“The common, or cubical coal, as it is called from the shape into which it breaks, does not bear the same obvious marks of vegetable origin in its structure; but where one species of coal can be so clearly demonstrated to be only altered vegetable matter, it would be bad philosophy to ascribe the other species to other causes. In the prodigious beds of coal, however, in Staffordshire, there is no want of vegetable traces; and even in the Newcastle coal the impressions of leaves and branches are frequently found, as well as in the freestone and slate-clay which intervene between its numerous strata. At Kilsyth, in Scotland, a very singular specimen was discovered; a tree standing upright, with its roots resting on a bed of coal, from which they could scarcely be distinguished, and its stem passing into a stratum of sandstone rock. The lower end was completely bituminated, and it burned with a clear flame; yet the upper part, though scarcely altered in the grain or apparent texture of the wood, was converted into sandstone similar to that by which it was enclosed. Round the stem there was a space of about an inch in thickness filled with coal, which renders it probable that the same process that converted the roots into coal acted upwards on the bark. The rock contains innumerable remains of plants; some of which are so perfect that their species have been made out, and no pencil could trace their delicate ramifications with greater nicety.
“In short,” continued my uncle, “it appears more than probable that every species of coal has proceeded from vegetable matter of different kinds, but under different circumstances; and that its chemical change was effected under the pressure of deep water. In one stage of that process it must have been in a soft pulpy state, like the lowest part of a deep peat-bog; for this is the only way that I can account for the impression of leaves, canes, seed-vessels, and shells, which are so commonly found on the external surface of coal.”
My uncle shewed us a beautiful specimen of a fern leaf, where the impression was as perfect as if it had been made with wax.
He then continued, “Sir James Hall thinks that peat may have been converted into coal by heat acting under great compression; and he has actually succeeded in making a substance very like it. When I have more leisure I will describe the ingenious process which he adopted, as well as some other experiments of the same nature, by which this distinguished philosopher discovered the means of fusing limestone, of imitating volcanic lava, and of forming solid sandstone from loose sand.
“But to return to our coals: the chief difference between the various kinds of coal which are applied to economical purposes, arises from the proportion of bitumen they contain. What is called caking coal yields about 40 per cent.; when burning it swells, agglutinates, and emits much smoke and gas, which inflame at a certain temperature. Cannel coal has only 20 per cent. of bitumen, and does not agglutinate or cake. It burns with a bright flame like a candle, from which circumstance it takes its name, cannel being the common pronunciation of candle in the North of England. The third sort I shall mention is called anthracite by mineralogists; but its common name is blind coal, or Kilkenny coal, from a district in Ireland, where there are vast beds of it. It contains little or no bitumen; it neither cakes nor flames, and gives out very little smoke. But as there are several varieties of coal between those principal species, much confusion has taken place in their names.”
16th.—When Mary and I were in the garden to-day, I observed a very odd appearance on the under surface of some of the leaves of a pear-tree; they appeared thickly set with strange little downy russet-coloured things like spines growing out of the leaf, perpendicular to it, and about a quarter of an inch in length, and very little thicker than a pin, with a protuberance or excrescence at the base.
Mary was amused at my surprise, and told me that they were the habitations of insects. She then took one of these tubes off the leaf, and on giving it a gentle squeeze, a minute caterpillar, with a yellowish body and black head, came out of the lower end; for the head is always downwards. We examined the place from which she had removed it, and I saw that there was a small hollow in the outer skin and pulpy part of the leaf, which had been eaten away by the caterpillar. It moves this little tube or tent from one part of the leaf to the other, and eats no other part than what the tent covers; and when these insects are abundant, Mary says that every leaf is covered with little withered specks, where they have feasted themselves.
The tube in which the caterpillar lives, is composed of silk, spun from its mouth almost as soon as it comes out of the egg, and as it increases in size it enlarges the tube, by slitting it in two, and introducing a strip of new materials. To preserve the perpendicular posture of its tent, this ingenious insect attaches several silken threads from the protuberance at the base to the surface of the leaf; but it has a still more singular device to protect the tent against any violence: it forms a vacuum in the protuberance at the base, which fastens it to the leaf as effectually as if an air-pump had been employed. This vacuum is caused by the insect’s retreating on the least alarm up the tube, which its body so completely fills that the space below is free of air, and the tube is pressed down like the exhausted receiver of an air-pump.
Mary easily convinced me of this when she seized it suddenly while the insect was at the bottom, the silken cords readily gave way, and the tube was detached by a very slight force; but when she touched it gently, giving the insect time to retreat, we found that a much stronger effort was required to loosen it. As if aware of the effect of the admission of air from below, this little philosopher carefully avoids gnawing quite through the leaf; and when he has eaten as deeply as he can venture, he cuts the cords of his tent and pitches it on a fresh part of the surface. When it has attained its perfect state, it becomes a small brown moth.
17th.-Mary has been trying a grand experiment, which has succeeded so well that mamma must have an account of it.