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 :
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.[1]
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 :
| Weight in air | 1.1294 | gram. |
| weight in„ alcohol at 15° C. | 0.8064 | gram.„ |
| Alcohol displaced | 0.3230 | gram.„ |
The equation will be :
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.
- ↑ M. Jolly's spring balance, as modified by Mr. C. F. Cross, is another useful form of instrument for this purpose.
