The burning of the substance must be perfect ; there must be no production of compounds intermediate between the original substance on the one hand, and tho carbonic acid and water on the other. That the oxygen of the air is insufficient for this purpose is evident, when we remember that smoke is nearly always produced under such circumstances, and smoke is partly composed of those inter mediate bodies, the production of which it is indispensable to avoid. The desired burning agent must, then, be one that will so readily yield its oxygen to organic matter at an elevated temperature, as to ensure complete oxidation of the substance to carbonic acid and water ; at the same time it must be itself unalterable by the high heat employed. These apparently somewhat antagonistic properties are possessed by the black anhydrous oxide of copper. [Correa.] About four or five grains of the powdered and dried substance are introduced Into a tube (a, fig. 1) made of very Infusible glass, about 15 to 20 inches long, nearly half an inch in diameter, drawn out to a point at one end (b, jig. 1), and previously about one-third tilled up with oxide of copper that has been dried by igniting to redness and allowing to cool in a well-closed, long, narrow bottle : a little more oxide of ',copper is then poured in, and the substance well mixed with the oxide by means of a wire twisted at one end into a corkscrew shape ; more oxide is again introduced, the wire passed once or twice through it to wash of adhering substance, and the tube finally filled up with oxide to within two Inches of its open extremity. On heating this tube and its con tents to redness, first near its open end (e, fig. 11, and then gradually throughout its whole length, the organic matter is thoroughly burnt into carbonic acid and water, the manner of separating and collecting which will presently be described.
With regard to the best material for heating the tube, much must depend upon circumstances. Till within the last few yearn charcoal has been the fuel almost always employed, some pieces of which are made red-hot in a furnace,and then placed, a few at a time, first round the anterior part of the tube (c, fig. 1), the latter being supported on sheet-iron crosspieces in the sheet-iron trough, dd, represented in the accompanying drawing.
Fig. I.
The combustion of charcoal, however, occasions a troublesome amount of dust, and, since the introduction of gas into laboratories, many furnaces have been contrived by means of which the glass tubes may be gradually and regularly heated with that fuel. The latest improvement in this respect is by Dr. Hofmann, the general arrange ment of whose funnies is seen in jig. 2.
In this apparatus the combustion of the gas is effected by means of a number of perforated clay burners, so grouped as to form a channel for the combustion tube, a system of step-cocks permitting to confine the heat to any special place in which it may be required at the time.
The arrangement by which the water and carbonic acid, formed during the operation from the organic substance, are collected, is very simple. A small glass tube, straight, as seen at e, fig. 3, or bent to the shape of the letter U, and having a bulb attached, as seen at e, fig. 2, is filled with pieces of fused chloride of calcium, and firmly fixed by a cork, or solid india-rubber plug, into the glass combustion tube. Imme diately at the opposite extremity of the chloride of calcium tube a piece of apparatus, known as Liebig's potash bulbs' (f, fig. 1), is attached by a piece of india-rubber tubing (g). The vapour of water and carbonic acid evolved from the tube are thus thoroughly absorbed, the former by the chloride of calcium, and the latter by a strong solution of caustic potash previously placed in the bulbs. The whole length of the tube having been brought to a red heat, and there main tained till no more bubbles of gas pass into the potash, the charcoal is removed from the extremity, the little point b broken of and air cautiously drawn through the whole arrangement by aspiration, with a piece of india-rubber tubing attached to the potash bulbs. In this last operation loss may be occasioned by the air carrying a little vapour of water away from the potash' solution, and hence the use of the little tube h, which contains a short stick of solid hydrate of potash. In place of driving out the residual vapour of water and carbonic acid by air, a few fragments of chlorate of potash may be placed in the posterior extremity of the tube ; and on heating this at the close of tho operation, evolution of oxygen takes place, and effects the desired object. Tho use of chlorate of potash here is attended with two or three advantages, but it is liable to be decomposed so rapidly as to blow some of the solution of potash altogether out of the bulbs. Another and greater modification of the process consists in having a considerable elongation of the tube in the place of the little capillary extremity above described : into this open end a little platinum or porcelain boat, containing the organic substance to be analysed, can bo placed, and a stream of pure and dry air or oxygen over it. On then gradually heating the glass tube in the neigh of the boat, the substance burns as it would do in air; but the products of combustion pass on over the ignited oxide of copper, and are collected respectively in the chloride of calcium tube and potash bulbs. At the same time any inorganic, non-volatile matter present in the organic substance remains in the boat as ash, and may readily be removed and examined.
Volatile liquids may be burnt in either of the above modifications of combustion tube, but are first inclosed in little bulbs having thin necks that can easily be broken.
With substances not readily burnt, chromate of lead is sometimes used in the place of oxide of copper.