may predominate even to the exclusion of combustible gases. In his investigation of the Welsh coals, Thomas analyzed 14 samples of gases emitted from crevices or "blowers" in the mines, and found that they contained from 47.37 to 97.65 per cent of methane, with over 94 per cent in all but two of them. Other earlier analyses of colliery gases have told essentially the same story." Methane is the principal gas of coal beds.

ARTIFICIAL COALS.

Various attempts have been made to prepare artificial coals, in the hope of gaining some information upon the genesis of the natural products. Two lines of research are represented in these efforts, but neither has yet led to any final conclusions.

In the first class of experiments it was sought to produce coals by pressure. W. Spring subjected peat to a pressure of 6,000 atmospheres, and transformed it into a hard, black, brilliant solid which was outwardly indistinguishable from coal. On the other hand, R. Zeiller, working with pressures of 2,000 to 6,000 kilograms to the square centimeter, found that peat and also the "ulmic acid " from the paper coals of Russia were merely compacted without change of chemical character. They retained their solubility in ammonia and showed no evidence of a true transformation into coal. Some experiments by Gümbel," who subjected lignite to pressures as high as 20,000 atmospheres, showed that even under such conditions no serious changes were produced, and that the vegetable structure was in great measure preserved.

In the second class of experiments heat is used as the transforming agent. In the ordinary process of charcoal burning, wood is heated out of free access of air, decomposition ensues, volatile matter is expelled, and a form of amorphous carbon finally remains in the kiln. Violette, who has studied this process with great care, found that when wood was heated nearly to 400° in a sealed tube, 78.5 per cent of it remained as a solid residue which had all the appearance of a fatty coal. In this case the volatile substances exerted a great pressure upon the contents of the tube, and a product very different from ordinary charcoal was formed. By heating wood under conditions which permitted the volatile matter to escape, he obtained a series of charcoals which varied in composition according to the temperature at which they were prepared. The experiments were

T.

* See G. Bischof, Edinburgh New Phil. Jour., vol. 29, p. 309; vol. 30, p. 127, 1840. Graham, Mem. Chem. Soc., vol. 3, p. 7, 1845. Lyon Playfair, Mem. Geol. Survey Great Britain, vol. 1, p. 460, 1846. A recent paper on the gases in coal is by F. G. Trobridge, Jour. Soc. Chem. Ind., vol. 25, p. 1129, 1906.

Bull. Acad. Belg., vol. 49, p. 367, 1880.

Bull. Soc. geol., 3d ser., vol. 12, p. 680, 1884.

4 Sitzungsb. Math. phys. Classe, K. bayer. Akad. Wiss., München, vol. 13, p. 141, 1883. Ann. chim. phys., 3d ser., vol. 32, p. 304, 1851.

conducted at temperatures ranging from 150° to the melting point of platinum, and his analyses of the products thus formed, 28 in all, show progressive changes, analogous to the changes observed in the passage from wood to anthracite. The charcoals, however, are not identical with coal, but differ from it both texturally and chemically. A finished charcoal is really the analogue of coke, being in fact the coke of wood; but in its preparation it is possible to trace, step by step, the breaking down of the original ligneous fiber. For that reason it is most desirable that the chemistry of charcoal burning should be studied much more in detail than it has been hitherto.

a

Violette's experiments with wood in sealed tubes were not the first of their kind. Early in the nineteenth century Sir James Hall obtained an artificial coal by heating wood in a closed cylinder of iron, and in 1850 or 1851 C. Cagniard-Latour performed essentially the same experiment in tubes of glass. These earlier experiments, however, were merely qualitative, for the products obtained were not analyzed.

In 1879 Fremy published an interesting series of observations, based upon experiments with carbohydrates other than cellulose, and upon the so-called "ulmic acid " from two sources. One example of ulmic acid was extracted from peat; the other was prepared from a constituent of woody tissue to which Fremy gave the name vasculose. The substances were all heated in sealed glass tubes to temperatures which seem to have been near 300° and yielded residues which behaved in all respects like coal. When heated to redness, they gave off water, gas, and tar and left behind a remainder of coke. These artificial coals had the following composition:

The similarity of these products to natural coals, especially in the last three examples, is evident.

The most recent experiments of this order seem to be those of S. Stein. He heated wood with water in sealed tubes, but at different temperatures, and partially analyzed the coaly substances thus obtained. His results are briefly as follows:

a Compt. Rend., vol. 32, p. 295, 1851.

seen.

Idem, vol. 88, p. 1048, 1879.

Chem. Centralbl., 1901, pt. 2, p. 950. From a Hungarian original which I have not

Here we have a series of products, ranging in composition from a substance near peat to one more closely resembling coal. Only, it must be observed, the hydrogen toward the end of the series is lower than in coals showing the same percentage of carbon. The parallelism between the artificial and the natural substances is therefore not quite complete. The natural inference from this conclusion is that agencies other than heat and pressure have taken part in the carbonization of vegetable matter, and these may have been microbian in character. The function of heat is to decompose the organic complexes; that of pressure is to retard the change and to prevent the escape of the volatile products; the combined effect must vary with variations in the intensity of the two agencies. If an exact adjustment of heat and pressure could be arranged, it is possible that a true artificial coal might be prepared, but this is a mere supposition.

