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62 THE MARINE AQUARIUM. 
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 espe- 
cially, 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. 
