------------+----------+----------+-----------+-----------+------------ When, as in this table, the fraction of an ounce is expressed by two places of decimals, it may be reduced to pennyweights (dwts.) by dividing by 5. For example, 0.40 of an ounce is 8 dwts. The fraction of a dwt. similarly expressed may be converted into grains with sufficient exactness by dividing by 4. Thus, 1.63 ozs. equal 1 oz. 12.60 dwts., or 1 oz. 12 dwts. 15 grains. In England it is usual to report in ounces and decimals of an ounce. The way to use the table is best shown by an example. Suppose a button of silver weighing 0.0435 gram was obtained from 20 grams of ore. Look down the 20-gram column of the table, and select the values corresponding to each figure of the weight, thus:-- 0.04 = 65.33 ozs. to the ton 0.003 = 4.90 " 0.0005 = 0.82 " ------ ----- 0.0435 = 71.05 " Add these together. The produce is 71.05 ozs., or 71 ozs. 1 dwt. to the ton. Or, suppose an ore is known to contain 1.24 per cent. of silver. Look down the 100-gram column, select the values, and add them together as before. 1.0 = 326.67 ozs. per ton 0.2 = 65.33 " 0.04 = 13.07 " ---- ------ 1.24 = 405.07 " This gives 405 ozs. 1 dwt. 10 grains to the ton. The calculation becomes more complicated when, as is frequently the case, the ore contains metallic particles. These show themselves by refusing to pass through the sieve when the ore is powdered. When they are present, a large portion, or if feasible the whole, of the sample is powdered and sifted. The weights of the sifted portion and of the "metallics," or prills, are taken; the sum of these weights gives that of the whole of the sample taken. It is very important that nothing be lost during the operation of powdering. Each portion has to be assayed separately. It is usual to assay a portion of the sifted sample, say, 20 or 50 grams, and to add to the produce of this its share of the "metallics." This way of calculating, which is more convenient than correct, is illustrated by the following example:-- Weight of whole sample 400 grams Made up of sifted portions 399 " " "Metallics" 1 " --- 400 " Twenty grams of the sifted portion, when assayed, gave 0.1050 gram of silver. The whole of the "metallics" scorified and cupelled gave 0.842 gram of silver. Since the 20 grams assayed was 1-20th of the whole, 1-20th part of the 0.842 gram of silver (from the metallics) must be added to its produce. We thus get 0.1471 gram (0.1050 + 0.0421). Referring to the 20 gram column, we get-- 0.1 = 163.33 0.04 = 65.33 0.007 = 11.43 0.0001 = 0.16 ------ ------ 0.1471 = 240.25 ounces per ton.