From one point of view the experiments with sealed tubes appear to be irrelevant. The change of woody fiber to peat or lignite is initiated at low temperatures and under nearly atmospheric pressure, conditions quite unlike those which either Violette or Stein adopted. As the rotted material becomes buried the pressure upon it increases; but, except where igneous intrusions have operated, there is nothing to show that especially high temperatures have been at work. The element of time, however, must be considered. The natural processes are carried on slowly; and it may be that the laboratory methods merely accelerate them. So far, then, the experiments are pertinent but inconclusive. They certainly do not cover all the ground. All that can be said is that moderate temperatures and pressures, operating for a long time, may produce results resembling those which are brought about hurriedly in the laboratory.

In order to account for what we might call the anthragenetic process, various hypotheses have been framed. J. F. Hofmann," for example, has used the analogy offered by the spontaneous combustion of grain, flax, and hay, and suggested that something of the same sort may happen in the buried materials from which coal is

a Zeitschr. angew. Chemie, p. 821, 1902.

formed. In that phenomenon heat is generated by fermentation. and when actual inflammation is prevented for lack of air a partial carbonization may occur. In cases of this kind heat is generated locally and an imperfect combustion occurs. Hofmann's suggestions are interesting, but, so far as the formation of coal is concerned, the evidence in their favor is very incomplete.

How far micro-organisms are active in the formation of coal is doubtful. They abound in the stagnant waters of swamps, and certainly have much to do with the earlier stages of vegetable decay. They start the process, but at the same time they generate antiseptic compounds which limit their activity. Peat, not far below the surface, is distinctly antiseptic and inimical to microbian life. Never theless, a number of authorities have argued strongly in favor of these organisms as principal agents in anthragenesis. B. Renault has found their remains in lignite and coal in significant abundance and variety.

THE CONSTITUTION OF COAL.

In the preceding pages, under other captions, I have cited a good deal of evidence relative to the substances found in coal or from which coal has been derived. Its vegetable origin is clear and needs no further discussion now; its present constitution is more difficult. to determine.

The question of constitution presents itself under two aspects, the one structural the other chemical. On the one side microscopic evidence is available, and it is seen that coal contains vegetable remains, micro-organisms, resinoid bodies, and so on. In some coals spores or spore cells are abundant; in others, as shown by Renault, remains of algæ are found. The lignites are obviously derived from woody fiber, and, in short, in many cases the proximate origin of the coals is not difficult to determine. Their structure, microscopic or macroscopic, tells a pretty clear story.

On the chemical side the problems are much less simple. The proximate constituents of coal are most imperfectly known and the little knowledge we have is mainly qualitative. The necessary in

Bull. Soc. ind. min., 3d ser., vol. 13, p. 865, 1899; vol. 14, p. 1, 1900. See also L. Lemière, idem, 4th ser., vol. 4, pp. 851, 1248, 1905, and vol. 6, p. 273, 1906. Also in Compt. Rend. VIII Cong. géol, internat., p. 502, 1900. Lemière regards the soluble or diastatic ferments, derived from living vegetation, as also operative in the process of vegetable decay.

See J. W. Dawson, Am. Jour. Sci., 3d ser., vol 1, p. 256, 1871. E. Orton (idem, vol. 24, p. 171, 1882) states that spore cases are abundant in the "sub-Carboniferous" rocks of Ohio, and are also found in the Devonian. On the microscopic structure of the natural hydrocarbons, resins, and coals, see Fischer and Rust, Zeitschr. Kryst. Min., vol. 7, p. 209, 1882. The important memoirs by Bertrand and Renault have already been referred to.

vestigations are difficult, the methods are not well formulated, and the available data are scattered and fragmentary. To what extent free carbon exists in coals is still an open question. It is probably absent from lignite and abundant in the extreme anthracites; but its quantitative determination can not be effected by any known analytical process.

There are two distinct lines of attack upon the problem in question. First, by means of solvents, to extract certain constituents of coal and to identify them. Some of these constituents, which are commonly small in amount, can be dissolved by gasoline, ether, benzene, chloroform, alcohol, and other organic solvents. The extractive matter thus obtained is, unfortunately, not simple, but seems to contain a mixture of substances whose nature is yet to be determined. By handling large quantities of material these bodies may be obtained in sufficient abundance for more complete investigation, and their separation into definite fractions is by no means hopeless."

Alkaline solvents, such as caustic soda, caustic potash, and ammonia, dissolve, as we have already seen, humic substances from peat and brown coal, but not from the older carbons. These substances are indefinite, but in time their nature may be determined, and their correlation with the ligneous carbohydrates ought then to become possible. If, however, as is supposed, some coals are derived from gelatinous algæ, the problem becomes more complex. The chemical constitution of those forms of vegetation is still very obscure. Up to the present time the mistake has been made, by chemists engaged in the study of coal, of assuming that the celluloses are the chief starting points-an assumption which is not unqualifiedly true. Carbons of animal origin also require attention. Much preliminary work of this kind remains to be done.

The direct separation of its constituents from coal is, however, possible only to a very limited extent. Hence the second line of attack, the conversion of these bodies into recognizable derivatives, is also essential. Not only do we need more experiments along the line developed by Donath, whose distinction between the lignites and the true coals has already been discussed, but much more needs to be done in the study of oxidation products, chlorine derivatives, etc. For example, in addition to the researches upon the nitrocompounds derivable from coal and the chlorination experiments reported to the

a On this subject see the authorities already cited. Also P. Siepmann, Preuss. Zeitschr. Berg-, Hütten-, u. Sal.-Wesen, vol. 39, p. 27, 1891. F. Muck (Die Chemie der Steinkohle, Leipzig. 1891) gives a good summary of earlier investigations by Dondorff, Reinsch, etc. An interesting memoir by W. C. Anderson (Proc. Phil. Soc. Glasgow, vol. 29, p. 72, 1897) also describes a number of important experiments.