A Brief History of a River Tank. 43 



glass ends are left untouched, they in time become quite 

 opaque with a dense coating of olive- coloured vegetation. This 

 is one of the grand features of the natural system. I may 

 introduce a thousand plants, Anacharis, Valisneria, Stratiotes, 

 etc., etc., and they may all perish. But those Dame Nature 

 introduces are sure to live. Being developed in situ they are of 

 constitutions adapted to the conditions which exist in the tank, 

 and though it requires a long time for a vessel, situated as this 

 is, to become richly clothed with suitable oxygen makers, some 

 supply of oxygen is secured from the very first, for I have seen 

 ciliated spores and beginnings of genuine vegetable deposits 

 within a few hours of the first furnishing of a tank. Hence it 

 was that, fortified by previous experiences of the natural 

 system, I did not hesitate to introduce the fishes as soon as the 

 tank was furnished, without waiting for the full development of 

 the microscopic forest, for I knew that before the fishes ex- 

 hausted the oxygen in the fresh river water, there would be 

 the beginning of a new supply for them, and there was never 

 any distress through that procedure. 



I have' italicised above the words " situated as this is." 

 The situation of this tank is the secret of its success. Near it 

 on one side is a window facing west. This window lights the 

 hall abundantly, but very few rays of light from it fall directly 

 on the tank. The only direct light which strikes upon the 

 tank comes from the fanlight over the door directly opposite, 

 and that is, of course, but moderate in amount. Now, in the 

 early days, when I wanted a quick growth of Oscillatoria, and 

 other oxygen makers, I had the blind drawn up at the side 

 window, and there was then no fern case there. The abundance 

 of light caused a speedy diffusion and germination of spores, 

 and as soon as I saw that the vegetation was likely to be too 

 plentiful, I had the blind drawn down, and intercepted' the 

 light that still came by means of a fern case. Thus by toning 

 down the daylight, and having my tank where an excess of 

 light was impossible, I secured a moderate, yet plentiful 

 growth of plants, and have never had one vegetable filament more 

 than needful, except on the front glass, where of course it 

 intercepts the view of the interior. It must not be supposed, 

 however, that the hall is dark in consequence, it is, in fact, 

 well lighted, and very cheerful. 



Now a word for the fishes. I find that for a vessel of 

 this sort, which though to me an experimental and scientific 

 affair, is in all other respects a piece of elegant furniture, 

 there are no fishes that may be kept with such certainty as 

 gold carp. My old friends of nine years' standing (or swim- 

 ming) have gone through several severe trials and seem none 

 the worse. I cannot say how much they have grown during 



