ON THE ASSAY OF COAL FOR CRUDE OIL, ETC. 
161 
Under tliese circumstances, a portion only is removed for examination, and a 
strong aqueous solution of common salt poured into tKe bottle ; the whole is 
then well shaken, and again heated as before described. The superior spe¬ 
cific gravity of the brine now enables the oil to rise to the surface. Alter 
standing a few hours, the brine is carefully removed by a siphon. The weight 
of the oil deducted from the weight of the original distillate, gives the weight 
of the water. The retort having cooled, the coke is remoA^ed and weighed, 
and a portion burnt for ash, or otherwise examined. And now the difference 
between the combined weights of oil, water, and coke, and the quantity of coal 
originally taken gives the weight of the gases evolved in the operation. From 
these weights the percentage or any other proportions are easily calculated. 
In English commerce, conventionality requires that the oil-value of a coal be 
stated by giving the number of gallons of oil yielded by one ton of the coal. 
This is readily accomplished by calculating from the centesimal proportions 
the number of pounds of oil per ton of coal; the specific gravity of the oil is 
then ascertained in the usual way ; and now the number of pounds, divided 
by the figures representing the specific gravity, indicate the number of gallons 
afforded per ton.^ 
The value of the crude oil is next determined:—A specimen of natural 
mineral oil, or petroleum, requires similar treatment. This is a matter of 
fractionation with or without previous distillation, Avith or without previous 
purification. As a general rule it is best to at once distil about fifty cubic 
centimetres of the rough oil in a small glass retort, a thermometer being 
inserted in the liquid and the distillate allowed to flow into a cubic cen¬ 
timetre measure. Ey watching the thermometer, and roughly noting 
the amount of products yielded at different temperatures as the thermo¬ 
meter rises to 250° C., and, after removing the thermometer, at tempera¬ 
tures above 250° C. a fair indication of the character of the oil is obtained. 
A specimen of ‘ once-run ’ oil is also thus secured, the appearance and 
specific gravity of which should be observed. The residue in the retort 
should have been so heated as to be but a carbonaceous, friable mass : 
when cold it may be removed and Aveighed. If the residue form only 
two or three per cent, of the crude oil, and the latter was not of very dark 
colour, then the crude oil may probably be submitted to the process of 
purification without previous distillation, but if the loss is ten or twelve 
per cent, it is an indication that the crude oil should be ‘ once-run ’ before 
purifying. Sometimes the contents of the retort towards the close of this 
first experimental distillation assume the appearance of melted pitch; in 
that case a portion should be removed by a glass rod, and if, when cold, it is 
hard, black, and lustrous, the distillation should be stopped, and the amount 
of the pitch (asphalte) ascertained by weighing the retort and contents and 
subtracting therefrom the weight of the retort previously noted; or, if the 
retort has not been already tared, as much of the contents as possible may 
be poured out, the retort then cleaned with naphtha, and, when dry, weighed. 
If the first specimen of pitch removed from the retort does not become hard, 
the distillation should be continued some time longer. The vapours AAhich 
are evoUed above 250° C. are very heavy and only distil with difficulty from 
such a retort as just described, even though the glass be in direct contact 
with an air-gas flame. After removing the thermometer, therefore, I usually 
insert in its place a glass tube, through which a current of carbonic acid ga 
* The heavier the oil the smaller amount of photogen it will yield, for the specific gravity 
of the latter should ran^e as low as from 0*790 to 0*825 : crude oils above 0*900, or “ once-run ” 
oils above 0*880 are not yet viewed with much favour in the market, unless, by their solidity, 
obviously containing much paraffin. Their value as lubricating oil will however probably rise 
to that of burning oil Avhen the use of the latter becomes more extended.—J. A. 
VOL. VIII. 
M 
