THE AQUARIUM" 71 



a beautiful effect. If the glass containing the weed be placed 

 in the dark the discharge of gas will cease. Similarly, if the 

 water be boiled for a considerable time, in order to drive out the 

 contained air, and then, after being allowed to cool, poured into 

 the vessel as gently as possible, to avoid aeration, no bubbles will 

 appear when the weed is placed in it. This shows us that light 

 is necessary for the evolution of the gas, and that it is only water 

 containing air which allows the process to go on. If we add the 

 contents of a bottle of soda-water to the vessel containing the 

 weed the evolution of gas is more rapid. The gas which is 

 contained in the soda-water, and which gives it its sparkle, is 

 carbonic acid, and we thus reach the presumption that carbonic 

 acid is the constituent of the air which the plant is utilising in 

 forming the bubbles of gas. 



The next point is to ascertain the nature of the gas given off 

 by the green plant in sunlight. This may be done without any 

 special apparatus, in the following way. Fill a shallow pie-dish 

 with water and place it in the window. Then take a wide- 

 mouthed bottle, place a small piece of weed in it, fill up with 

 water, and cork carefully. The bottle should be filled to over- 

 flowing, and care taken to avoid more than a mere bubble of air 

 being left in. Then invert the bottle in the pie-dish, removing 

 the cork when the mouth is completely submerged. Place a 

 weight on the upturned base of the bottle to prevent capsizing, 

 and leave the whole in the sun for some days if necessary. As 

 the bubbles rise they will displace the water, and the gas will 

 thus accumulate at the base of the bottle. If the bottle is of 

 no great size the water will soon be virtually all displaced. If 

 it is not desired to wait so long as this the bottle, with the aid 

 of a little dexterity, can be corked under water, without the 

 admission of air, and if then suddenly inverted will contain part 

 water and part gas, which latter has now, of course, risen to the 

 top. Take a splinter of dry wood and light it. After it has burnt 

 for a few minutes, blow it out, open the bottle quickly, and thrust 

 the glowing end of the wood into it. The wood will burst again 

 into flame, showing that the gas evolved by the green plant in 

 sunshine is oxygen. 



The next point is to provide a vessel filled with lime-water, 



