Specific gravity bottles determine liquid densities by measuring the difference between an empty and filled bottle and dividing by an equal volume of water to find the specific gravity of the substance. These bottles are also known as a density bottle or relative-density bottles. Specific gravity bottles are typically constructed from glass to provide clear views of the contents in containers that can withstand volatile liquids and semi-solid bitumens. Specific gravity bottles may include caps, which aid in preventing evaporation that could damage results. Some specific gravity bottles have graduated necks for easier measurements. The bottles come in a variety of sizes and shapes depending on the material to be measured.