LECTURE III.

Recapitulation-Statical and dynamical aspects of organic chemistry— Destruction and construction of constituent molecules-Tendency of oxidation to produce molecules with fewer and fewer carbon and hydrogen atoms-Final production of carb-anhydride CO2, and water H2O—Destructive or analytic phase of organic chemistry—Natural synthesis of organic compounds attended by deoxidation-Liberation of oxygen by growing vegetables-Tendency of deoxidation to combine separate carbon and hydrogen atoms into complex molecules-Vegetable tissue and secretion formed by deoxidation of carbonic anhydride and water-Imperfect knowledge of intermediate products— Formation of nitrogenised tissues-Ammonia in its relation to plant life-Correlations of ammonia, nitrous acid, and nitrogen-Deoxidation of nitrous acid by plants-Manurial equivalency of nitrous acid and ammonia-Existence of nitrogen in natural organic products as a residue of ammonia-Artificial synthesis of organic bodies-Combination of constituent molecules with one another-Elementary formation of constituent molecules-Historical remarks on organic synthesisAlleged incompetency of chemical, and necessity for vital action— Artificial production of all organic compounds by purely chemical means--Kolbe's indirect formation of acetic acid from carbon, hydrogen, and oxygen, in 1845-Subsequent advances by Berthelot and others-Oxidation of hydrogen into water, and of carbon into carbonic anhydride-Evolution of light and heat-Deoxidation of water and carbonic anhydride into hydrogen and carbon-Similar separations of oxygen from hydrogen and carbon effected by living plant and by artificial processes-Comparison of deoxidising vegetal and oxidising animal functions-Nature of forces concerned in respective actions.

(44.) I STATED in my last lecture that chemists were acquainted with a great number of monobasic organic acids, containing two atoms of oxygen in their respective molecules, and that these acids were capable of being arranged in two principal classes,

known as the aromatic class and the fatty class, and exemplified in

The acids of these two series presented, I told you, a marked parallelism in their constitution, seriation, and properties; and, moreover, when submitted to the action of the same chemical reagents, underwent precisely analogous metamorphoses. I dwelt still more upon the mutual resemblance manifested by consecutive members of the same series, and pointed out that even the most remote members were distinguished from one another by gradational differences only. I observed, also, that each one of these primary monobasic acids, fatty or aromatic, was associated with a more or less complete set of congeners, which differed from it in constitution and properties, but were correlated with it by the circumstance of their containing the same number of carbon atoms, and still more markedly by their derivation from, and convertibility into it and one another-that acetic acid C2H4O2, for instance, was associated with the less oxidised bodies, olefiant gas C2H4, alcohol C2H6O, and aldehyd C2HO, as well as with the more highly oxidised glycolic and oxalic acids, C2H4O ̧ and C2H2O4, respectively—to such an extent, indeed, that they might all be regarded as varieties of one and the same primitive molecule. I further went on to say that the complex tissue products of the animal and vegetable kingdoms were built up of the residues of these fatty and aromatic acids, and of their respective congeners; so that, upon breaking up such tissue products into their constituent molecules, we were, in the great majority of instances, able, even at the present time, to refer the constituent

molecules to their appropriate positions in certain definite series and groups; and had every reason to believe that with increasing knowledge we should be able to make the assignment in every instance.

(45.) Moreover, in my first lecture I pointed out to you that organic chemistry had a statical aspect which related to the composition of bodies, and a dynamical aspect which related to their changes of composition. But in all that I have hitherto observed I have had regard principally to the statical aspect of the question. I have glanced, indeed, at the mutual metamorphosis by oxidation and deoxidation of compounds belonging to the same natural group; and have referred more fully to the combination of their several residues with one another in forming complex tissue products, and to the separation of the completed residues from one another in the breaking up of these products; but I have not yet considered the mode in which the primary constituent molecules are themselves produced, or yet the mode in which, when once produced, it is possible for us to destroy them, and to these points I will now direct your attention.

