Liebig, who was one of the first chemists to dispute the correctness of Berzelius’ hypothesis, by showing that the natural organic bases never gave any indication of the presence in them of ready formed ammonia, replaced it by the suggestion that they might be bodies into the composition of which amidogen (H2N) entered, and that these, instead of being compounds of ammonia and an organic group, might be derivatives from ammonia; or ammonia in which an atom of hydrogen had been displaced by an equivalent organic radicle.
The labours of subsequent chemists, notably those of Messrs Wurtz and Hofman, have developed Liebig’s theory, and have proved the analogy in structural arrangement between ammonia and the greater number of organic bases; whilst they have further shown, not only in one, as supposed by Liebig, but for all three of the hydrogen atoms in ammonia, may be substituted certain compound radicles.
ORGAN′IC SUBSTANCES. We have reserved a notice of the method of estimating the quantity of carbon, hydrogen, oxygen, and nitrogen, in organic compounds, until now, in order to present them to the reader in a more useful and connected form. The operation essentially consists, in respect of the first three, in causing the complete combustion of a known quantity of the substance under examination, in such a manner that the carbonic acid and water thus produced shall be collected, and their quantity determined. From these the proportions of their elements are easily calculated. The estimation of the quantity of nitrogen (as is also the case with chlorine, phosphorus, sulphur, &c.) requires a separate operation. The two great classes of organic bodies (azotised and non-azotised) are readily distinguished from each other by heating a small portion with some solid hydrate of potassium, in a test tube. If nitrogen is present, it is converted into ammonia, which may be recognised by its characteristic odour and its alkaline reaction.
1. Estimation of the CARBON, HYDROGEN, and OXYGEN.—a. The method of Prof. Liebig, now almost exclusively adapted for this purpose, is as follows:—The substance under examination, reduced to powder, is rendered as dry as possible, either by the heat of a water bath or by exposure over concentrated sulphuric acid, in vacuo; 5 or 6 gr. of it are then weighed in a narrow open test tube, 2 or 3 inches long, and to ensure accuracy this tube and any little adhering matter is again weighed after its contents have been removed—the difference between the two weights being regarded as the true quantity of the substance employed in the experiment. A ‘combustion tube,’ of hard white Bohemian glass (·4 to ·5 inch diam.; 14 to 18 inches long), is next taken, and about 2-3rds filled with black oxide of copper, prepared by the ignition of the nitrate, and which has been just re-heated to expel hygrometric moisture. Nearly the whole of
this oxide, whilst still warm, is then gradually poured from the tube and triturated with the organic sample in a dry and warm mortar, after which the mixture is transferred to the combustion tube, and the mortar being rinsed out with a little fresh oxide, which is added to the rest, the tube is, lastly, nearly filled with some warm oxide fresh from the crucible. The contents of the tube are next arranged in a proper position by a few gentle blows, so as to leave a small passage for the evolved gases from the one end of the tube to the other. (See engr.)
The ‘combustion tube’ with its ‘charge’ is next placed in a ‘furnace’ or ‘chauffer’ of thin sheet iron (see middle figure above). Its open end is then connected with a ‘drying tube’ filled with fragments of fused chloride of calcium, and carefully weighed. This tube is, in its turn, connected with a small glass (‘Liebig’s potash bulbs’) containing solution of pure potassa of sp. gr. 1·27, also carefully weighed. The junction with the first is made by means of a perforated cork; that with the second by means of a small tube of india rubber tied with silk, the whole being made quite air-tight. The apparatus is then tested by sucking a few bubbles through the liquid with the dry lips, when, if the level of the solution of potassa in the two legs continues unequal for some minutes, the joints are regarded as perfect. The whole arrangement