388 Proceedings of the Royal Physical Society. 



better than anything previously tried. Besides introduc- 

 ing air into the tanks, this arrangement ensures that the 

 water also is thoroughly aerated as it enters. Figs. 4 and 

 5 show two arrangements which have been tried. These 

 two are most applicable for comparatively shallow tanks, 

 while the one shown in Fig. 6 may be used in tanks of 

 any depth. The water, in issuing from the fine pointed 

 tube, draws in with it a current of air through the open tube, 

 and the two form a mass of froth in the lower one. The out- 

 let is contracted a little so as to confine this froth and make 

 it enter the tank in the form of a stream. This arrangement 

 gives even smaller bubbles than before. By the arrangement 

 shown in Fig. 6 the mixed air and water are made to enter 

 the tank at the bottom, so that the bubbles of air have to 

 pass through the whole of the water before they can make 

 their escape. This ensures perfect aeration, and some of the 

 bubbles are so small as to require a lens to see them. By 

 these improved methods the daily circulation has been re- 

 duced to 620 to 630 gallons, and the motor only uses 340 

 gallons to pump this quantity. Thus the cost of circulation 

 has been reduced from 14s. a month to 5s. I should mention 

 that the 14s. a month does not represent the cost when the 

 vulcanite jets were in use ; these were soon found to be so 

 expensive, that they quickly gave way to the plain glass 

 tube arrangement, and I kept no account of the daily circula- 

 tion at that time. 



Altogether, the water in my aquarium, which has never 

 been changed since the commencement, was never in such 

 good condition as it is at present, and never contained as 

 great a quantity of microscopic life. 



Although this system is so generally applicable and so 

 satisfactory, it must not be supposed that I recommend its 

 universal application. Probably no single system of aeration 

 is equally advantageous for all marine organisms. Although 

 pure and well aerated water is a necessity for nearly all, it is 

 also necessary to be acquainted with the natural habitat of 

 any particular form before one is able to arrange a system of 

 circulation suited to its requirements. For instance, Anemonia 

 sulcata {Antheus cereus), one of the commonest anemones in 



