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the formation of the natural oil. He thought that in the Derby- 
shire mine the oil had been formed by the coal having been 
subjected to a gentle heat from beneath, and by the vapours thus 
formed being cooled by the layer of sandstone above. Following 
up this idea, he applied a gentle heat to various classes of coal 
in air-tight vessels; and the result was that he obtained liquid 
products more or less resembling the natural oil, together with 
a quantity of permanent gas. During these trials Mr. Young 
found that he could obtain from each ton of Boghead coal 120 to 
130 gallons of crude oil. In 1862 this material was exhausted ; 
and since then paraftin oil has been almost exclusively obtained 
from the bituminous shales existing in the coal measures, which 
yield, on an average, 32 gallons of crude oil per ton. With the 
Boghead coal, rich in organic matter, 600° to 700° Fahr. was the 
most advantageous temperature; but with the shales now used 
in the most improved retorts, about 800° Fahr. is believed to be 
the best temperature. 
The improved Henderson retort, which was shown in a 
diagram, is oblong in cross section and 15 feet long, containing 
about 18 cwt. of shale. A set of four is placed vertically in 
each chamber. Below the retorts there is a furnace; and by 
opening a door at the bottom of each retort, the spent shale 
drops into it. At first this mass, which contains carbon to the 
extent of 12 to 14 per cent., appears quite black. The products 
of combustion are conducted by a flue to the upper part of the 
oven; and are led off from near the bottom of the retorts, being 
the part exposed to the least heat. In order to remove these 
vapours from the regions of higher temperature as they form, 
and so prevent their decomposition into gaseous products, a large 
volume of superheated steam is delivered into the upper part of 
the retort. urther, by prolonging the action of the steam after 
all the oily matter is extracted, from 20 to 30 per cent. more 
ammonia is obtained. The hydrogen of the water takes hold of 
the nitrogen of the shale, forming ammonia; while the oxygen 
of the steam unites with carbon, eausing partial combustion, 
and producing carbonic oxide and carbonic acid. ‘The vapours 
generated are passed through a coil of pipes surrounded by water. 
‘Those which are condensed to the liquid state by this means are 
the oils, together with water, which is charged with ammonia. 
The vapours remaining uncondensed (amounting to about 2000 
cubic feet per ton) are sometimes purified and sold in the neigh- 
pourhood for illuminating purposes; yielding a light equal to 25 
candles. In other cases, these gases are passed up through a 
tower filled with porous material (such as coke), down which a 
quantity of heavy paraffin oil is allowed to trickle. This heavy 
oil absorbs the light hydrocarbons in the gas, thereby reducing the 
luminosity to approximately }-candle power. In this way about 
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