A device often adopted in practice where a large number of assays of one kind are made, and the report is given as so many ounces or pounds to the ton, is that known as the _assay ton_. The assay ton may be any arbitrary and convenient weight, but its subdivisions must bear to it the same relations as pounds and ounces bear to the actual ton. On the other hand, in a laboratory where many kinds of work are performed, different sets of weights of this kind would only tend to confusion, even if they were not unnecessary. With a set of gram weights and its subdivisions anything may be done. If it is desired to report as pounds to the ton, then, since there are 2240 lbs. to the ton, a weight of 2.240 grams may be taken as the assay ton, and each 0.001 gram yielded will equal 1 lb., or 22.4 grams may represent the ton, and each 0.01 gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton; if we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1 oz. to the ton. In some cases it may be convenient to have, in addition to the usual gram weights, one or other of the "assay tons" mentioned above, but generally it is better to work on a purely decimal system, and convert when required into ounces per ton, &c., either by actual calculation or by reference to a set of tables. PRACTICAL EXERCISES. The student should practise such calculations as the following:-- 1. Calculate the percentages in the following cases:-- (a) Ore taken, 2 grams; copper found, 0.2155. (b) " 1.5 gram; iron found, 0.8340. (c) " 30 grams; lead found, 23.2. 2. Calculate the parts per thousand in the following:-- (a) Bullion taken, 1.1 gram; silver found, 1.017. (b) " 1.14 gram; silver found, 1.026. (c) " 0.6 gram; gold found, 0.5500. 3. Calculate parts per 100,000 in the following:-- (a) Ore taken, 20 grams; silver found, 0.0075. (b) " 50 grams; gold found, 0.0026. (c) Water taken, 500 c.c.; solids found, 0.1205. 4. Calculate cwts. to the ton in the following:-- (a) Ore taken, 5 grams; tin found, 2.816. (b) " 5 grams; tin found, 3.128. (c) An ore with 68.2 per cent. of tin. 5. Calculate lbs. to the ton in the following:-- (a) An ore with 3.28 per cent. oxide of tin. (b) Ore taken, 20 grams; oxide of tin found, 1.67. 6. Calculate ozs. (troy) to the ton in the following:-- (a) Ore taken, 50 grams; gold found, 0.0035. (b) " 20 grams; silver found, 0.0287. (c) " 25 grains; silver found, 0.0164. 7. Calculate in grains per gallon:-- (a) 0.51 gram per litre. (b) 24.6 parts per 100,000. (c) Solution taken, 100 c.c.; copper found, 0.0045 gram. (c) " 50 c.c.; iron found, 0.165 gram. 8. Convert into ozs. (troy) per ton:-- (a) 7 loths per centner. (b) 30 grams per quintal. (c) 15 parts per 100,000. FOOTNOTES: [1] Ether or carbon bisulphide.