Miscellany.

Miscellany.

Why Coal Burns.

If you take a lump of coal out of the coal-scuttle you find yourself in possession of an irregular lump of black stone, which usually soils the hand that holds it to a greater or less extent, and which generally presents but one obvious feature—namely, that it clearly consists of thin parallel layers, some of which are usually shiny and glistening, while others are more dull and earthy in appearance. In consequence of this structure, as every one knows who has ever stirred a fire, it is comparatively easy to break up a piece of coal in one direction (the direction corresponding with that of the component layers), but repeated blows from the poker may be vainly used if the refractory lump be attacked in the opposite direction (the direction at right angles to layers). Now, as before remarked there is nothing whatever about a piece of coal which would in any way indicate its combustible nature, and perhaps the first question that we should feel disposed to ask is, why does coal burn ? To answer this question we must call in the help of our chemical friends ; but we can get an intelligible reply without dipping very deeply into the theory of combustion. The chemist tells us, then, that coal is composed principally of the elementary substance which is termed carbon, and which is seen in its purest form in lamp-black, charcoal, and the wonderfully dissimilar black lead and diamond. He further tells us that carbon, when raised to a certain temperature, has the strongest desire to unite itself with the gas called oxygen, which is present in a large amount in our atmosphere, this union being attended with the production of light and heat, and resulting in the formation of the invisible and poisonous gas which is technically called carbonic-acid gas. When, therefore, we burn a piece of coal in the fire-place, what happens, roughly stated, is (1) that the carbon of the coal enters into direct union with the oxygen of the air, emitting heat and light in so doing, the carbonic-acid gas thus produced escaping up the chimney in an invisible form ; and (2) that the earthy and inincombustible matter present in greater or less amount in all coals is left in the grate unburned, in the form of ashes and cinders.— Science for A //.