Mr; W. Thompson on MttrMi P^varh. 388 



a filter by withdrawing the impurities from the water and thus 

 keeping it sweety not knowing the fact to be that they con- 

 sumed the carbonic acid gas, and gave off oxygen for the sup- 

 ply of the animals. Thus it was left to others to find out and 

 account for what accident had thrown in my way, and to adopt 

 that principle which my ignorance had prevented me from availing 

 myself of. Having said thus much, I will proceed to give the 

 results of my experience on the subject ; first, as to prolonging 

 the health and life of the captives ; and secondly, as to a few of 

 the species most easily kept in confinement. And first as to pro- 

 longing their life, no one will doubt that after supplying them 

 with proper air, one of the main points to ensure success, is as 

 far as artificial means will allow to assimilate their state of cap- 

 tivity to their state of freedom, and this has been my chief 

 object in the experiments I have made. I began at first by 

 changing the water every tide during the day, but only for the 

 littoral species, such as Trochus umbilicatus and cinerarius, Lit- 

 torina, Rissoa and Purpura amongst shells, and Carcirms Porcel- 

 lana and Athanas amongst the Crustacea ; this I found very 

 troublesome, and I then thought the same end might be attained 

 by placing weed-covered pieces of stone in the pans in such a 

 way as that their apices might rise above the water, thus giving 

 the animals power to suit themselves as to the depth of water ; 

 this I found succeeded well ; I therefore only changed it once a 

 day, and eventually every other day ; I then extended it to a week, 

 but unsuccessfully, until accident caused me to place some plants 

 of Delesseria and Rhodymmia in the vessel as detailed in the 

 former portion of this paper. 



From my experiments, I suggest the following plan, in 

 respect to the manufacture of Vivariums. Never having seen 

 one it is possible some of my suggestions may already be in use. 

 I should propose that the bottom be made of well-seasoned 

 varnished wood, for the facility it affords of attaching battens to 

 it, and that supposing the tank to be 12 feet long, the bottom 

 should incline upwards, say at half the length, with a ledge of a 

 foot wide at the top (I send a sketch, to elucidate my meaning, 



A. The ledge. B. The incUne. C. C. Taps. D. General level of water. 



12 feet. 



\b ■loaliJ,'^ 



on a scale of 3 feet to an inch). When the tank is full there 



