XX.

Assay of Sulphurets in Ores.

It is often very necessary to know the quantity of sulphurets contained in ores, to be able to arrange accordingly the processes of smelting them.

To one part of ore, 50 to 100 grains, one part of powdered glass and two of borax are required.

The borax is first mixed with the ore, and when placed in the Hessian crucible, or in the lead and copper one, fig. 4, Plate II., both are covered with the glass. The crucible is then exposed to a bright red-heat, as with copper and lead, and on cooling the button is broken out, which may contain in the shape of sulphurets, copper, lead, iron,

&c., according as these exist in the ore, and it may then be examined for these metals by the means already given in their respective assays.

XXI.

Mode of testing the Calorific Power of Coa and other Fuels.*

It must not be supposed that the amount of heat which a fuel is capable of producing is entirely dependent on its chemical composition.

Different means have been adopted to determine the efficacy of coal as a fuel. Thus, Despretz has made experiments of this kind by discovering the amount of water which a certain quantity of coal will elevate in temperature one degree of the centigrade thermometer. The varying temperature of the water, however, must ever prevent an accurate result. Rumford has proposed to determine the same by the

*Karsten's Eisenhüttenkunde, vol. ii., p. 219, § 476.

direct use of a thermometer in a closed vessel. Berthier has, however, proposed one that seems equally simple, and serviceable for all practical purposes.

According to his proposition, one gramme of the coal (or other fuel; for it is applicable to all) should be thoroughly pulverized. All coals are readily reduced to such a state. Wood should be used as shavings, or rasped. The powdered substance is then mixed with some litharge, but more than it is capable of reducing,-not less than twenty times its own weight nor more than forty. The quality of the fuel will enable the assayer, after a little practice, to determine à priori with considerable accuracy what will be the requisite quantity. The mixture is placed in a clay crucible (Plate II., fig. 4 or 5) and carefully covered with about twenty to thirty grammes of litharge. The crucible must not be filled over onehalf. This is then placed in a heated muffle and covered. The heat should not be too

severe and rapid. The contents will boil up. When the melting has thoroughly taken place, strong heat should be applied for ten minutes, so that the button may collect. The crucible is then taken out and cooled slowly, then broken, and the button extracted and weighed.

This process rests upon the fact that the carbon will reduce the litharge; and having, therefore, assumed one number for the amount of litharge reduced, from which to calculate our experiments, it will be easy to compare the different varieties of fuel. When many experiments of this kind are made, the assayer will do well to establish a certain number as the basis. This will vary according to the general character of his coals. This number it will be well to assume as low as possible, to avoid the inconvenience of fractions. He would therefore adopt a very inferior variety of coal as the one with which to compare the others in the following manner, though