since by rivers and cut down by the waves of the sea, but we can say that wherever there is coal now, there they must have been.

But what is it that has changed these beds of dead plants into hard, stony coal? In the first place you must remember they have been pressed down under an enormous weight of rocks above them. We can learn something about this even from our common lead pencils. At one time the graphite or pure carbon, of which the blacklead (as we wrongly call it) of our pencils is made, was dug solid out of the earth. But so much has now been used that they are obliged to collect the graphite dust, and press it under a heavy weight, and this makes such solid pieces that they can cut them into leads for ordinary cedar pencils.

Now the pressure which we can exert by machinery is absolutely nothing compared to the weight of all those hundreds of feet of solid rock which lie over the coal-beds, and which has pressed them down for thousands and perhaps millions of years; and besides this, we know that parts of the inside of the earth are very hot, and many of the rocks in which coal is found are altered by heat. So we can picture to ourselves that the coal was not only squeezed into a solid mass, but often much of the oil and gas which were in the leaves of the plants was driven out by heat, and the whole baked, as it were, into one substance. The difference between coal which flames and coal which burns only with a red heat, is chiefly that one has been baked and crushed more than the other. Coal which flames has still got in it the tar and the gas and the oils which the plant stored up in its leaves, and these when

they escape again give back the sunbeams in a bright flame. The hard stone coal, such as anthracite, on the contrary, has lost a great part of these oils, and only carbon remains, which seizes hold of the oxygen of the air and burns without flame. Coke is pure carbon, which we make artificially by driving out the oils and gases from coal, and the gas we burn is part of what is driven out.

We can easily make coal-gas here in this room. I have brought a tobacco-pipe, the bowl of which is filled with a little powdered coal, and the broad end cemented up with Stourbridge clay. When we place this bowl over a spirit-lamp and make it very hot, the gas is driven out at the narrow end of the pipe and lights easily (see Fig. 55). This is the way all our gas

is made, only that furnaces are used to bake the coal in, and the gas is passed into large reservoirs till it is wanted for use.

You will find it difficult at first to understand how coal can be so full of oil and tar and gases, until you have tried to think over how much of all these there is

in plants, and especially in seeds-think of the oils of almonds, of lavender, of cloves, and of caraways; and the oils of turpentine which we get from the pines, and out of which tar is made. When you remember these and many more, and also how the seeds of the club-moss now are largely charged with oil, you will easily imagine that the large masses of coal-plants which have been pressed together and broken and crushed, would give out a great deal of oil which, when made very hot, rises up as gas. You may often yourself see tar oozing out of the lumps of soft coal in a fire, and making little black bubbles which burst and burn. It is from this tar that James Young first made paraffin oil, and the spirit benzone comes from the same source.

In the ages that have passed since the vegetation that now forms our coal was deposited its slow decomposition, perhaps under conditions of great heat and pressure, has resulted in vast natural accumulations of this coal-oil and also of coal-gas in the interior of the earth.

The great storehouses that contain these valuable products of the ancient coal-forests are only to be found where the bending of the stråta makes great caverns. The rocks and earth above the rocks constituting the domes over the great natural cisterns or tanks often press, as may well be supposed, with enormous weight upon the inclosed coal-oil or gas.

When these oil wells, as they are called, were first discovered, and before any efficient means of restraining the flow had been contrived, the oil frequently burst forth, and, carrying away the barriers erected

against it, overflowed the country, tainting the air, befouling the soil, and poisoning all the streams in its neighbourhood.

straws, well casings blown high in air, and the oil in columns as thick as a man's body has spouted

up for days fully two hundred feet above the surface of the earth, forming, as it flowed toward the sea, rivers of oil many miles in length. The force of coal-gas escaping from the coal-gas wells in Indiana, Pennsylvania, and Ohio, has been known to blow out drills of nearly a ton in weight, and to burst the doubly-riveted tanks and heavy iron mains which were used in attempting to confine it, so that it was for a time thought that nothing could be contrived that would withstand its pressure. The roar of the escaping gas could be heard for miles, and schools had to be closed and all business suspended in the vicinity of the wells. If the gas was set on fire, as sometimes happened, the roaring was increased to such an extent that workmen who were obliged to remain in its neighbourhood were made deaf for life, and the light from the well could in some cases be seen for forty miles around.

Until the last few years, however, the very existence of most of these great reservoirs of potential energy was unsuspected, and although coal-oil skimmed from the surface of pools in oil-bearing localities was sometimes employed to a limited extent, mostly as a medicine, it is only of late years it has been found in quantities sufficient to allow its extended use. Yet so rapid has its applications to uncounted domestic, mechanical, and industrial purposes advanced that it may already justly claim to materially modify our progress in the arts and sciences.

Not only is mineral oil now used to cook our food, to light our houses, and to drive our engines, but the manufacture of a great number of articles and of