Page:Handbook of Precious Stones.djvu/35

 y the weight by which the substance when weighed with brass weights will appear too light. The true weight, W, in vacuo will then be :

$$W = w + y$$

Now, with the true weight, W, in vacuo, the specific gravity may be calculated according to the equation previously given. To furnish a notion of the value of this correction, it may be stated that a fragment of rock crystal weighing 10 grams will become 10.0031 grams, a grain of 3 parts in 10,000.

When the specific gravity of a small gem is to be taken, an assay balance of great accuracy may be advantageously employed.

In this case the full advantage of the delicacy of the instrument cannot be secured if water, which has a high surface-tension, be the liquid in which the stone is weighed, the friction between it and the stone and immersed pan being too great. Alcohol considerably diluted with water answers well. A fair quantity is prepared and preserved in a well-stoppered and capped bottle. Its specific gravity is best ascertained by means of Dr. Sprengel's tube. In the following example of an experiment a dilute alcohol of sp. gr. .8488 at 15° C., and containing about 80 per cent, by weight of absolute alcohol, was used :

The equation will be :

$$\frac{1.1294\times .8488}{.323} = 2.9676 = \textrm{sp. gr. of phenakite}$$

The one objection to this use of diluted alcohol consists in the tendency which it has to change its density by loss of alcohol; on this account pure toluene, a liquid hydrocarbon, having a density of .869 at 15°C., affords a convenient substitute for spirit.

8445.