» THE MARINE AQUARIUM. 99 



Common table salt 81 parts. 



Epsom Salts • ' 



Chloride of Magnesium 10 " 



Chloride of Potassium 2 " 



One pound avoirdupois of this compound will make 

 nearly three gallons of sea water. But the best mode of 

 mixing the salt and water is by means of a little glass 

 instrument called a hydrometer, which cau be procured of 

 dealers in chemical apparatus. If one of these that is used 

 to examine acids, and called an acidimclre, is procured, it 

 will be found to bo numbered on its graduated scale in 

 numbers running from the top downwards. Now, when it 

 is placed in pure fresh water it should float at zero, but, 

 in sea water, at a point somewhere about three and a half 

 or between the degrees marked three and four. If then 

 we dissolve all our salt, say three pounds, in as much 

 water as will just float the hydrometer and keep adding 

 water until we find it floats at the point mentioned, we 

 shall have it of the right strength. "When the salts are to 

 be mixed, it is best to do so in a tub or some other vessel, 

 and not in the tank itself, for the reason that it is pre- 

 ferable to let it stand for a day to allow all sand and 

 dust that might have been in the salts to settle, when it 

 will become perfectly clear and bright and can be intro- 

 duced into the tank. Artificial sea-water has one disadvan- 

 tage, and that is, that it has to be seasoned, in other 

 words, it has to be used about a week before we can 

 introduce our animals, and for that purpose we put in as 

 much Vina and Entcromorpha as we can and leave it in 

 the sun, when we shall see the plants give off abundance 



