external air to pass through the vessels, or buildings containing the coal, from which any of the above-mentioned substances are to be extracted. After being kindled, the coals are decomposed by a slow, but imperfect combustion, without dissipating the ingredients. The residuum in the oven, forms excellent cinders, or coke; while the volatile particles are condensed in reservoirs, placed at proper distances.
It is a circumstance worthy of notice, that not less than 70 kinds of coal are brought to the London market; the value and prices of which differ, in general, from 1s. to 10s. and sometimes even 15s. in the chaldron, according to their qualities. About 45 of these various sorts are imported from Newcastle, and the remainder from Sunderland; the whole of which may be divided into four classes:
The first class contains only six kinds of coal; called Wall's-end, Bigg's-main, Walker's, Heaton-main, Willington, and Hebburn-main. The prices of these sorts vary, according to their abundance in the market, from 1s. to 3s. per chaldron; but they are generally upon a par, except the Wall's-end, which is mostly 6d. or 1s. dearer than the others.
The second class includes three sorts; all of which run large. They light and burn like a candle, and produce white ashes. These are usually mixed either with some of the first class, or with any of the strong sorts of the second, third, and fourth classes; because they run large, and make them burn in a more lively manner. These three sorts are, Hartley, Coupen-main, and Blythe; and their price is generally from 2s. to 4s., more or less, below that of Wall's-end, according to their scarcity or abundance in the market. Next to these are twelve sorts, which possess nearly the same qualities as the best coals, but are in general smaller, and seldom vary more than 2s. in the chaldron, though they are usually from 3s. to 4s. in price under the Wall's-end.
The third class consists nearly of the same number as the second, and is likewise divided into two sorts: the first of which burns quickly, and produces white ashes; the other is very strong and good, but, at the same time, very small, and is used by smiths and manufacturers. The prices of this class of coals are generally from 4s. to 6s. per chaldron, more or less, under that of the Wall's-end, according to their abundance or scarcity.
Lastly, the fourth class contains all the remaining kinds of coal: they differ also in quality; some burn light, produce white ashes, are slaty, and very indifferent; others are small and strong, but not good enough for smiths. The price of these varies greatly, especially of the lighter kind. It is, in general, from 8s. to 10s. and even 15s. lower than the Wall's-end. These different classes, and particularly some of the inferior sorts, are frequently mixed together, and thus afford an opportunity of changing the prices of coals; this, however, is always to the loss of the consumer, who loses 10s. or more in the quality, in the hope of saving 4s. or 6s. in the price.
The following is a striking instance of the great variation to be found in the quality of coal: in weighing different kinds of that fossil, there was the surprizing difference of 30lbs. in the weight of
