 " Familiar Letters on Chem- 
 istry," 10; origin of, 173 
 
 Faraday: friendship with Liebig, 
 199 ; letter from Liebig, 200 
 
 Fat, Production of, from carbo- 
 hydrates, 147 
 
 Fermentation, 64 ; Liebig's 
 theory of, 66 ; supposed in- 
 fluence of oxygen on, 70 ; 
 Vitalistic theory of, 72 ; 
 Pasteur's work on, 72 
 
 Ferments, 65 
 
 Food of plants, Source of, 116 
 
 , Respiratory, 141 ; non- 
 nitrogenous, function of, 144 
 
 Foods, Classification of, 141- 
 145 ; plastic, 141 
 
 Formulae, Method of checking, 
 159 
 
 Gay-Lussac, Work with, 18 
 Gerhardt. Contest of, with 
 
 Liebig, 43, 44 
 
 Giessen, Appointment to pro- 
 fessorship at, 19; the teach- 
 ing at, 175 
 
 Glucosides, Discovery of, 35 
 " Ground absorption," 110 
 
 Hofmann : Faraday lecture, 10 
 Humboldt : kind assistance to 
 Liebig, 18 
 
 Humus, 88 ; real use of, 98 
 Husbandry, Natural laws of, 
 113 
 
 Isomerism, Discovery of, 29 
 Kastncr, 14 
 
 Laboratory, The Giessen, 175 
 Land, Deterioration of, 115 
 Lavoisier on relations of plants 
 
 and animals, 99 
 
 Liebig a pioneer in science, 20 ; 
 " Condenser," the, 24 ; and 
 Wohler, friendship of, 36; 
 character of, 37; importance 
 of what he did for physiology, 
 152-170 ; the first " extension 
 teacher," 173 ; as a teacher, 
 178; literary work, 195; 
 memoirs by, 195; dominant 
 characteristics, 197 ; and his 
 pupils, 205 ; last years, 209 ; 
 letters, 211; last letter to 
 Wohler, 214 ; last letter from 
 Wohler, 215 
 
 Meat, Extract of, 163 
 
 extract, Value of, 165 
 
 , Roast, why superior to 
 
 boiled, 162 
 
 Memorial addresses, 10 
 
 Mineral manures, Experiments 
 with, 107; difficulty of apply- 
 ing, 109 
 
 Minerals in plants, error con- 
 cerning their origin, 103 
 
 Mirrors, Silvering of, 63 
 
JUSTUS VON LIEB1G. 
 
 219 
 
 Molecular weights, Methods of 
 finding, 23 
 
 Mulder : similarity of nitro- 
 genous components of plants 
 and animals, 136 
 
 Munich, Life at, 212 
 
 Nitrogenous constituents of 
 
 plants and animals, Similarity 
 
 of, 134 
 
 foods, Necessity of, 138 
 Nitrogen, Sources of, for plants, 
 
 101-106; elimination of, by 
 
 animals, 153 
 
 Oil of bitter almonds, 33 
 Organic analysis, Method of, 21 
 - Chemistry, Export on 
 Present State of (in 1840), 81 
 
 Paris, Opportunities for prac- 
 tical study at, 17 ; studies and 
 experiences at, 17; teaching 
 at, 17; visit to, in 1867, 210 
 
 Plants and animals, Relations 
 of, 84, 138 
 
 , Components of, 100 ; 
 
 produce the blood of animals, 
 135 ; "accumulation offeree " 
 by, 143 ; their mineral com- 
 ponents, 107 
 
 Platen : references to Liebig as 
 a youth, 15 
 
 Potassium cyanide, Method of 
 making, 40 
 
 Reading, Liebig's early, 13 
 Roscoe, Obituary notice by, 10 
 
 Rotation of crops, Effect of, on 
 soil, 120 
 
 Scientific disputes, Object of, 
 
 43 
 
 Soup, 163 
 Spontaneous combustion, Views 
 
 on, 189 
 Substitution, Discovery of, 58 
 
 Technical education, what it 
 means, 185-187 
 
 Tyrol, Characteristic adven- 
 tures in, 205 
 
 Universities, German, How re- 
 search is taught at, 177 
 Uric acid, Investigation of, 35 
 
 Vegetables and animals, Rela- 
 tions of, 84 
 
 Vegetable tissues, Components 
 of, 85 
 
 Vinegar plant, 75 
 
 " , Quick, process," 71 
 
 Vital force, 167 
 
 Vogel, Memorial address by, 10 
 
 Wander- Year, 14 
 
 Windier, S. C. H., letter, The, 
 
 60 
 Wohler, Early life of, 25 ; and 
 
 Liebig, meeting of, 29 ; and 
 
 Liebig begin joint work, 30 ; 
 
 character of, 37 ; letter from, 
 
 37 ; work on urea, 129 ; 
 
 friendship with, in late years, 
 
 209 
 
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