Should any difficulty arise in the way of getting the salt water direct from the sea, it may be made artificially by dissolving ‘sea salt’ in the proper proportion of fresh water, or even by purchasing the different salts contained in the sea separately, and then adding them to fresh water in proportionate quantities. The composition of sea water is as follows:-- Water 96·47 per cent. Sodium chloride 2·70 ” Magnesium chloride ·36 ” Magnesium sulphate (Epsom salts) ·23 ” Calcium sulphate ·14 ” Potassium chloride ·07 ” Traces of other substances ·03 ” 100·00 and it will be seen from this table that artificial sea water may be made by adding about three and a half pounds of sea salt, obtained from the sea by the simple process of evaporation, to every ninety-six and a half pounds of fresh water used. In making it there may be some difficulty in determining the weight of the large volume of water required to fill an aquarium of moderate dimensions, but this will probably disappear if it be remembered that one gallon of water weighs just ten pounds, and, therefore, one pint weighs twenty ounces. If the sea salt cannot be readily obtained, the following mixture may be made, the different salts being purchased separately:-- Water 96-1/2 lbs. Sodium chloride (common salt) 43-1/4 ozs. Magnesium chloride 5-3/4 ” Epsom salts 3-3/4 ” Powdered gypsum (calcium sulphate) 2-1/4 ” Although in this mixture the substances contained in the sea in very small quantities have been entirely omitted, yet it will answer its purpose apparently as well as the artificial sea water prepared from the true sea salt, and may therefore be used whenever neither sea salt nor the natural sea water is procurable. Assuming, now, that the aquarium has been filled with sea water, it remains to introduce the animal and vegetable life for which it is intended; and here it will be necessary to say something with regard to the amount of life that may be safely installed, and the main conditions that determine the proportion in which the animal and vegetable life should be present in order to insure the greatest success.