A more legitimate method of calculation is as follows:--Calculate separately the produce of each fraction as if they were from different ores. Multiply each produce (best stated in per cents.) by the weight of the corresponding fraction. Add together the products, and divide by the weight of the whole sample. Taking the same example for illustration, we have:-- _Metallics._--Weight 1 gram. 1 gram of it yielded 0.842 grams of silver. .'. Produce = 84.2 per cent. Produce multiplied by the weight is still ~84.2~. _Sifted Portion._--Weight 399 grams. 20 grams of it yielded 0.105 gram of silver. .'. Produce = 0.525 per cent. Produce multiplied by weight (0.525 × 399) is ~209.475~. Add together; and divide by 400, the weight of the whole sample-- 84.2 209.475 ------- 400) 293.675 (0.7342 0.7342 is the total produce of the ore in per cents. Referring to the 100-gram column in the table we find 239.84 ounces to the ton as the produce. 0.7 = 228.67 0.03 = 9.80 0.004 = 1.31 0.0002 = 0.06 ------ 239.84 Comparing this with the result calculated by the first method--viz., 240.26, we see that that was 0.38 oz., or between 7 and 8 dwts. too high. With ores containing "metallics" it is of great importance to powder the whole of the selected sample without loss during the process; and of even greater importance to well mix the sifted portion, of which the last portions to come through the sieve are apt to be more than ordinarily rich through the grinding down of some portions of the metallic prills. ~Remarks on Cupellation.~--Cupellation is at once the neatest and the most important of the dry methods of assaying. Its purpose is to remove easily oxidisable metals, such as lead and copper, from silver and gold, which are oxidisable with difficulty. Metals of the first class are often spoken of as _base_, and gold and silver as _noble_ metals. When lead is exposed to the action of air at a temperature a little above redness, it combines with the oxygen of the air to form litharge, an oxide of lead, which at the temperature of its formation is a _liquid_. Consequently, if the lead rests on a porous support, which allows the fused litharge to drain away as fast as it is formed, a fresh surface of the lead will be continually exposed to the action of the air, and the operation goes on until the whole of the lead has been removed. Silver or gold exposed to similar treatment does not oxidise, but retains its metallic condition; so that an alloy of lead and silver similarly treated would yield its lead as oxide, which would sink into the support, while the silver would remain as a button of metal.