72 SPECIFIC OEAVITT. whole must then he left to cool down to nearly the temperature of the air before weighing. The vessel containing the water should be as large as con venience will allow. When it is too small, the results are apt to be erroneous, in consequence perhaps of a circulation of the water when its temperature differs slightly from that of the air 261. When the substance floats in water, the double hook must be made to clasp it, and be heavy enough to cause the whole to sink. Proceed to weigh as before ; and, approximately, w + x Or, u being the weight of the air displaced by the mineral, Q. — + W w + w + X To find r in this case, we must subtract the apparent weight of the hook together with the mineral in water, from the apparent weight of the hook alone in water Minerals soluble in water must be weighed in oil, the weight O ^ ri - at , of water, must be ascertained. 262. The following method also admits of considerable ac- curacy:— A glass cup, like the annexed figure, with its fio 79 rim ground truly plane, provided with a lid of glass, ' ' also ground plane is to be filled with water, closed, and any water adhering to the outside carefully wiped off. The cup is then to be weighed in any delicate balance hunv put the mineral into the cup, close it with the lid, wipe off the water ad hering to the outside of the cup, and weigh it as before. Let « be the weight of the mineral; * the weight of the cup filled with water; y the weight of the cup containing the mineral, and filled up with water. Then, approximately, o = w U> + X — y Or, u being the weight of the air displaced by the mineral, a = w + « W + X -f u — y ' The escape of water between the rim of the cup and its lid may be prevented by coating the under surface of the lid with a very thin film of grease, or turpentine. Glass cups should be provided of different sizes, and the smallest used 11 • i uovu I!) capable of containing the mineral. 263. The same cup may be used in finding the specific gravity of a liquid. ^ Let x be the weight of the lid and cup filled with the liquid
