Imágenes de páginas
PDF
EPUB

100° additional of surface temperature to raise this to tropical heat. This would necessitate a temperature of 2,000° at the depth of sixty feet, a condition of things, it would seem, utterly incompatible with the existence of luxuriant vegetation on the surface.

The second mode of accounting for it is by means of distribution of land and water upon the earth's surface. Land, as compared with water, is both a better absorber and better radiator of heat, i. e., will both heat faster under the influence of a source of heat, as the sun, and cool faster when that source is withdrawn. This is familiarly illustrated by land and sea breezes. Again: the earth at the equator receives more heat from the sun than it radiates, while at the poles, on the contrary, it radiates more than it receives from the sun, the overplus in both cases being balanced by the currents of ocean and atmosphere. If these currents could be prevented, the equator, for a time, would get progressively warmer, and the poles progressively colder. We may evidently, then, look upon the earth as a body heating at the equator and cooling at the poles. Now, when we recollect the great absorbing and radiating power of land, as compared with water, it is easy to see that the mean temperature of the earth's surface may be materially affected by the distribution of these elements with reference to the two points in question. For instance, if the water be all collected in a belt about the equator, and the poles be occupied entirely by land, we would have the conditions most unfavorable for heating at the equator and most favorable for cooling at the poles. The result would be a considerable lowering of the mean temperature. If, on the contrary, the waters be gathered into polar oceans, leaving an equatorial belt of land, the conditions would be most favorable for heating at the equator and most unfavorable for cooling at the poles, and the mean temperature would consequently rise. It is estimated that these two extreme conditions would bring down the mean temperature to 32°, or raise it to tropical heat. It is not to be supposed that such extreme conditions ever existed ; but any approximation to such conditions—for instance, a decided predominance of land towards the equator or poles—would produce the same effects to a corresponding degree. Now, it is possible that the greater heat of the coal period may be due to some such distribution of land and water.

The fatal objection to this explanation is that we find no coal in tropical regions. As every coal field presupposes a large river, and therefore a considerable extent of land, the distribution of coal may be looked upon as in a general way indicative of the distribution of land during the period. It would seem from this that the larger bodies of land existed in temperate and arctic rather than in tropical regions.

But if it is impossible by distribution of land and water to account for the greater mean temperature, it is at least easy in this way to account for the greater humidity and uniformity of climate which we have found equally to characterize this period. I have already alluded to the fact that the palæozoic seas were probably very wide and the land correspondingly small in extent and low, and that such a condition of things, on account of the very limited condensation and precipitation of vapor, would produce a very humid climate. Now, water

severe.

being both a bad absorber and bad radiator of heat, both heating very slowly and cooling very slowly, it is evident that a great predominance of that element would produce, also, a very uniform climate. The difference of temperature between pole and equator, and between winter and summer, would be less than at present.

Some geologists think, with Mr. Lyell, that this uniformity and humidity of climate is sufficient to account for the coal vegetation without the necessity of a higher mean temperature than now exists. If the present mean temperature was distributed more equably both over the earth surface and over the year, the effect would be to produce cooler equator, it is true, but also much warmer high latitudes, and particularly the winters of high latitudes would be much less

The evidence is, however, it seems to me, in favor of some elevation of the mean temperature also. It is difficult to conceive how any uniformity of distribution of the present mean temperature, such as would be produced by the predominance of water, could rai the winter temperature of Mellville island to the point necessary for the luxuriant growth of tree ferns. Some increase of temperature from internal cause seems to be necessary. I suppose, therefore, that if the temperature of the earth from internal causes was slightly elevated, say 10°, so that the mean temperature from 60° should become 70°, and then this mean temperature distributed over the earth surface as uniformly as possible, by means of a wide extent of ocean, we should have all the conditions necessary to produce the phenomena of coal vegetation. It will be recollected, too, that we have much independent evidence of the cooling of the earth from an original very high temperature.

With reference to the highly carbonated condition of the atmosphere, we may suppose this to be the result of the greater activity of carbonic acid producing causes, or else we may refer it to the original constitution of the air—the natural process by which carbonic acid is given to the air, decomposition, combustion, respiration of animals, and volcanoes, carbonated springs, &c. It will be admitted by all that the first three may be neglected, since they return to the air only what had been previously taken from it. The carbonic acid supplied to the air by volcanoes and carbonated springs, according to Bischoff, is so inconsiderable that, unless we suppose these sources much more active than now, it would take millions of years to affect materially the constitution of the air. But even this refuge is taken away, when we recollect that volcanoes and springs derive their carbonic acid from carbonates, and chiefly from carbonate of lime, or common limestone. But limestones, according to the testimony of all who have carefully studied them, and particularly according to the recent microscopic observations of Sorby, are entirely of animal origin, i. e. entirely made up of broken fragments of shells, corals, crinoids, sometimes recognizable under the microscope, sometimes reduced to impalpable powder. This carbonate of lime is evidently derived from sea-water. Whence, then, does sea-water derive its carbonate of lime? The lime is derived, beyond doubt, from igneous rocks, the carbonic acid probably from the atmosphere, through the animal and vegetable kingdoms, since lime exists in igneous rocks not as a carbonate but as

a silicate. It would seem to follow, then, that springs and volcanoes, also, only return to the atmosphere what had been previously taken from it. The only difference between these sources and the three first is, that while decomposition, combustion, and respiration return to the air what had been taken from it but a little while before, springs and volcanoes return to the air what had been taken from it during some previous geological epoch. Thus the atmosphere becomes the great original source of all the carbonic acid in the world.

But whatever be the cause of the excess of carbonic acid in the atmosphere during the coal period, we cannot fail to see an evident and beneficent design in its removal. Carbonic acid, as is well known, is as poisonous to animals as it is nourishing to plants. Previous to the coal period there lived none but aquatic animals of low order. These, on account of low vitality, sluggish circulation, and little necessity for rapid and constant oxygenation of the blood, have great endurance of carbonic acid. But now the earth was prepared to receive airbreathing animals, the atmosphere must be purified for the purpose. This was accomplished by the astonishing vegetation of the coal period. But observe, and never cease to admire and wonder, that the self-same providential act which purified the atmosphere and rendered the earth a fit habitation for reptiles and birds, had reference also to the coming of man countless ages after, and laid up materials for his use. In the carbon thus silently extracted from the atmosphere was buried a mechanical energy which, after a sleep of millions of years, was to rise again as the great physical regenerator of the human race.

ORIGIN OF COAL.

It is now universally admitted among geologists that coal is entirely of vegetable origin. There was a time, however, and that not many .years ago, when the vegetable or mineral origin of coal was a question warmly contested by the best geologists; but its vegetable character is now so firmly established and so universally admitted that the history of the controversy has lost its interest. I will not, therefore, tire you with its details, but proceed to state the evidence upon which the universal belief is founded.

First, then, the enormous profusion of fossil plants, in the form of impressions of leaves, trunks and branches of trees, fruits, &c., found in immediate connexion with a coal seam, affords strong presumption in its favor. In the second place, this presumption is strengthened, and becomes, in fact, almost certainty in the case of trunks of trees retaining their external conformation, and under the microscope their internal structure even to the minutest sculpturing upon their cell walls, and yet turned to perfect coal. It might possibly be objected that it may be a substitution of one substance for another, similar to what takes place in petrification, where we find, also, the external conformation and internal structure perfectly preserved, but the organic matter all gone, that the ancient trunk having been buried in bituminous matter and thoroughly impregnated therewith, as particle by particle the woody matter was removed by decomposition the bituminous matter took its place, and thus perfectly imitated its

structure. But this objection is forever set aside, when, in the third place, we subject even the most structureless coal to microscopic scrutiny. The distinguished American microscopist, Professor Bailey, of West Point, has been able to detect the unmistakable evidences of vegetable structure even in the hardest anthracite. In fact it may be affirmed that there is no coal which, under careful examination, will not reveal a vegetable structure.

Again : All the stages of gradation between perfect wood and perfect coal may be traced with the greatest certainty. We find the first stage of this process in the blackened semi-bituminized logs of our peat bogs and deltas of the present epoch. The next stage we find in the lignites or brown coal of the tertiary period; the next the highly bituminous coal of the oolite; then the coals of the true carboniferous; and lastly, the anthracites of the same and lower strata. Thus we may trace the whole embryology of coal from its immature to its most perfect condition—may trace and identify all the intermediate links of the chain of conditions of which wood and coal form the extreme limits. But not only in external form and appearance, but also in chemical composition we can trace these several stages. Wood consists of carbon, hydrogen, and oxygen; coal consists of the same elements but in different proportions. In coal the proportion of carbon is greater and of oxygen and hydrogen less than in wood. Now, if we compare the chemical composition of wood, peat, lignite, bituminous coal and anthracite, we find a progressive decrease in the proportion of oxygen and hydrogen, until, in anthracite, we find the carbon almost pure, and absolutely pure in graphite, if we acknowledge this as of similar origin. This chemical evidence is, it seems to me, absolutely demonstrative.

Lastly, direct experiment proves that peat, which we know to be of vegetable origin, may, by strong pressure, be made to assume the hardness, the density, the general appearance, and all the useful properties of coal.

Assuming, then, the vegetable origin of coal as a basis of argument, we will proceed to speak of, and to account for, the principal varieties of coal.

All coal consists of two parts, the one combustible the other incombustible. It is easy to separate these from one another. If a piece of coal is thrown into the fire the combustible portion passes away in the form of gases, the incombustible remains behind in the form of ash. Now, the relative proportion of these two vary infinitely in different coals. We have every stage of gradation between pure shale and pure coal, between pure incombustible and almost as pure combustible. In the purest coal the amount of ash is only 1 to 2 per cent.; others, more impure, contain 5, 10, 20, 50 per cent. of ash. At this point coal loses the property of ready combustion, and with it loses also the name of coal in popular language. But the geologist recognizes no remarkable change at this particular pointno scientific reason why the name should change from coal to shale, as there is no corresponding change of nature. From this point, under the name of shaly coal, black slate, &c., the amount of ash may continue to increase and the amount of combustible matter to

We may

decrease, until, in pure shale or slate, the whole becomes incombustible.

Now, wood consists also of combustible matter and ash, but the amount of ash in wood is much less than in coal-the wood of elm contains about 2 per cent.; willow, i half per cent.; beech, } per cent; oak and pine about i per cent. The leaves and bark of trees, however, contain much more than this. The fully matured leaves of the beech, willow, and elm contain, severally, 6.6, 8, and 11 per cent. of ash. It is probable, then, that 2 to 3 per cent. is a fair average of the per centage of ash in dry vegetable matter. But even if the coal is perfectly pure, that is, formed of vegetable matter without foreign admixture, we should find a higher proportion of ash than in the wood from which it was formed, for, as we have already seen, wood loses hydrogen and oxygen in the process of change into coal. The weight therefore diminishes, but the absolute amount of ash remains the same, and consequently the relative amount increases. safely say, then, that if coal contains not more than 5 per cent. of ash it may be considered quite pure; but if it contains more than 10 per cent. it is probably impure, i. e., mixed with foreign matter. This foreign matter being evidently the mud or clay upon which the carbonaceous matter was originally laid down or by which it was afterwards covered. Hence we find the purest coal in the largest seams and in the middle portions equally removed from the floor and roof. As we pass towards the roof of a seam the coal passes by imperceptible degrees into black slate, which is, in fact, mud, more or less mixed with carbonaceous matter.

So much for the varieties of coal depending upon purity or impurity, upon the relative proportion of earthy, incombustible, inorganic matter, and of combustible organic matter.

But, aside from the earthy matter, the combustible or organic matter of coal consists of two proximate elements mechanically mixed, viz: carbon and bitumen; charcoal is nearly pure carbon; common tar or pitch is very similar both in chemical composition and in general appearance to bitumen. If, then, we conceive a piece of charcoal, carefully burnt so that the vegetable structure is perfectly retained, to be thoroughly impregnated with pitch or tar, we should have a substance extremely similar to common coal. These two ingredients of coal may also be easily separated from one another. This is constantly done in the process of coking and in the manufacture of illuminating gas. The more volatile bitumen is driven off in the form of gas or collects in the pipes as coal tar and the carbon remains as coke. Now, the relative proportion of these two ingredients also vary infinitely in different coals. We may have a coal of pure carbon, or a coal of pure bitumen, or a coal containing these two in every proportion. It is the relative proportion of these which give rise to the principal varieties of coal. A coal of pure carbon is called anthracite; with a small amount of bitumen, say 10 to 20 per cent., it is called dry coals or semi-bituminous coal; when there is 20 to 30 per cent, of bitumen it is called bituminous or coking coal; when the per centage is above this and the coal burns with a strong blaze and melts, it is called fat coals. Besides these there are certain varieties depending

« AnteriorContinuar »