70 



with the proportion of dissolved material, we are enabled to deter- 

 mine the degree of salinity by finding the density of the solution. 

 We can apply this principle to the aquarium, as a means of 

 determining whether the water contains the correct amount of sea 

 salt, also for testing any artificial salt water that has been prepared 

 for the aquarium. 



Probably some of our readers are acquainted with some form 

 of hydrometer an instrument used for finding the density of 

 any liquid ; but we will describe a simple substitute that may be of 

 use to the owner of a marine aquarium, especially if the salt water 

 for the same is artificially prepared. Melt a little bees-wax, and mix 

 it with fine, clean sand. Then, remembering that the wax is lighter 

 than water, and consequently floats, while sand is considerably 

 heavier, and sinks, adjust the above mixture until a solid ball of 

 it is just heavy enough to sink very slowly in sea water. Now 

 make two such balls, and then cover one of them with a light 

 coating of pure wax. "We have now two balls, one of which will 

 float in sea water, and the other sink, and these may be used at 

 any time to test the density of the water in, or prepared for, the 

 aquarium. If the water is only a little too salt, both balls will 

 float ; while, if not sufficiently rich in saline matter, both will sink. 



We must conclude this chapter by making a few remarks on 

 the important matter of the regulation of light and temperature. 

 Direct sunlight should always be avoided, except for short and 

 occasional intervals, not only because it is liable to raise the 

 temperature to a higher degree than is suitable for the inmates of 

 the aquarium, but also because an excess of light and heat tends 

 to produce a rapid decomposition of organic matter, and a con- 

 sequent putrid condition of the water, and this dangerous state is 

 most likely to occur when both light and temperature are high at 

 the same time. 



The water should always be cold ; and as it is not always easy to 

 estimate the temperature, even approximately, by the sensation 

 produced on immersing the fingers, it is a good plan to have a 

 small thermometer always at hand, or placed permanently in the 

 aquarium. In the summer time the water should be kept down to 

 fifty-five degrees or lower, and in winter should never be allowed 

 to cool much below forty. There may be some difficulty in main- 

 taining a temperature sufficiently low in summer, but a small piece 

 of ice thrown in occasionally to replace the loss due to evaporation, 

 especially on very hot days, will help to keep it down. 



