The amount and the importance of the laboratory work done by Liebig in Giessen were very great Without considering here the work done by his students under his direction, of which no doubt a very large part was conceived by him, and in the execution of which he constantly contributed his assistance and advice, we shall look only at what appears under his own name. During the twenty-six years he spent at Giessen as ordinary professor, he contributed to scientific journals more than two hundred papers, about twenty of which were records of joint work, chiefly with Wöhler. During the same time he published his works on organic analysis, organic chemistry, chemistry applied to physiology and agriculture, his Chemical Letters, and many smaller treatises. From 1832 he was joint editor of the Annalen der Pharmacie, from 1837 of the Handwörterbuch der reinen und angewandten Chemie, and from 1847 to 1856 of the Jαhresbeι∙icht der Chemie. These statements give some idea of the amount of his work; of its importance and of its effect on the history of science we shall speak later.

In 1845 he was raised to the hereditary rank of baron under the title of Freiherr von Liebig. In 1852 he accepted the invitation of the Bavarian Government to the ordinary professorship of chemistry in the university of Munich. This office he held till his death in 1873.

In private life Liebig was hospitable, courteous, and kindly. Honoured by all the great scientific societies of the world, and regarded by almost every one as the great authority in chemistry, he assumed no airs of superiority, and lived the simple and quiet life of a German professor. Liebig’s influence on the history of chemistry may be considered under five heads :—(1) the effect of the opening of the Giessen laboratory, and of Liebig’s constant efforts to induce other universities to follow this example; (2) the improvements introduced by him in methods of investigation and in apparatus; (3) the discovery of new facts; (4) the development of theory; and (5) the application of chemistry to physiology, agriculture, and the arts.

We have already spoken of the first. Under the second head by far the most important change introduced by Liebig was his method of organic analysis. Organic substances were analysed, and analysed with accuracy, before 1830, but such analyses could then be carried out only by highly skilled chemists, and involved great labour and the use of cumbrous apparatus. Liebig’s method of organic analysis, which was published in all its details in 1831, and which (with important but secondary improvements) is that still used, made it easy for any advanced student to make a fairly accurate and very useful analysis of an organic substance. Analysis is to the chemist what astronomical methods for determining longitudes and latitudes are to the geographical explorer. Without it many interesting and useful discoveries may be made, but it is only when complete and accurate analyses are made of all the new substances produced in the course of a research that the research becomes fully available to other explorers. If Liebig had contributed nothing to organic chemistry but his method of analysis, he would still have been in a perfectly true sense the founder of modern organic chemistry. Many other improvements of apparatus are due to him; we need only mention the simple form of condenser called by his name, and constantly used by every chemist, and the easy and accurate method for determining the quantity of urea in a solution, which was the first step towards introducing precise chemical methods into practical medicine. This is also the proper place to refer to his analyses of the natural alkaloids» and his discovery of the method of determining their equivalents by the analysis of their chloroplatinates. In the third place we have to consider the new facts discovered by Liebig. The very great addition to our knowledge of organic chemistry made by Liebig naturally throws into the shade his contributions to inorganic chemistry, but we ought to remember his numerous analyses of mineral waters and his contributions to the difficult question of the accurate separation of cobalt and nickel. It is, however, in organic chemistry that Liebig’s great discoveries were made. These discoveries are so intimately connected with his chemical theories that we may most conveniently consider them along with the fourth head, his contributions to the development of chemical theory.

