If the average quantities of steam raised by coals which are capable of reducing the same amount of lead, be now compared, as shown in Table III., it will be observed that although on the

whole the amount of lead may be taken as an approximative test of the value of fuel, it can by no means be relied on as an exact measure of that value.

The same may be observed with reference to the amount of fixed carbon as shown in Table IV. for differences of 5 per cent.

The total amounts of carbon contained in the different coals classed together for every 5 per cent of difference, and an average taken of the amount of steam raised from these, is shown in Table V., which, although it bears out the general fact of the

carbon being in proportion to the steam raised, does not warrant the carbon being made an exact measure of the heating power.

This branch of our subject is still in a most unsatisfactory position, and there is no known scientific method by which to estimate the commercial value of a coal.

The following summary of experiments upon the introduction of vapour into the fire, by Dr. Fyfe, are interesting. As the mean of 13 observations, when a well-regulated and properly-divided current of vapour was conducted over the flame, I lb. of Scotch coal vapourised 9.49 lbs. of water from 0° C. Another kind of coal gave as the mean of 10 observations, which varied from 9.23 to 13.34, 10.97 lbs. of water from 0° C. A direct comparative trial, made with a steam boiler, showed that 537 lbs. of coal, assisted by the action of vapour, effected as much as 812 lbs. without it, and for this purpose 4 lbs. of the vapour were conducted through the fire for every 100 lbs. that were liberated. If these be deducted, we obtain instead of 10.97, only 10.53 as the actual effect produced, which is still 37 per cent more than was obtained by the unassisted coal.

The Hessian Society for the Promotion of Arts and Manufactures examined experimentally the value of wood, peat, and coal burnt under six different well-arranged coppers, and found

Scotch Coals.

that 1 lb. of cleft beech wood two years felled converted 2.075 lbs. of water from 0° C. into vapour, 1 lb. of peat evaporated 1.991 lb., and 1 lb. of small coal 5.201 lbs. of water.

SECONDARY PRODUCTS OBTAINED FROM FUEL.

In conformity with the arrangement of this work we have now to review the other branches of chemical manufacture in which fuel is either employed as the raw material for the production of other compounds, or the object of which is to convert the waste products from the manufacture of artificial fuel into useful commercial articles.

Taking the various kinds of fuel in the order they have hitherto been handled, and commencing with the products obtained from wood, we have to describe the methods of obtaining pyroligneous acid or wood-vinegar, with its accompanying wood- or pyroxylic spirit, and the various compounds with metallic bases which are generally connected with its production.

Pyroligneous Acid-Wood-Vinegar.—Glauber, in his works (Furnis novis philosophicis, 1648, and Mirac. Mundi, 1658), includes spiritum acidum amongst the products of the dry distillation of wood, which is simply common vinegar, and is consequently called Acetum lignorum. Boyle likewise mentions acetum among the same products in his Chemista Scepticus (1661); and Boerhaave aceta acetosa in his Elementis Chemic (1732). There can be therefore no doubt that the existence of an acid liquid amongst the products of the dry distillation of wood was known in the middle of the 17th century, although it is not very certain that the acid was clearly recognised as acetic, because the expression acetum was applied at that time as a generic term for acids, and not restricted to vinegar. As late even as the end of the last century, in the times of Lavoisier and Fourcroy, the acide pyro-ligneux was considered a distinct acid, until in the year 1800 the latter chemist proved, conjointly with Vauquelin, that the acid of wood was impure acetic acid.

The manufacturing process for obtaining vinegar (acetic acid) from the products of the distillation of wood is the result of the researches of Stolze (given in his work on the Purification and Application of Crude Pyroligneous Acid) and of Mollerat, few modifications of their original process having been introduced in the manufacture.

The general nature of the products resulting from the dry distillation of wood has been already noticed, and it has been shown how one portion of the carbon of the wood remains as a solid residue in the form of charcoal, while the remainder escapes partly in the form of permanent gases, and partly as vapours which condense into liquids at the ordinary atmospheric temperature. Two distinct liquids are constantly obtained, which separate into layers in the receivers. The tar, which is the heavier, is of a brownish-black_colour, thick, opaque, of a peculiar odour, and composed of a number of substances as yet very imperfectly known. Some of these are volatile, oily bodies (Empyreumatic oil, Pyrelain, Pyrostearine), and hold in solution another class of substances which have a strong resemblance to the vegetable resins (Empyreumatic resin, Pyrretine). This solution also contains extractive matter, and more or less of the substances named hereafter, particularly acetic acid. The lighter liquid consists of an aqueous solution of numerous substances, some of which are likewise very imperfectly known, as Schweitzer and Weidmann's Xylite (Gmelin's Lignon) and Mesite; others have been more fully investigated, amongst which are acetic acid, wood-spirit (hydrated oxide of methyl), acetate of methyl, creosote, and acetone.

The acetic acid has a powerful affinity for the resinous constituents of the tar, with which it forms compounds that are not easily decomposed. For this reason the crude aqueous liquid, or raw wood-vinegar, always retains a considerable quantity of resin in solution, which is accompanied by the empyreumatic extractive matters which are themselves soluble in water. These impurities cling pertinaciously to the crude vinegar, and are the cause of its brown colour and peculiar empyreumatic taste. The remarkable preservative and anti-putrescent properties of crude vinegar are also due in some measure to these admixtures, as well as to the presence of creosote, which it often contains to the extent of 1 per cent.

These products are obtained in every process of charring, although their relative quantities are subject to great variation. There is therefore no scientific objection to the two processes of charcoal-burning and vinegar-making being carried on conjointly, if other circumstances did not create practical difficulties in the way of such a combination. In the manufacture of charcoal, the cost of carriage generally obliges the burner to

locate himself where wood is abundant, and change his position when it becomes exhausted, while in doing this he is generally placing himself at a great distance from a market for the volatile products, and in situations where the requisite apparatus and materials for collecting these products are not easily obtained or conveyed. A fixed plant becomes essential where the volatile products are to be thoroughly economised, and this is incompatible with the charring of large quantities of wood at once under a moveable covering, as is practised in the meiler process; and lastly, the quality of the charcoal must suffer if the process is so conducted as to afford the largest possible quantity of tar and pyroligneous acid. Wherever, therefore, the charcoal is the chief object of the manufacture, it is generally carried on without attention being paid to the collecting of tar, and when the vinegar and tar are to be collected, the charcoal becomes a secondary source of profit. The latter process, indeed, is very seldom combined with the manufacture of charcoal for fuel.

It is reasonable to suppose that as wood is an organic structure made up of numerous chemical ingredients, each of these several constituents must exercise a special and unequal amount of influence upon the production of vinegar during the process of distillation; and indeed experiment has proved that the acid is chiefly derived from the woody fibre (cellulose) and the bodies allied to it. Payen is of opinion that it is not so much the cellulose itself, as the encrusting matter of the cells, which is the source of the vinegar; having found that pure cellulose yielded less acid than the entire wood, in which there is abundance of the latter substance. Stolze observed that different species of wood, thoroughly exhausted of the soluble matters and resin, still yielded unequal quantities of vinegar, although, what is remarkable, they afforded very nearly the same total amount of distilled products, namely, about 47 per cent. The less bark there is upon the wood, the greater is the yield of vinegar, and trunk wood consequently produces more acid than brushwood.

The following results are selected from Stolze's experiments: