CHAPTER XIV.

271. _What is coal?_

Coal is a "_vegetable fossil_."

272. _What is meant by a vegetable fossil?_

It is a substance _originally vegetable_, which, by pressure and other agencies within the earth, has been brought to a condition approaching that of _mineral_ or earthy matter.

273. _Why do we know that coal is of vegetable origin?_

By the _chemical components_ of its substance; and also by the _vegetable forms_ that are found abundantly in coal beds.

Professor Buckland, in his _Bridgewater Treatise_, speaking of the impressions of plants found in the coal mines, says; "The finest example I have ever witnessed is that of the coal mines of Bohemia. The most elaborate imitations of living foliage upon the painted ceilings of Italian palaces bear no comparison with the beauteous profusion of extinct vegetable forms with which the galleries of these instructive coal mines are overhung. The roof is covered as with a canopy of gorgeous tapestry, enriched with festoons of most graceful foliage, flung in wild irregular profusion over every part of its surface. The effect is heightened by the contrast of the coal-black colour of these vegetables with the light ground-work of the rock to which they are attached. The spectator feels himself transported, as if by enchantment, into the forests of another world; he beholds trees, of forms and characters now unknown upon the surface of the earth, presented to his senses almost in the beauty and vigour of their primeval life; their scaly stems and bending branches, with their delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of countless ages, and bearing faithful records of extinct systems of vegetation which began and terminated in times of which these relics are the infallible historians."

[Verse: "Surely every man walketh in a vain show; surely they are disquieted in vain: he heapeth up riches, and knoweth not who shall gather them."--PS. XXXIX.]

274. _What are the chemical components of coal?_

They consist of _carbon_, _hydrogen_, _oxygen_, and _nitrogen_. The proportions of these elements vary in different kinds of coal. Carbon is the chief component; and the proportions may be stated to be, generally, _carbon_, 90 per cent.; _hydrogen_, from 3 to 6 per cent.; the other elements enter into the compound in such small proportions, that, for all ordinary purposes, it is sufficient to say that coal consists of _carbon_ and _hydrogen_, but chiefly of _carbon_.

275. _What is charcoal?_

Charcoal consists almost entirely of _carbon_. It is made from _wood_ by the application of heat, without the admission of air. The hydrogen and oxygen of the wood are expelled, and that which remains is charcoal, or _carbon_ in one of its purest states.

276. _What is animal charcoal?_

Animal charcoal, like vegetable charcoal, consists of _carbon_ in a state approaching purity. It is made from the _bones of animals_, heated in iron cylinders. It is commonly called _ivory black_.

277. _What is the purest form of carbon known?_

The purest form of _carbon_ is the _diamond_, which may be said to be absolutely pure.

Hence we derive another of the beautiful lessons of science--a lesson which teaches us to _despise nothing that God has given_. The soot which blackens the face of a chimney-sweep, and the diamond that glistens in the crown of the monarch, consist of the same element in merely a different atomic condition. What a lesson of humility this teaches to Pride! The haughty beauty as she walks the ball-room, inwardly proud of the radiance of her gems as they rise and fall upon her breast, little thinks or knows that _every breath that is expired around her wafts away the like element of which her treasures are composed_. That even in our own flesh and bones the same abounding substance lies hid; and that the buried tree of the primitive world, and the little flower of to-day, are both the instruments of giving this singular element to man!

278. _What is coke?_

Coke is coal, divested of its hydrogen and other volatile parts, by a similar process to that by which charcoal is produced. It forms the residue after hydrogen gas has been made from coals. It consists almost entirely of _carbon_.

[Verse: "Oh that men would praise the Lord for his goodness, and for his wonderful works to the children of men."--PSALM CVII.]

279. _Why do burning coals produce yellow flame?_

Because the _hydrogen_ which they contain is combined with some proportion of _carbon_, which imparts a bright yellow colour to the flames.

280. _Why do some of the flames of a fire appear much whiter than others?_

Because the quality of coals, and the conditions under which they are burnt, are liable to variation. Some coals yield a _heavy_ hydrogen, called _bi-carburetted hydrogen_, which burns with a much brighter flame than _carburetted hydrogen_.

281. _Why does bi-carburetted hydrogen burn with a whiter flame than the common coal gas?_

Because it is combined with a larger proportion of _carbon_, to which it owes its increased luminosity.

282. _Why do some of the flames of a fire appear blue?_

Because the hydrogen which is escaping where those flames occur is _pure hydrogen_, destitute of carbon.

283. _Why does the fire sometimes appear red, and without flame?_

Because the volatile gases have been driven off and consumed, and combustion is continued by the _carbon_ of the coals and the _oxygen_ of the air.

284. _What effect has the burning of a fire upon the composition of the air?_

It is found that in burning 10lb. of coal the oxygen contained in 1,551 cubic feet of air is altogether absorbed. It is therefore necessary to keep the atmosphere of a room, in which a coal fire is burning, fresh and pure, to supply 155 cubic feet of fresh air for every pound of coal that is consumed.

[Verse: "O Lord how manifold are thy works, in wisdom hast thou made them all: the earth is full of thy riches."--PSALM CIV.]

285. _Why does wood which is "green" hiss and steam when it is burnt?_

Because it contains a large amount of water, which must be evaporated before combustion can proceed.

286. _What is the effect of this evaporation?_

A great deal of heat is unprofitably expended in driving off the water of the fuel.

287. _Why does poking a fire cause it to burn more brightly?_

Because it opens avenues through which the air may enter to supply _oxygen_.

288. _Why do "blowers" improve the draft of air through a fire?_

Because, by obstructing the passage of the current of air _over_ the fire, they cause additional air to pass _through_ it, and therefore a greater amount of _oxygen_ is carried to the coals.

289. _What is smoke?_

Unconsumed particles of _coal_, rendered volatile by heat, and driven off.

290. _What is soot?_

_Carbon_ in minute particles, driven off with other volatile matters and deposited on the walls of chimneys.

291. _Why do fresh coals increase the quantity of smoke?_

Because they contain volatile matters which are easily driven off; and because, also, they reduce momentarily the heat, so that those matters that first escape cannot be consumed.

292. _Why do charcoal and coke fires burn clearly and without flame?_

Because the _hydrogen_ has been previously driven off from those substances.

293. _Why is it difficult to light charcoal and coke fires?_

Because they contain no _hydrogen_ to produce _flame_, and assist combustion.

[Verse: "He hath made his wonderful works to be remembered: the Lord is precious and full of compassion."--PSALM CXL.]

A new plan of kindling fires has lately been recommended. Coals are to be laid in the _bottom_ of the fire-place to a considerable depth, then the paper and wood are to be laid on, and then a little coals and cinders over them. This plan of "laying in" the fire is precisely the _reverse_ of that which has been pursued for many years. The theory is, that when the coals in the bottom are ignited, a more even combustion is kept up, whilst the smoke and gas which would otherwise escape, and become as so much waste fuel, is burnt up, and produces heat. We have heard the plan strongly recommended by persons who have tried it, and who testify to the great economy of fuel to which it conduces.