The notion of compound radicals is to be found in chemistry as far back at least as the time of Lavoisier. Lavoisier says, “Some experiments of my own and some made by Μ. Hassenfratz have convinced me that in general nearly all the vegetable acids, such as tartaric, oxalic, citric, malic, acetic, pyrotartaric, pyromucic acids, have for their radical hydrogen and carbon, but united so as to form a single base, that all these acids differ from one another by the difference in proportion of these two substances and the degrees of oxidation.” Berzelius adopted this view and expressed it thus,— “We find the difference between organic and inorganic bodies to be that, while in inorganic nature all the oxidized substances have a simple radical, all the organic substances have compound radicals. Just as ammonia is an alkali with a compound radical, but possessing nevertheless the greatest analogies with the mineral alkalies which have simple radicals, so we find the greatest analogy between the mineral and the organic acids, so that the relations of potash and soda to acetic, oxalic, citric acids are the same as their relations to sulphuric, nitric, phosphoric acids.” These views were published by Berzelius in 1817. In 1815 Gay-Lussac had discovered cyanogen, and shown that this compound of carbon and nitrogen is the radical of prussic acid and its salts, in the same sense as chlorine is the radical of hydrochloric acid and the chlorides. Ampere had indicated a theory of the constitution of the ammonia salts, which Berzelius worked out in detail, according to which those salts contain a compound radical, ammonium, playing in them the part of potassium in the potash salts. Finally, Davy suggested in 1815 that the hydrated acids which correspond in function to hydrochloric acid should be regarded as the true acids, and proposed to represent them as compounds of hydrogen with a radical.

Such were the knowledge and the theoretical position of chemists on this question when Liebig became professor; the dates to which we have referred correspond to the time when as a schoolboy he was devouring the contents of chemical journals at Darmstadt, and no doubt he then became acquainted at first hand with the discoveries and speculations of Ampere, Davy, Gay-Lussac, and Berzelius.

We have seen that his first investigation referred to the fulminates. He continued his researches upon the compounds of cyanogen and the substances connected with them formed frequent subjects of his researches during his whole life. In this region of organic chemistry he made many important discoveries, of which the limits of this article do not allow a detailed account; we can only mention melone, mellam, ammeline, ammelide, and melanine, as substances discovered and investigated by him. In the course of these investigations he discovered the precise nature of the chemical changes which occur in the manufacture of bromide of potash. In 1831 he examined the action of chlorine upon alcohol, among other substances discovered chloral, and commenced the series of investigations into the derivatives of alcohol and ether to which we shall refer immediately. In 1832 Wohler and Liebig published the results of their joint research on the oil of bitter almonds and its derivatives. This research may be said to stand at the head of modern organic chemistry. For elegance of method and for clearness of insight it is unrivalled, and will always remain a model of what such an investigation should be. They showed in the clearest manner that the compound C 7 H 8 O (here and elsewhere in this article we use the symbols now in common use, instead of those employed by the authors), to which they gave the name of benzoyl, is the constant part, or radical, of a great series of compounds. The importance of this investigation was generally recognized. Berzelius hailed it as the advent of real daylight on the subject of organic chemistry, and even suggested the names proin or orthrin (from πρωί and Hpθpos) for the newly discovered radical.

We now come to the long controversy as to the constitution of alcohol and ether, which engaged so much of Liebig’s time and energy. Gay-Lussac had shown in 1815 that alcohol and ether might, as far as their composition is concerned, be represented as compounds of olefiant gas and water, and further that, if we represent the quantities by volume of gas or vapour, ether contains one volume of olefiant gas and half a volume of water vapour, while alcohol contains equal volumes of the two. In 1828 Dumas and Boullay published an elaborate memoir on the preparations and properties of the ethers, in which they further developed the ideas suggested by Gay-Lussac. They regarded olefiant gas as the radical of all the etherial compounds, as ammonia is of the ammoniacal salts, and formulated them thus :— [table]

We have given only a sample of their tables, leaving out among others some substances in reference to the composition of which they had fallen into error, error which Liebig detected and used as an argument, valid enough then no doubt, but of little interest now.

