80 THE BOOK OF NATURE STUDY 



Whatever care is taken with the marine aquarium estab- 

 lished at a distance from the sea, there must always come 

 a time when a fresh supply of water becomes imperatively 

 necessary. If there is no means of obtaining this, the attempt 

 may be made to manufacture artificial sea water, though this can 

 never be so satisfactory as the natural product. Gosse's formula, 

 which he used with considerable success, is as follows : 



Common tablesalt, 3^ ouncesl . 



Epsom salts, J ounce /Avoirdupois. 



Chloride of magnesium, 200 grainsl 

 Chloride of potassium, 40 grains/ 



These salts should be placed in a jar, and about four quarts of 

 river water added, until the density of the resultant fluid, as 

 tested by a hydrometer, is just over 1026, pure water being taken 

 as 1000. The cost of the salts named is small. After shaking 

 the mixture up until all the salts are dissolved, the fluid should 

 be filtered, and growing seaweed placed in it for a few days before 

 it is used in the aquarium. 



Before leaving the subject of the water, it may be well to 

 note that there are not a few seaside places where the water on 

 the shore is quite unsuitable for aquarium purposes, being much 

 contaminated with mud. In these cases it may be necessary 

 to take a boat and go out some distance before the crystal clear- 

 ness which is desirable is obtainable. This is particularly necessary 

 where an attempt is to be made to keep the more delicate forms 

 of life. 



In concluding this brief consideration of the aquarium it 

 may be well to repeat that the keeping of such a tank as that 

 described above, whether for freshwater or marine animals, can 

 never be an easy matter ; much care and experience is required, 

 and a very brief neglect may have disastrous results. On the 

 other hand, the keeping of a few animals or plants in captivity 

 for a time in a pie-dish, or similar receptacle, is not difficult, and 

 from the teacher's point of view is often far more useful. Better 

 have half a dozen dishes, each containing a small but flourishing 

 colony of animals, than one ambitious tank where the high death- 

 rate of the inhabitants is a constant heartbreak, and the survival 



