with minute fragments of sand. This deposit proves that the chemicals we obtain are not pure; and, perhaps, it is not desirable that excessive purity should be obtained, but it certainly is desirable to keep such matters out of the tank. When the salts have been stirred up once or twice, so as to dissolve them thoroughly, test them for the last time with the hydrometer, till it registers 1.027 or thereabouts; it may safely range from 1.026 to 1.028 without interfering with the success of the experiment. Hydrometers, registered for sea-water, are not everywhere obtainable, and the specific gravity-bulb, sold by Mr. Lloyd, for a shilling, answers the purpose just as well. Mr. Cox, of 100, Newgate Street, has lately supplied me with an hydrometer of a register of 1.000 to 1.050, made in Paris—the cost was seven shillings. I prefer it to the bulb because it can be put to other uses.
A Caution to the Uninitiated.—Some beginners have attempted the preservation of marine specimens in solutions of common bay-salt, and have expressed surprise that they perish rapidly in a solution of salt obtained from the sea. Anyone at all acquainted with chemistry would readily predict, that there could be no more certain way of killing the creatures than the adoption of such a plan of preserving them. When bay-salt is prepared, many of the more soluble materials, chloride of magnesium especially, remain behind in the mother liquor, because the chloride of sodium crystallizes first, hence bay-salt alone does not produce sea-water; we must have the aid of the experienced chemist, or turn chemists and prepare it for ourselves.
