566 L I E B I G
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 in
teresting 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 dis
coveries 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 M. 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. Ampère 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 Ampère, 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 Wöhler 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 5 (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 com
pounds. The importance of this investigation was generally recog
nized. Berzelius hailed it as the advent of real daylight on the
subject of organic chemistry, and even suggested the names prom
or orthrin (from (
Greek characters) and (Greek characters)) 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 defiant 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: –
Base.
Acid.
Water.
NH 3
HOI
C,H 4
HC1
Nitrite of ammonia
2NH 3
N 2 3
HoO
2CJI 4
N 2 0g
II..O
Ether ..
2C.,H 4
H.>O
CoHj
11 20
NHj
2II..O
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 sal- ammoniac, not as NH 3 ,HC1, but as NH 4 ,C l. 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 docs not. According to Dumas and Boullay alcohol and ether are both hydrates, but in Liebig's view ether is (C 2 H B ) 2 and alcohol (C 2 H 5 2 0,H 2 0. 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 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 Regnault 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-
