from the weight in air we will have the _loss of weight in water_, and this equals the _weight of an equal volume of water_, which is precisely what we got by our bottle method. We now need only divide the weight in air by the loss of weight in water, and we shall have the specific gravity of the stone. [Illustration: FIG. 6.] To actually weigh the stone in water we must use a fine wire to support the stone. We must first find how much this wire itself weighs (when attached by a small loop to the hook that supports the balance pan and trailing partly in the water, as will be the case when weighing the stone in water). This weight of the wire must of course be deducted to get the true weight of the stone in water. The beaker of water is best supported by a small table that stands over the balance pan. One can easily be made out of the pieces of a cigar box. (See Fig. 6.) The wire that is to support the stone should have a spiral at the bottom in which to lay the gem, and this should be so placed that the latter will be completely submerged at all times, but not touching bottom or sides of the beaker. Example of data, and calculation, when getting specific gravity by the method of weighing in water: Weight of stone = 4.02 carats ----------- Weight of stone (plus wire) in water = 3.32 carats Weight of wire = .30 carat ----------- True weight of stone in water = 3.02 carats ----------- Loss of weight in water = 1.00 carat Weight of stone 4.02 Specific gravity = --------------- = ---- = 4.02 Loss in water 1.00 Here the specific gravity, 4.02 would indicate some corundum gem (ruby or sapphire), and the other characters would indicate at once which it was. The student who means to master the use of the two methods given in Lessons V. and VI. should proceed to practice them with stones of known specific gravities until he can at least get the correct result to the first decimal place. It is not to be expected that accurate results can be had in the second decimal place, with the balances usually available to jewelers. When the learner can determine specific gravities with some certainty he should then try unknown gems.