(46.) If we treat the more complex members of our series of fatty acids, for instance, with powerful oxidising agents, we obtain bodies in which the number of the constituent atoms of hydrogen and carbon becomes progressively less and less, until we arrive at bodies containing only two, and finally at bodies containing only one carbon-atom. In some cases these successive oxidation products are found to contain the same number of atoms of oxygen as the bodies from which they were produced, though in the majority of instances they contain a greater number, and consequently belong to more oxygenised series. But whether they contain the same or a greater number of oxygen atoms, we find the number of their atoms of carbon and hydrogen become gradually less and less, their molecules pertaining to simpler and simpler groupings. For example, the following intermediate compounds, among many others, have been successively obtained by oxidising stearic acid C18H36O2, with nitric acid of moderate strength: :

(47.) The tendency of oxidation, then, is to separate the constituent carbon and hydrogen atoms from one another, until at last there is left only the most stable mono-carbon compound known to chemists, namely, carbonic anhydride, or, as it is frequently called, carbonic acid. No matter what the complexity of the original molecule, the chemist eventually succeeds in transforming it by oxidation, though a series of less and less complex molecules, into carb-anhydride, or oxide of carbon, on the one hand, and water, or oxide of hydrogen, on the other-the identical bodies out of which the vegetable organism directly, and the animal organism indirectly, builds up those complex bodies which we have designated proximate organic principles. As was observed by Gerhardt some twenty years ago, 'one of the two extremities of the scale of organic compounds is occupied by albumin, and gelatin, and fat, and cerebral matter; the other extremity by carbonic acid, and water, and ammonia; while an infinity of bodies are included in the interval. The chemist, by treating the superior substances with oxidising agents, gradually descends the scale of complexity, converting these substances into more and more simple products, by successively burning off a portion of their carbon and hydrogen.'

(48.) Thus, then, we have presented to us one important aspect of organic chemistry, namely, its analytic or destructive aspect; that aspect upon which, until of late years, the attention of chemists was almost exclusively directed; that aspect, indeed, which was at one time considered to be the only possible aspect that could ever be presented. To quote again from the same distinguished chemist, of whom I am always proud to avow myself

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a pupil: 'I show,' said Gerhardt, writing in 1842, 'how the chemist does everything that is contrary to living nature—that he burns, destroys, works by analysis—the vital force alone operates by synthesis and reconstructs the edifice destroyed by chemical forces.' But, in reality, there is another side to the shield; there is a constructive as well as a destructive, a synthetic as well as an analytic organic chemistry; and to this view of the subject I will now direct your attention.

(49.) I need scarcely remind you of the mode in which vegetable structures are built up. The minute seed grows into the gigantic tree, the great mass of which is made up of carbon, hydrogen, and oxygen, that the living organism has stored up from the carbonic acid and water with which it has been supplied throughout the period of its existence, and which it has inter-combined into the various forms of vegetable tissue. Now, this storing up of carbon, hydrogen, and oxygen-this formation of vegetable compounds-is attended throughout by an evolution of oxygen. The proportion of oxygen contained in carbonic acid and water being greatly in excess of the proportion contained in vegetable tissue and secretion, we have throughout the growth of every plant a constant deoxidation of carbonic acid and water— the carbon, hydrogen, and necessary oxygen being retained in the substance of the plant, the oxygen in excess of the requirements of the plant being discharged into the atmosphere. Let me recall to your recollection one of the original experiments of Priestley upon this subject. He showed, for example, that under exposure to sunlight, a quickly-growing leafy plant, immersed in an atmosphere which by the combustion of fuel had been freed from oxygen and charged with carbonic acid, gradually restored that atmosphere to its pristine condition, by an absorption and subsequent decomposition of its carbonic acid into oxygen gas evolved from the leaves, and carbon retained within the vegetable structure. Here we have an imitation of the experiment. Α bunch of fresh mint has been thrust into this narrow cylinder of dilute carbonic acid water standing in the small pneumatic trough, and the whole exposed to sunlight. You perceive that