Re: How do you measure specific gravity of a solid which floats in water?

Dear Jennifer,
The appropriate instrument to measure specific gravity of powders, 
especially if they float on water, is a GAS PYCNOMETER. It works along the 
same principles of volume displacement, as a liquid pycnometer, although 
the gas pressure has to be controlled. Such instruments are commercially 
available at various degrees of automation (see e.g.  http://micromeritics.com/catalog_index.html). 
Probably a manual instrument (Beckman) is to be found in a corner of a 
cupboard of any powder analysis lab. 
Another possibility, if urgency prevails over accuracy, is to use an 
ordinary pycnometer (glass vial) filled with a low-density liquid 
(methanol, heptane, "Isopar G"..) instead of water. Some skill is needed 
due to the volatility of these liquids. Their density is also very 
temperature-dependent.
Best Regards
Werner Sieber