See that the joints are tight, and then commence the combustion by lighting the burners nearest the U-tube; make the first three or four inches red hot, and gradually extend the heat backwards the length of the tube, but avoid too rapid a disengagement of gas. When gas ceases to come off, open the pointed end of the tube and draw a current of dried air through the apparatus.
The carbon dioxide is absorbed in the potash bulbs, and their increase in weight multiplied 0.2727 gives the amount of carbon in the substance taken.
The increase in weight in the calcium chloride tube will be due to the water formed by the oxidation of the combined hydrogen. If this last is required the increase in weight multiplied by 0.111 gives its amount.
COALS.
The determination of the actual carbon in coals and shales is seldom called for; if required, it would be performed in the way just described.[118] The ordinary assay of a sample of coal involves the following determinations—moisture, volatile matter, fixed carbon, ash, and sulphur. These are thus carried out:—
Determination of Moisture.—Take 3 grams of the powdered sample and dry in a water-bath for an hour or so. The loss is reported as moisture. Coals carry from 1 to 2 per cent. If the drying is carried too far, coals gain a little in weight owing to oxidation, so that it is not advisable to extend it over more than one or two hours.
Determination of Volatile Matter.—This determination is an approximate one, and it is only when working under the same conditions with regard to time, amount of coal taken, and degree of heat used, that concordant results can be arrived at. It is a matter of importance whether the coal has been previously dried before heating or not, since a difference of 2 per cent. may be got by working on the dried or undried sample. Take 2 grams of the powdered, but undried, sample of coal, place in a weighed platinum crucible, and support this over a good Bunsen burner by means of a thin platinum-wire triangle. The heat is continued until no further quantity of gas comes off and burns at the mouth. This takes only a few minutes. The cover is tightly fitted on, and when cold the crucible is weighed. The loss in weight, after deducting the moisture, gives the "volatile matter," and the residue consists of "fixed carbon" and "ash."
Determination of Ash.—The coke produced in the last operation is turned out into a porcelain dish and ignited over a Bunsen burner till the residue is free from particles of carbon. Calcination is hastened by stirring with a platinum wire. The operation may be done in a muffle, but this gives results a few tenths of a per cent. too low. The dish is cooled in a dessicator, and weighed. The increase in weight gives the amount of "ash," and the difference between this and the weight of the coke gives the "fixed carbon."
The assay is reported as follows:—