In 1833 Liebig proposed a quite different theory, one which stands to Dumas and Boullay’s in the same relation which the ammonium theory holds to the ammonia theory of the constitution of the ammoniacal salts. Just as Ampère and Berzelius regarded salammoniac, not as NH 3 ,HC1, but as NH 4 ,C1, so Liebig proposed for muriatic ether the formula C 2 H 5 ,C1 instead of C 2 H 4 ,HC1, The really cogent argument which he brought forward, the argument which we can now best appreciate, is that, while alcohol contains combined water, ether does not. According to Dumas and Boullay alcohol and ether are both hydrates, but in Liebig’s view ether is (C 2 H 6 ) 2 O and alcohol (C 2 H 5 ) 2 O,H 2 O. If we wish to understand this argument we must recollect that to the chemists of that time oxidized hydrogen was water, and Liebig’s arguments are as sound now as they were then, for the most recent views represent alcohol as C 2 H 5 —0—H, and this contains oxidized hydrogen, or, as we may say, half a molecule of water, and Liebig’s tests do not distinguish preformed water, but hydrogen and oxygen combined as they are in water. Much debate and investigation followed, in the course of which the relations of all the substances derived from alcohol were thoroughly and practically studied in a fuller and more careful manner than would have been possible had there not been a theoretical point to defend and to attack. The enormous number of facts discovered by Liebig put him in" a very favourable position as the advocate of his theory. Chemists could not do without a knowledge of these facts, and they could only get this knowledge through Liebig’s papers, in which the facts were expressed in the language of his theory.

In 1835 Régnault began a series of most important researches into the compounds derived from olefiant gas. He showed that many of these substances might be classified and their relations explained by the assumption of a radical C 2 H 3 , to which he gave the name of aldehydene. Liebig saw that according to his view of the meaning of the word radical, as “the unchanging constituent in a series of compounds,” the same radical may be assumed in alcohol and in acetic acid, and in 1839, in a note published in the Annalen, he puts the matter in a very clear light. He says, “Ether and ammonia have in their compounds a certain resemblance which was·

first indicated by Dumas and Boullay, and the view of the constitution of the ammonia salts which is generally held in France was the reason why ether was considered the first hydrate of olefiant gas, alcohol as the second hydrate, &c.; in Germany and other countries the water necessary for the constitution of the salts of ammonia with oxygen acids was considered as an integral part of the base; it was assumed that this water forms with the ammonia oxide of ammonium (NHA 2 O, and this view in a certain sense smoothed the way for another, according to which the existence of organic oxides, capable of neutralizing acids, appeared very probable, as a necessary complement to the organic acids which chemists had long been inclined to regard as oxides of organic radicals. Ether was in these countries regarded as an organic oxide, and this difference of view excited a ten years’ strife, as an immediate result of which· we may regard the discovery of a great number of compounds which enriched science with innumerable important observations. No region of organic chemistry has been so thoroughly and so completely studied as the compounds connected with ether; and now, when the existence of organic oxides is no longer denied, the support of the opposite opinion has come to an end, although it cannot be said that the question itself has been experimentally decided. If we compare in the light of our present knowledge the ammonia compounds with the ether compounds, we at once see that the opposing views were fundamentally the same in the two cases. The disputes took place because we were not at one as to the interpretation of the phenomena. The etherand ammonia compounds assume in fact the same form if amidogen is regarded as the unchangeable radical of the ammonia compounds, and acetyl [Regnault’s aldehydene] as the starting point of the ether compounds. The two sets of compounds differ only in so far as we must ascribe to acetyl the power of forming acids, a power which amidogen does not possess?’

He then gives a table containing in two columns the ammonia and the ether compounds, in which C 2 H 3 corresponds to NH 2 , C 2 II 4 to NH 3 , and C 2 H 5 to NH 4 , and adds, “These formulae require no explanation; they have been developed in order to show the extraordinary resemblance of the ammonia and the ether compounds, and to show why it was that many chemists regarded olefiant gas as the first member of the series of ether compounds. Both of the formerly antagonistic theories have, as may be easily seen, from this point of view the same foundation, and all further questions as to the truth of the one or other view is thus of course set at rest.”

It was during the course of the controversy which then closed that Liebig had discovered aldehyde, chloral, and, simultaneously with Soubeiran, chloroform, besides numerous other substances of less general interest, and developed the theory now generally received of the formation of ether by the action of sulphuric acid on alcohol.

In the very short sketch given above of the discussion as to the constitution of ether, we mentioned that Liebig’s ethyl theory was to some extent borrowed from Berzelius. Berzelius had suggested that ether was the oxide of a radical (Liebig’s ethyl), but he was at first inclined to regard alcohol, not as the hydrate oxide of the same radical, but rather as the oxide of another one, which with our symbols would be written C 2 H β ∙ But there was a deeper difference than...