132 



SUMMARY OF CURRENT RESEARCHES, ETC. 



third tube I conducts the air from the bottle to the aquarium, while 

 the water escapes from the bottle through the tap at the bottom. All 

 that is necessary is to regulate the flow into and out of the bottle in 

 such a way that the water shall be at a constant level. "When this 

 has once been experimentally ascertained the aquarium may be left 



Fig. 27. 



without fear day and night. If the bottle were allowed to get empty 

 the aeration would of course stop, while if it were filled the fresh 

 water would pass into the aquarium. In order to supply the loss 

 from evaporation a little fresh water should be added from time to 

 time, which will prevent the necessity for renewing with salt water. 



The apparatus will pass 22^ litres of air per hour through an 

 aquarium of 90 litres at an expenditure of water of 36 litres. In 

 this case the exit tube for the air, 5 mm. in diameter, is plunged 11 cm. 

 into the aquarium. If the tube is plunged lower, say 36 cm., the 

 pressure of the water which obstructs the exit of the air is greater, 

 and 45 litres of water would be expended in passing 16 litres of air, 

 i. e. 9 litres of water more, and 6^ litres of air less. In the author's 

 opinion, apart from the increase in the expenditure of watei-, it is un- 

 desirable that the air tube should go to the bottom of the aquarium, as 

 the disturbance to the water which is thus caused is unfavourable to 

 the development of delicate animals. 



To ensure that the air-bubbles shall be small, the air tube is 

 terminated by a small sphere with half-a-dozen very small orifices at 

 its equator, and enveloped with two or three thicknesses of muslin. 



